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MIXED-PHASE
CLOUDS
Observations and Modeling
Edited by
CONSTANTIN ANDRONACHE

Elsevier
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The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom
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or mechanical, including photocopying, recording, or any information storage and retrieval system, without
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Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/
permissions.
This book and the individual contributions contained in it are protected under copyright by the
Publisher (other than as may be noted herein).
Notices
Knowledge and best practice in this field are constantly changing. As new research and experience broaden
our understanding, changes in research methods, professional practices, or medical treatment may
become necessary.
Practitioners and researchers must always rely on their own experience and knowledge in evaluating
and using any information, methods, compounds, or experiments described herein. In using such
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any liability for any injury and/or damage to persons or property as a matter of products liability,
negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas
contained in the material herein.
Library of Congress Cataloging-in-Publication Data
A catalog record for this book is available from the Library of Congress
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A catalogue record for this book is available from the British Library
ISBN: 978-0-12-810549-8
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Typeset by SPi Global, India

CONTRIBUTORS
Andrew S. Ackerman
National Aeronautics and Space Administration, Goddard Institute for Space Studies, New York,
NY, United States
Constantin Andronache
Boston College, Chestnut Hill, MA, United States
Joseph Finlon
University of Illinois at Urbana-Champaign, Urbana, IL, United States
Jeffrey French
University of Wyoming, Laramie, WY, United States
Ann M. Fridlind
National Aeronautics and Space Administration, Goddard Institute for Space Studies, New York,
NY, United States
Kalli Furtado
Met Office, Exeter, United Kingdom
Dennis L. Hartmann
University of Washington, Seattle, WA, United States
Robert Jackson
Argonne National Laboratory, Environmental Sciences Division, Lemont, IL, United States
Olivier Jourdan
Universit
e Clermont Auvergne, Clermont-Ferrand; CNRS, Aubière, France
Daniel T. McCoy
University of Leeds, Leeds, United Kingdom
Steven D. Miller
Colorado State University, Fort Collins, CO, United States
Guillaume Mioche
Universit
e Clermont Auvergne, Clermont-Ferrand; CNRS, Aubière, France
Yoo-Jeong Noh
Trude Storelvmo
Yale University, New Haven, CT, United States
ix

Ivy Tan
Yale University, New Haven, CT, United States
Thomas F. Whale
Mark D. Zelinka
Lawrence Livermore National Laboratory, Livermore, CA, United States
x Contributors

PREFACE
The objective of this book is to present a series of advanced research topics on mixed-
phase clouds. The motivation of this project is the recognized important role clouds play
in weather and climate. Clouds influence the atmospheric radiative balance and hydro-
logical cycle of the Earth. Reducing uncertainties in weather forecasting and climate pro-
jections requires accurate cloud observations and improved representation in numerical
cloud models. In this effort to better understand the role of cloud systems, the mixed-
phase clouds present particular challenges, which are illustrated in this book.
The book has two parts, covering a wide range of topics. The first part, “Observa-
tions,” contains articles on cloud microphysics, in situ and ground-based observations,
passive and active satellite measurements, and synergistic use of aircraft data with space-
borne measurements. The second part, “Modeling,” covers numerical modeling using
large eddy simulations to analyze Arctic mixed-phase clouds, and global climate models
to address cloud feedbacks and climate sensitivity to mixed-phase cloud characteristics. It
is my hope that this book will give some indication of the enormous power and future
potential of increasing refined observation techniques and numerical modeling at mul-
tiple scales to solve the complex problems of the role of cloud systems in Earth Sciences.
The publication of this book would not have been possible without the help, interest,
and enthusiasm of the contributing authors. I would like to thank all of the authors and
their supporting institutions for making this project possible. I am particularly grateful to
Ann Fridlind, Michael Folmer, Daniel McCoy, Ivy Tan, and Michael Tjernstr€
om who
offered many useful suggestions during the review process. Finally, it is a great pleasure to
acknowledge Candice Janco, Laura Kelleher, Louisa Hutchins, Tasha Frank, Anitha
Sivaraj, and Anita Mercy Vethakkan from Elsevier for their willing, dedicated, and con-
tinuous help during the project.
Boston Massachusetts
xi

CHAPTER 1
Introduction
Boston College, Chestnut Hill, MA, United States
Contents
1. Observations 2
2. Modeling 5
3. Concluding Remarks 7
Acknowledgments 7
References 7
Clouds have a significant influence on the atmospheric radiation balance and hydrolog-
ical cycle. By interacting with incoming shortwave radiation and outgoing longwave
radiation, clouds impact the energy budget of the Earth. They also have an important
role in the Earth’s hydrological cycle by affecting water transport and precipitation
(Gettelman and Sherwood, 2016). The interaction of clouds with atmospheric radiation
depends on hydrometeor phase, size, and shape. Under favorable humidity conditions,
the cloud phase is determined largely by the temperature, condensation nuclei, and ice
nuclei in the atmosphere. When the cloud temperature is above 0°C, clouds are formed
of liquid water droplets. Ice clouds consist of ice crystals and can be found at temperatures
well below 0°C. In some clouds, supercooled liquid droplets coexist with ice crystals,
most frequently at temperatures from 35°C to 0°C. These are mixed-phase clouds,
which are particularly difficult to observe and describe in numerical weather prediction
(NWP) and climate models.
Mixed-phase clouds cover a large area of the Earth’s surface, and are often persistent,
with a liquid layer on top of ice clouds. Many observations have documented the pres-
ence of these clouds in all regions of the world and in all seasons (Shupe et al., 2008).
They tend to be more frequent at mid- and high-latitudes, where temperatures are
favorable to the formation and persistence of supercooled liquid clouds. Global clima-
tology is available from CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Sat-
ellite Observations (CALIPSO) data, accumulated in recent years (Stephens et al., 2002;
Winker et al., 2009; Zhang et al., 2010). Earlier observations, based on aircraft in situ
measurements, detected single layer mixed-phase clouds characterized by a layer of
supercooled liquid droplets at the top of an ice cloud (Rauber and Tokay, 1991). Over
the last decades, in situ observations using instrumented aircraft (Baumgardner et al.,
2011), have provided very detailed insights in cloud microphysics and dynamical
1
Mixed-Phase Clouds © 2018 Elsevier Inc.
https://doi.org/10.1016/B978-0-12-810549-8.00001-5 All rights reserved.

conditions that form and maintain these clouds. Such data are essential for calibration of
ground-based and spaceborne remote sensing instruments, as well as for the validation of
numerical models.
Given the importance of mixed-phase clouds in a number of applications, such as the
prediction and prevention of aircraft icing, weather modification, and improvement of
NWP and climate projections, a series of research programs have contributed to rapid
progress in these areas. Selected results are illustrated in this volume, accompanied by
references to the most recent studies. The chapters of this book present research on var-
ious aspects of mixed-phase clouds, from cloud microphysics to GCM simulations.
Chapters 2–6 focus mainly on observational aspects, while Chapters 7–10 illustrate
modeling work from small scales using LES to a global scale using GCMs. The next sec-
tions give a short description of each chapter.
1. OBSERVATIONS
Chapter 2 discusses the relevance of ice nucleation to mixed-phase clouds, and current
research on ice nuclei particles (INPs) in the atmosphere. The existence of mixed-phase
clouds is possible because liquid water droplets can exist in a supercooled state at tem-
peratures as low as 38°C. For lower temperatures, in the absence of INPs, the process
of homogeneous ice nucleation can start. The coexistence of liquid water droplets and
ice particles in mixed-phase clouds requires specific microphysical and dynamical con-
ditions. When a cloud consisting of supercooled liquid water droplets evolves to a state
containing some ice crystals, the process of ice nucleation is involved. Despite decades of
research, the process of heterogeneous ice nucleation is not sufficiently known (Phillips
et al., 2008, 2013; DeMott et al., 2011; Atkinson et al., 2013). A better characterization of
the heterogeneous ice nucleation process is needed for the understanding of mixed-phase
clouds. This chapter reviews a series of topics relevant for the study of mixed-phase
clouds. First, the modes of heterogeneous ice nucleation are described, with a focus
on deposition ice nucleation and freezing ice nucleation. Second, the ice nucleation
in the atmosphere—particularly in mixed-phase clouds—is summarized and discussed.
Third, the experimental methods for examining ice nucleation are presented with a focus
on wet and dry dispersion methods. Fourth, the nucleation theory is concisely explained
in both homogeneous and heterogeneous cases. Fifth, the properties of good hetero-
geneous ice nucleators are discussed, including the direct measurement of INP concen-
tration in the atmosphere. This information on direct measurements is particularly
important for (a) providing atmospheric model input data, and (b) allowing comparisons
between models and observations, thus contributing to the understanding of the ice
nucleation processes in the atmosphere.
Chapter 3 introduces a method for the detection of liquid-top mixed-phase (LTMP)
clouds from satellite passive radiometer observations. While in situ measurements of
2 Mixed-Phase Clouds

mixed-phase clouds provide detailed information for these clouds, such observations are
limited and insufficient for many applications. Satellite remote-sensing techniques are
efficient for the continuous monitoring and characterization of mixed-phase clouds.
Active satellite sensor measurements, such as CloudSat and CALIPSO have the capability
to observe detailed vertical structures of mixed-phase clouds. Nevertheless, they are lim-
ited to a spatial domain along the satellite path (Stephens et al., 2002; Winker et al., 2009)
and have limited applicability for some short-term purposes. Thus, there is great interest
in developing methods for mixed-phase clouds detection using passive radiometry. If
adequate methods are developed, satellite remote sensing will provide an ideal venue
for observing the global distribution of mixed-phase clouds and the detailed structures
such as LTMP clouds. This chapter introduces a method of daytime detection of LTMP
clouds from passive radiometer observations, which utilizes reflected sunlight in narrow
bands at 1.6 and 2.25 μm to probe below liquid-topped clouds. The basis of the algorithm
is established on differential absorption properties of liquid and ice particles and accounts
for varying sun/sensor geometry and cloud optical properties (Miller et al., 2014). The
algorithm has been applied to the Visible/Infrared Imaging Radiometer Suite (VIIRS) on
the Suomi National Polar-orbiting Partnership VIIRS/S-NPP and Himawari-8
Advanced Himawari Imager (Himawari-8 AHI). The measurements with the active sen-
sors from CloudSat and CALIPSO were used for evaluation. The results showed that the
algorithm has potential to distinguish LTMP clouds under a wide range of conditions,
with possible practical applications for the aviation community.
Chapter 4 illustrates some of the problems associated with the microphysical proper-
ties of convectively forced mixed-phase clouds. Field experiments are conducted using
aircraft with particle measurement probes to obtain direct observations of the microphys-
ical properties of clouds. Such experiments have been carried out to study various types of
cloud systems, including supercooled clouds and mixed-phase clouds. One particular
subset of these clouds is the convectively forced mixed-phase clouds. Analysis of obser-
vations based on retrievals from CloudSat, CALIPSO, and Moderate Resolution Imag-
ing Spectroradiometer (MODIS) show that about 30%–60% of precipitating clouds in
the mid- and high-latitudes contain mixed-phase (M€
ulmenst€
adt et al., 2015). In this
chapter, authors describe in detail the methodology used in aircraft campaigns, what
quantities are typically measured, the importance of particle size distribution (PSD) of
hydrometeors, and its moments. The primary in situ measurement methods reviewed
include bulk measurements, single particle probes, and imaging probes, with references
to recent field campaigns ( Jackson et al., 2012, 2014; Jackson and McFarquhar, 2014).
Examples of observations made during the COnvective Precipitation Experiment
(COPE) in southwest England during summer 2013 are presented, with a detailed anal-
ysis of liquid water content (LWC), ice water content (IWC), and PSD characterization.
In general, the microphysical properties of convective clouds can be widely variable due
to numerous factors that include temperature, position in the cloud, vertical velocity,
3
Introduction

strength of entrainment, and the amount of cloud condensation nuclei loaded into the
cloud. The study illustrates that determining IWC from the airborne measurement is
much more challenging than determining LWC. Therefore, reducing the uncertainty
in IWC from airborne cloud microphysical measurements remains an important research
priority.
Chapter 5 provides an overview of the characterization of mixed-phase clouds from
field campaigns and ground-based networks. Earlier field campaigns focused on measure-
ments of the microphysical and dynamical conditions of mixed-phase cloud formation
and evolution (Rauber and Tokay, 1991; Heymsfield et al., 1991; Heymsfield and
Miloshevich, 1993). These studies contributed to solving problems such as aircraft icing
and cloud seeding for weather modification. In situ aircraft measurements documented
the presence of mixed-phase clouds with a layer of supercooled liquid water on the top of
an ice cloud. The US Department of Energy (DOE) Atmospheric Radiation Measure-
ment (ARM) program and its focus on the role of clouds in the climate system facilitated
many field missions. Some were directed to observations in Arctic regions, aiming to
establish a permanent observational station in Barrow, Alaska (Verlinde et al., 2016).
Advances in ground-based remote sensing capabilities developed by the ARM program,
aided by field campaigns, produced accurate methods to observe atmospheric processes
related to water vapor, aerosol, clouds, and radiation. The ability to detect and charac-
terize mixed-phase clouds at ARM sites provided the basis for developing additional
observation stations in other parts of the world. One significant development in Europe
was the Cloudnet program, which established a standard set of ground-based remote
sensing instruments capable of providing cloud parameters that can be compared with
current operational NWP models (Illingworth et al., 2007). Developments following
the Cloudnet program and the expansion of ARM capabilities and collaborations have
resulted in a more comprehensive approach for monitoring cloud systems— including
mixed-phase clouds—at a variety of sites, enabling the evaluation and improvement
of high-resolution numerical models (Haeffelin et al., 2016).
Chapter 6 focuses on the characterization of mixed-phase clouds in the Arctic region,
using aircraft in situ measurements and satellite observations. Data from the CALIPSO
and CloudSat satellites are used to determine the frequency of mixed-phase clouds.
Results show that mixed-phase clouds exhibit a frequent and nearly constant presence
in the Atlantic side of the Arctic region. In contrast, the Pacific side of the Arctic region
has a distinct seasonal variability, with mixed-phase clouds less frequent in winter and
spring and more frequent in summer and fall. The vertical distribution of mixed-phase
clouds showed that generally, they are present below 3 km, except in summer when these
clouds are frequently observed at mid-altitudes (3–6 km). Results indicate that the North
Atlantic Ocean and the melting of sea ice influence the spatial, vertical, and seasonal var-
iability of mixed-phase clouds (Mioche et al., 2015, 2017). The microphysical and optical
properties of the ice crystals and liquid droplets within mixed-phase clouds and the

associated formation and growth processes responsible for the cloud life cycle are eval-
uated based on in situ airborne observations. Lastly, the authors show that the coupling of
in situ mixed-phase clouds airborne measurements with the collocated satellite active
remote sensing from CloudSat radar and CALIOP lidar measurements are useful in val-
idating remote sensing observations.
2. MODELING
Chapter 7 provides an overview of numerical simulations of mixed-phase boundary layer
clouds using large eddy simulation (LES) modeling. Atmospheric turbulent mixing
characterizes boundary layer clouds, and the LES modeling has been extensively used
to represent the coupling between dynamical and mixed-phase microphysical processes.
Many detailed LES and intercomparison studies have been based on specific cloud sys-
tems observed during field campaigns (McFarquhar et al., 2007; Fridlind et al., 2007,
2012; Morrison et al., 2011). The focus of this chapter is mainly on modeling results from
the three major field campaigns on which intercomparison studies have been based: the
First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment-
Arctic Cloud Experiment (FIRE-ACE)/Surface Heat Budget in the Arctic (SHEBA)
campaign (Curry et al., 2000), the Mixed-Phase Arctic Cloud Experiment (M-PACE)
(Verlinde et al., 2007), and the Indirect and Semi-Direct Aerosol Campaign (ISDAC)
(McFarquhar et al., 2011). The chapter presents detailed results from each case study
and discusses outstanding questions about fundamental microphysical processes of Arctic
mixed-phase clouds.
Chapter 8 presents efforts toward a parametrization of mixed-phase clouds in general
circulation models. Observations show that mid- and high-latitude mixed-phase clouds
have a prolonged existence, considerably longer than most models predict. A series of
simplified physical models and LES simulations have been applied to data from aircraft
observations to understand the factors that lead to the longevity of mixed-phase clouds.
The results from many case studies indicate that the persistence of mixed-phase condi-
tions is the result of the competition between small-scale turbulent air motions and ice
microphysical processes (Korolev and Field, 2008; Hill et al., 2014; Field et al., 2014;
Furtado et al., 2016). Under certain situations, this competition can sustain a steady state
in which water saturated conditions are maintained for an extended period of time in a
constant fraction of the cloud volume. This chapter examines previous work on under-
standing this mechanism and explains how it can be elaborated into a parametrization of
mixed-phase clouds. The parametrization is constructed on exact, steady state solutions
for the statistics of supersaturation variations in a turbulent cloud layer, from which
expressions for the liquid-cloud properties can be obtained. The chapter reviews the
implementation of the parametrization in a general circulation model. It has been shown
to correct the representation of Arctic stratus, compared to in situ observations, and
5
Introduction

improve the distribution of liquid water at high latitudes. Some important consequences
of these enhancements are the reduction in the recognized radiative biases over the
Southern Ocean and improvement of the sea surface temperatures in fully coupled cli-
mate simulations.
Chapter 9 introduces and examines cloud feedback in the climate system. The
reflected shortwave (SW) radiation by the oceanic boundary layer (BL) clouds leads to
a negative cloud radiative effect (CRE) that strongly affects the Earth’s radiative balance.
The response of the BL clouds to climate warming represents a cloud feedback that is
highly uncertain in current global climate models. This situation impacts the uncertainty
in the estimation of equilibrium climate sensitivity (ECS), defined as the change in the
equilibrated surface temperature response to a doubling of atmospheric CO2 concentra-
tions. This chapter considers cloud feedback, with a focus on the mid- and high-latitudes
where cloud albedo increases with warming, as simulated by global climate models. In
these regions, the increase in cloud albedo appears to be caused by mixed-phase clouds
transitioning from a more ice-dominated to a more liquid-dominated state (McCoy et al.,
2014, 2015, 2016). The chapter discusses problems in constraining mixed-phase clouds in
global climate models due to: (a) uncertainties in ice nucleation—a fundamental micro-
physical process in mixed-phase clouds formation, and (b) current difficulties in measur-
ing the cloud ice mass. Another feature of global climate models is that they use a
parameterization of mixed-phase clouds. A frequent approach is to use a phase partition
with temperature based on aircraft measurements. One serious limitation of this method
is that it cannot account for the regional variability of ice nuclei (IN) (DeMott et al.,
2011). Comparisons with satellite data suggest that this behavior appears to be, at least
to some extent, due to an inability to maintain supercooled liquid water at sufficiently
low temperatures in current global climate models.
Chapter 10 addresses the impact of mixed-phase clouds’ supercooled liquid fraction
(SLF) on ECS. The ECS is a measure of the ultimate response of the climate system to
doubled atmospheric CO2 concentrations. Recent work involving GCM simulations
aimed to determine ECS due to changes in the cloud system in a warming climate. This
chapter examines the impact of mixed-phase clouds SLF on ECS using a series of
coupled climate simulations constrained by satellite observations. It follows a series of
recent studies on mixed-phase cloud feedback as determined by GCM simulations
(Storelvmo et al., 2015; Tan and Storelvmo, 2016; Tan et al., 2016; Zelinka et al.,
2012a,b). This study presents non-cloud feedbacks (Planck, water vapor, lapse rate,
and albedo) and cloud feedbacks (cloud optical depth, height, and amount). The cloud
phase feedback is a subcategory within the cloud optical depth feedback. It relates to how
the repartitioning of cloud liquid droplets and ice crystals affects the reflectivity of
mixed-phase clouds. Results suggest that cloud thermodynamic phase plays a significant
role in the SW optical depth feedback in the extratropical regions, and ultimately influ-
ences climate change.

3. CONCLUDING REMARKS
The recent research on mixed-phase clouds presented in this volume, as well as the
selected references for each chapter, provide an overview of current efforts to appreciate
cloud systems and their role in weather and climate. Understanding the role of clouds in
the atmosphere is increasingly imperative for applications such as short-term weather
forecast, prediction and prevention of aircraft icing, weather modification, assessment
of the effects of cloud phase partition on climate models, and accurate climate projections.
In response to these challenges, there is a constant need to refine atmospheric observation
techniques and numerical models. These efforts are sustained by many evolving research
programs and by a vibrant community of scientists. The book “Mixed-phase Clouds:
Observations and Modeling” provides the essential information to help readers under-
stand the current status of observations, simulations, and applications of mixed-phase
clouds, and their implications for weather and climate.
ACKNOWLEDGMENTS
I want to express my sincere gratitude to all of the authors and reviewers who contributed to this volume.
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9
Introduction

CHAPTER 2
Ice Nucleation in Mixed-Phase Clouds
Thomas F. Whale
Contents
1. The Relevance of Ice Nucleation to Mixed-Phase Clouds 13
1.1 Modes of Heterogeneous Ice Nucleation 14
1.2 Ice Nucleation in the Atmosphere 14
1.3 Ice Nucleation in Mixed-Phase Clouds 15
2. Experimental Methods for Examining Ice Nucleation 16
2.1 Wet Dispersion Methods 17
2.2 Dry Dispersion Methods 17
3. Nucleation Theory 18
3.1 Homogeneous Ice Nucleation 18
3.2 Heterogeneous Ice Nucleation 21
4. Properties of Good Heterogeneous Ice Nucleators 26
4.1 The Traditional View of Heterogeneous Ice Nucleation 26
5. What Nucleates Ice in Mixed-Phase Clouds? 30
6. Field Measurements of Ice Nucleating Particles 34
7. Summary 34
Acknowledgments 35
References 35
1. THE RELEVANCE OF ICE NUCLEATION TO MIXED-PHASE CLOUDS
At atmospheric pressure ice Ih is the thermodynamically stable form of water below 0°C.
Pure water does not freeze at 0°C because the stable phase must nucleate before
crystal growth can occur. Liquid water can supercool to temperatures below 35°C
(Herbert et al., 2015; Koop and Murray, 2016; Riechers et al., 2013) before ice nucle-
ation occurs homogenously. On some level, this fact must be responsible for the exis-
tence of mixed-phase clouds. If liquid water supercooled only slightly, much of the
variability and interest caused by the coexistence liquid water droplets and ice particles
would not occur. The progress of a cloud from consisting entirely of supercooled liquid
water to a state also containing ice must at some point involve an ice nucleation process.
Despite decades of research, heterogeneous ice nucleation remains poorly understood.
Improved insight into the process is of great importance for understanding of mixed-
phase clouds.
13

1.1 Modes of Heterogeneous Ice Nucleation
There are several pathways by which ice can form on a heterogeneous ice nucleating
particle (INP). These are known as modes. Historically, several different sets of defini-
tions have been used for these modes. Notably, the definitions of Vali (1985) and
Pruppacher and Klett (1997) are a little different. Recently, Vali et al. (2014) led an online
discussion by the ice nucleation community on terminology and published a document
outlining new definitions (Vali et al., 2015). These definitions that are described here are
used throughout this chapter.
The two principle modes of ice nucleation are deposition and freezing. Deposition ice
nucleation is defined as ice nucleation from supersaturated vapor on an INP or equivalent
without prior formation of liquid (a phase transition from gas to solid). Freezing ice
nucleation is defined as ice nucleation within a body of supercooled liquid ascribed to
the presence of an INP, or equivalent (a phase transition from liquid to solid). Freezing
nucleation is subdivided into immersion freezing, where the entire INP is covered in
liquid water, contact freezing, where freezing is initiated at the air-water interface as
the INP comes into contact with supercooled liquid water and condensation freezing,
where freezing occurs concurrently with formation of liquid water. It is challenging
to differentiate condensation freezing from both deposition nucleation and immersion
freezing in a strict physical sense, as the microscopic mechanism of ice formation is
not known in most cases. It is entirely plausible that many, most, or all cases of deposition
nucleation are preceded by formation of microscopic quantities of water which then
freezes, followed by depositional growth (Christenson, 2013; Marcolli, 2014). Mecha-
nisms of this sort are known to occur for organic vapors (e.g., Campbell et al., 2013;
Kovács et al., 2012). Similarly, it is not clear how condensation freezing differs from
immersion freezing in cases where liquid water does form prior to freezing (which
may be most or all cases). Happily, it is thought that immersion mode freezing is likely
to be the dominant freezing mode in most mixed-phase clouds (Cui et al., 2006; de Boer
et al., 2011) so we need not concern ourselves with nucleation of ice below water sat-
uration. The remainder of this chapter is therefore solely concerned with immersion
mode ice nucleation, where particles are clearly immersed in water. The following sec-
tion briefly describes the relevance of immersion mode ice nucleation to the atmosphere
in general, to determine the role of ice nucleation in mixed-phase clouds within the
broader field of ice nucleation studies.
1.2 Ice Nucleation in the Atmosphere
Clouds are made up of water droplets or ice crystals, or a mixture of thereof, suspended in
the atmosphere. By interacting with incoming shortwave radiation and outgoing long-
wave radiation, they can impact the energy budget of the earth and thereby play a key role
in the earth’s climate. They also strongly influence the earth’s hydrological cycle by

controlling water transport and precipitation (Hartmann et al., 1992). The magnitude of
the impact of clouds on the global energy budget remains highly uncertain despite
decades of research (Lohmann and Feichter, 2005). The latest Intergovernmental
Panel on Climate Change (IPCC) report suggests a net cooling effect from clouds of
20 Wm2
(Boucher et al., 2013).
Much of this uncertainty stems from the poorly understood nature of interactions
between atmospheric aerosol and clouds (Field et al., 2014). Atmospheric aerosol consists
of solid or liquid particles suspended in the air. There are many different types of aerosol
in the atmosphere. Primary aerosol is emitted directly from both natural and anthropo-
genic sources as particles, and includes mineral dust, sea salt, black carbon, and primary
biological particles. Secondary aerosol forms from gaseous precursors that are often
emitted by plants and oceanic processes. Clouds form when moist air rises through
the atmosphere and cools down. Typically, water droplets form on aerosol particles called
cloud condensation nuclei (CCN)(Pruppacher and Klett, 1997).
As the majority of clouds are formed via processes involving aerosol particles, cloud
properties such as lifetime, composition, and size are highly dependent on the properties
of the aerosol particles with which the cloud interacts. These effects are known as aerosol
indirect effects (Denman et al., 2007). Cloud glaciation, which is dependent on the ice
nucleation properties of the aerosol in clouds, (Denman et al., 2007) is one of these
effects. In the latest IPCC report, these effects have been grouped together, and confi-
dence in the assessment of the impact of aerosol-cloud interactions is rated as low. The
potential scale of the impact ranges from a very slight warming effect to a relatively sub-
stantial cooling of 2 Wm2
(Field et al., 2014).
There are two overarching categories of tropospheric clouds in which ice nucleation
is most relevant. These are cirrus clouds and mixed-phase clouds. Cirrus clouds form in
the upper troposphere at temperatures below 38°C, and consist of concentrated
solution droplets, which can be frozen via immersion mode ice nucleation, or ice formed
by deposition nucleation. Mixed-phase clouds form lower down in the troposphere
between 0°C and about 38°C (the approximate temperature of homogeneous ice
nucleation). Ice formation in these clouds is generally thought to be controlled by
immersion mode ice nucleation (Cui et al., 2006; de Boer et al., 2011) although the con-
tact mode may also play a role (Ansmann et al., 2005).
1.3 Ice Nucleation in Mixed-Phase Clouds
Ice nucleation processes have the potential to alter mixed-phase cloud properties in sev-
eral ways. Liquid water clouds may occasionally supercool to temperatures where
homogenous freezing is important before any ice is formed, below about 35°C
(Herbert et al., 2015), but generally glaciate at warmer temperatures (Ansmann et al.,
2009; Kanitz et al., 2011). This indicates heterogeneous ice nucleation controls
15

mixed-phase cloud glaciation in many cases. Satellite observations have indicated that at
20°C about half of mixed-phase clouds globally are glaciated (Choi et al., 2010).
The presence of ice crystals in a cloud can change its radiative properties significantly
compared to a liquid cloud and the size and concentration of ice crystals are also impor-
tant (Lohmann and Feichter, 2005). Cloud thickness, spatial extent, and lifetime can also
alter radiative forcing and can potentially depend on INP concentration. Precipitation
processes are closely linked to ice formation as ice I is more stable than liquid water below
0°C. As such, ice particles in mixed-phase clouds tend to grow at the expense of super-
cooled liquid water droplets. This process is known as the Wegener-Bergeron-Findeisen
process and is thought to be the most important route for precipitation from mixed-phase
clouds as larger particles will fall faster than smaller ones (Pruppacher and Klett, 1997).
Clouds which contain relatively small ice crystal concentrations and more supercooled
water are more likely to precipitate as the ice crystals can grow to larger sizes than they
might have if ice crystal concentrations were higher. As a result, lifetime of these clouds
might be shorter than it would otherwise have been. Additionally, ice multiplication
processes can result from the fragmentation of ice formed through primary ice nucleation
processes and increase the concentration of ice crystals in clouds by several orders of mag-
nitude (Phillips et al., 2003). The best understood of these is the Hallett-Mossop process
which occurs from 3°C to 8°C (Hallett and Mossop, 1974) although other processes
have also been posited (Yano and Phillips, 2011). These various processes, and others,
interact in complex and generally poorly understood ways, contributing to the large
uncertainty on the radiative forcing due to aerosol-cloud interactions (Field et al.,
2014). These interactions between aerosol, clouds, and liquid in mixed-phase clouds
need to be understood quantitatively to properly understand and assess the impact of
clouds on climate and weather. This chapter focuses on experimental methods for quan-
tifying concentrations of INPs, ways of describing the efficiency of INPs, what is known
about the identity of INPs in the atmosphere, and the progress of studies into fundamental
understanding of why certain substances nucleate ice efficiently.
2. EXPERIMENTAL METHODS FOR EXAMINING ICE NUCLEATION
The majority of quantitative studies of how efficiently a particular material nucleates ice
have been conducted with the goal of determining what species nucleate ice in the atmo-
sphere. The atmospheric science community has employed a wide variety of techniques.
There are two overarching families of techniques for determining the immersion mode
ice nucleating efficiency of nucleators. These are wet dispersion methods and dry disper-
sion methods (Hiranuma et al., 2015). Wet dispersion methods involve dispersion of
INPs into water, which is then frozen. Dry dispersion methods involve the dispersion
of aerosol particles into air, where they are then activated into water droplets before
freezing. Techniques have also been divided into those which use droplets supported

on the surface or suspended in oil, and those which use droplets suspended in gas (Murray
et al., 2012) which are largely synonymous with wet and dry dispersion techniques,
respectively. Almost invariably, raw ice nucleation data takes the form of a fraction of
droplets frozen under a given set of conditions. Typical variables are temperature, cooling
rate, droplet size, and nucleator identity and concentration of the nucleator in droplets.
2.1 Wet Dispersion Methods
Most wet dispersion techniques are droplet freezing experiments, also known as droplet
freezing assays. These involve dividing a sample of water into multiple sub-samples and
cooling these individual samples down until they freeze. For studies of heterogeneous ice
nucleation a nucleator is suspended in the water prior to sub-division, or pure water
droplets are placed onto a nucleating surface. The temperature at which droplets freeze
is recorded, typically by simultaneous video and temperature logging. Different droplet
volumes have been used, ranging from milliliters to picoliters (Murray et al., 2012; Vali,
1995). Droplets are typically either placed on hydrophobic surfaces (e.g., Lindow et al.,
1982; Murray et al., 2010) or in wells or vials (e.g., Hill et al., 2014). In these cases, freez-
ing is usually observed visually, often through a microscope. Emulsions of water droplets
in oil can also be frozen, and freezing events recorded via microscope (e.g., Zolles et al.,
2015) or by using a calorimeter (Michelmore and Franks, 1982). Recently, microfluidic
devices have been used to create mono-disperse droplets for studying ice nucleation
(Riechers et al., 2013; Stan et al., 2009).
Droplet freezing techniques typically use linear cooling rates, although isothermal
experiments have also been conducted (Broadley et al., 2012; Herbert et al., 2014;
Sear, 2014). Larger droplets up to milliliter volumes have typically been used for investi-
gations of biological ice nucleators while the smallest droplets have been used for studies of
homogeneous ice nucleation. The majority of studies of atmospherically relevant INPs
have been conducted using smaller, nano- to picoliter-sized droplets (Murray et al., 2012).
Other techniques that use wet dispersion to produce droplets include those that freeze
single droplets repeatedly many times in order to establish the variation in freezing tem-
perature in that single droplet (Barlow and Haymet, 1995; Fu et al., 2015). Wind tunnels
are similar in that they support single suspended droplets in an upward flow of air of
known temperature (Diehl et al., 2002; Pitter and Pruppacher, 1973). Freezing pro-
babilities are determined by conducting multiple experiments. Droplets are typically pre-
pared by wet dispersion then introduced into the airflow but could also be dry dispersed.
Similarly, droplets can be suspended by electrodynamic levitation (Kr€
amer et al., 1999).
2.2 Dry Dispersion Methods
Cloud expansion chambers are large vessels in which temperature, humidity, and aerosol
contents are controlled, usually with the goal of simulating clouds (Connolly et al., 2009;
17

Emersic et al., 2015; Niemand et al., 2012). Experiments involve pumping the chamber
out to reduce temperature thereby inducing ice nucleation in the chamber. The ice
nucleation efficiency of aerosols in the chamber can be determined from the appearance
of ice crystals. In order to conduct experiments in the immersion mode the INPs must
activate as CCN before ice nucleation occurs.
Continuous Flow Thermal Gradient Diffusion Chambers (CFDCs) flow air-
containing aerosols through a space where temperature and humidity are controlled using
two plates coated in ice (Garimella et al., 2016; Rogers, 1988; Stetzer et al., 2008). Typ-
ically, aerosol size distributions and concentrations are characterized going into the area
of controlled supersaturation with respect to ice and the number of ice crystals coming
out the other end it also determined. In this way a droplet fraction frozen can be deter-
mined. Alternatively, a pre-conditioning section can be used to ensure that all aerosol
particles prior are activated as CCN prior to entry to the ice nucleation section of the
instrument, thereby ensuring that all freezing is immersion mode (L€
u€
ond et al., 2010).
3. NUCLEATION THEORY
While there is no satisfactory overarching theory for nucleation phenomena (Sear, 2012)
there are various theories and descriptions used to describe ice nucleation. This section
describes theories and descriptions used for describing immersion mode ice
nucleation data.
3.1 Homogeneous Ice Nucleation
Homogenous nucleation is nucleation that does not involve a heterogeneous nucleator.
In the atmosphere, cloud water droplets can supercool to temperatures below 35°C.
While heterogeneous ice nucleation is probably more common in most mixed-phase
clouds, homogeneous nucleation is also thought to be a factor (Sassen and Dodd,
1988) and mixed-phase clouds have been observed at sufficiently cold temperatures to
support this (Choi et al., 2010; Kanitz et al., 2011). Many laboratory experiments have
also investigated homogenous nucleation (Murray et al., 2010; Riechers et al., 2013; Stan
et al., 2009) and it has been shown that classical nucleation theory (CNT) can describe
laboratory data for homogenous nucleation well (Riechers et al., 2013).
3.1.1 Classical Description of Homogenous Ice Nucleation
The following is a derivation of CNT adapted from work by Pruppacher and Klett
(1997), Mullin (2001), Debenedetti (1996), Murray et al. (2010), and Vali et al.
(2015). Supercooling occurs because of a kinetic barrier to the formation of solid clusters
large enough for spontaneous growth. This stems from the increasing energy cost of
forming interface between ice and supercooled water as the size of a cluster grows. At
the cluster size where the energy gain of adding a water molecule exceeds the energy

cost of forming an interface between the ice and supercooled water spontaneous growth
will occur. This can be expressed as:
ΔG ¼ ΔGs + ΔGV (1)
Where ΔG is the overall change in Gibbs free energy of the ice cluster, ΔGs is the surface
free energy between surface of the particle and the bulk of the supercooled water, and
ΔGV is volume excess free energy. ΔGs and ΔGV are competing terms, ΔGV being neg-
ative while ΔGs is positive. Gs can be expressed as:
Gs ¼ 4πr2
γ (2)
where r is the radius of the solid cluster and γ is the interfacial energy between ice and
water. Gv can be expressed as:
Gv ¼
4πr3
3v
kBT lnS (3)
where v is the volume of a water molecule in ice, kB is the Boltzmann constant, T is the
temperature, and S is the saturation ratio with respect to ice. Adding Eqs. (2) and (3) gives
the total Gibbs free energy of the barrier to nucleation:
ΔG ¼
4πr3
3v
kBT lnS + 4πr2
γ (4)
The two terms of Eq. (4) are opposing so the free energy of ice formation passes through a
maximum, as shown in Fig. 1. The maximum value corresponds to the size of the critical
nucleus, ri
∗.
Critical nucleus size can be calculated by differentiating Eq. (4) with respect to ri
∗
and setting dΔG/dri ¼ 0 before rearranging for ri yields:
Fig. 1 Schematic of ice germ radius against Gibbs free energy.
19

r∗
i ¼
2γv
kBT lnSΔGv
(5)
Eq. (5) can be used to calculate the temperature dependence of critical radius size. S can
be calculated using parameterizations from Murphy and Koop (2005) along with the
value for γ from Murray et al. (2010). It can be seen that the size of the critical nucleus
increases sharply with rising temperature in Fig. 2.
By substituting back into Eq. (4), ΔG∗ at temperature T can be calculated:
ΔG∗ ¼
16πγ3
v2
3 kbT lnS
ð Þ2 (6)
To determine nucleation rate, the Arrhenius style Eq. (7) can be applied.
Jhom ¼ A exp
ΔG∗ T
ð Þ
kT

(7)
Jhom is the nucleation rate, A is the pre-exponential factor, and k the Boltzmann constant.
Combining Eqs. (6) and (7) Eq. (8) can be written down.
ln Jhom ¼ lnA
16πγ3
v2
3k3T3 lnS
ð Þ2 (8)
Hence, a plot of ln Jhom against T3
(lnS)2
will be linear with an intercept of lnA and,
over a narrow temperature range, slope:
m ¼
16πγ3
v2
3k3
(9)
Since v is known, this allows γ to be determined from experiments determining Jhom.
0
5
10
15
20
25
30
35
40
45
50
–40 –35 –30 –25 –20 –15 –10 –5 0
Critical
nucleus
radius
(nM)
Temperature (°C)
Fig. 2 Critical radius size for Ice Isd as a function of temperature.

Jhom has units of nucleation events cm3
s1
. In larger volumes of water nucleation is
therefore more probable. In an experiment looking at a large number of identical droplets
held a constant temperature where a single nucleation event within a droplet is assumed
to lead to crystallization of that droplet a freezing rate R(t) can be determined. R(t) is a
purely experimental value that has units of events s1
. It can be determined for any
droplet freezing experiment, heterogeneous or homogeneous. Application to heteroge-
neous experiments is discussed in the following section. R(t) can be calculated using:
R t
ð Þ ¼
1
N0 NF
dNF
dt
(10)
where NF is the total number of frozen droplets at time t and N0 is the total number of
droplets present, frozen or unfrozen. If V is the volume of the droplets Eq. (11) can be
written down.
Jv ¼
R t
ð Þ
V
(11)
where Jv is the volume nucleation rate. if the droplets are free of impurities so that
nucleation is via the homogenous mechanism then:
Jhom ¼ Jv ¼
R t
ð Þ
V
(12)
Following on from this, for constant temperature the fraction of droplets NL that remains
unfrozen at time t can therefore be calculated using:
NL ¼ N0 exp JhomVt
ð Þ (13)
In cases where droplets are constantly cooled, rather than being held at a steady temper-
ature to small increments of it is necessary to apply Eq. (13) to small time intervals over
which changes in temperature are small. In this way, Jhom(T) can be determined.
3.2 Heterogeneous Ice Nucleation
Immersion mode heterogeneous ice nucleation takes place when an external entity
lowers the energy barrier preventing ice nucleation. As a result, the probability of a
nucleation event occurring at any given supercooled temperature can be far higher in
the presence of a suitable heterogeneous ice nucleator. The observed outcome is that
heterogeneous ice nucleation takes place at higher temperatures than homogenous
ice nucleation in otherwise equivalent systems. Different nucleators nucleate ice with
varying efficiency (Hoose and M€
ohler, 2012; Murray et al., 2012). The following
sections detail methods for describing immersion mode heterogeneous ice nucleation
efficiency.
21

3.2.1 Application of CNT to Heterogeneous Nucleation
In classical nucleation theory, the temperature dependent heterogeneous nucleation rate
coefficient can be related to the energy difference by:
Jhet T
ð Þ ¼ Ahet exp
ΔG∗φ
kT

(14)
where Ahet is a constant and φ the factor by which the heterogeneous energy barrier to
nucleation is lower than the homogenous barrier. This equation is identical to Eq. (7),
except that the height of the energy barrier is lowered by a factor φ calculated using:
φ ¼
2 + cosθ
ð Þ 1 cosθ
ð Þ2
4
(15)
where θ is the contact angle between a spherical ice nucleus and a flat surface of the nucle-
ator. It is possible to calculate contact angles if Jhet(T) is known therefore. It is not clear
what contact angles mean physically although they give an indication of a material’s ice
nucleating ability.
3.2.2 Single Component Stochastic Models
The simplest CNT based models are a type of single component stochastic (SCS) model.
Jhet(T) is usually measured per surface area of nucleator meaning it has units of events
cm2
s1
. These models use a single nucleation rate (Jhet) to describe a nucleator’s behav-
ior. Jhet is in principle calculated in the same way as Jhom from Eq. (10) except that a rate per
surface area of nucleator, Js is used:
Jhet ¼ Js ¼
R t
ð Þ
A
(16)
Jhet can be related to CNT as described as in the above section (e.g., Chen et al., 2008) to
account for temperature dependence of Jhet but a simple linear temperature dependence
can also be used (e.g., Murray et al., 2011).
These models are not usually appropriate as they assume that all droplets in an exper-
iment nucleate ice with the same rate. Although there are examples of nucleators which
show good agreement with a single component model, notably KGa-1b kaolinite
(Herbert et al., 2014; Murray et al., 2011) it is clear that this is not the case for many
materials (Herbert et al., 2014; Vali, 2008, 2014). Jhet often does not equal R/A
(Herbert et al., 2014; Vali, 2008, 2014). As a result, various other models of ice nucleation
have been developed. Multiple component stochastic models are an extension of single
component models.
3.2.3 Multiple Component Stochastic Models
Multiple stochastic models (MCSMs) have been developed to describe the observed var-
iation in nucleation rates between droplets. These models divide a population of droplets,

or sites, into sub-populations with different single component rates. There are a number
of different variations on this theme. Some use distributions of efficiencies described by
CNT (L€
u€
ond et al., 2010; Marcolli et al., 2007; Niedermeier et al., 2014; Niedermeier
et al., 2011), while others use linear dependences (Broadley et al., 2012). All use multiple
different curves, representing different sites, droplets, or particles and sum the freezing
probabilities of all these to generate a total nucleation rate at a given temperature. Such
descriptions therefore retain time dependence and account for variability between
droplets.
3.2.4 Singular Models
Singular models of ice nucleation assume that each droplet in an ice nucleation exper-
iment contains a site that induces it to freeze at a specific characteristic temperature
(Vali and Stansbury, 1966). The justification for this approach is that it is typically
observed that variability in freezing temperature for a single droplet frozen and thawed
multiple times is much smaller than the range in freezing temperature of a population of
droplets with identical nucleator content (Vali, 2008; Vali and Stansbury, 1966). The
concept was originally put forward by Levine (1950). Typically, concentration of sites
is related to either droplet volume or surface area of nucleator. The differential nucleus
spectrum, k(T), which can be calculated from the output of ice nucleation experiments
using:
k T
ð Þ ¼
1
V N0 NF T
ð Þ
ð Þ

dNF T
ð Þ
dT
(17)
where V is the droplet volume used in the experiment, N0 is the total number of
droplets in the experiment, and Nf (T) is the number of droplets frozen at temperature T.
By integrating this expression the cumulative nucleus spectrum, K(T) can be derived:
K T
ð Þ ¼
1
V
ln 1
NF T
ð Þ
N0

(18)
K(T) has dimensions of sites per volume. Recently, it has become common to deter-
mine the surface area of nucleator contained in each droplet in order to calculate the
ice active site density ns(T), which is a measure of the number of sites per unit surface
area of nucleator (Connolly et al., 2009). ns(T) is related to K(T) by:
ns T
ð Þ ¼
K T
ð Þ
A
(19)
where A is the surface area of nucleator per droplet. To calculate ns(T) directly from
droplet experimental data the following expression can be used:
ns T
ð Þ ¼
1
A
ln 1
NF T
ð Þ
N0

(20)
23

Site-specific models of ice nucleation can also conceivably use other units besides nucle-
ator surface area and droplet volume, for instance, the number of nucleation sites per cell
or per particle can be calculated, if the number of these entities per droplet is known.
Singular models ignore time dependence. According to a site-specific model at
constant temperature, no freezing will take place. This is generally not the case but it
is often true that freezing does not follow the sort of exponential decay that would be
predicted by a single component model (Sear, 2014).
3.2.5 The Framework for Reconciling Observable Stochastic
Time-Dependence (FROST)
To overcome the difficulty that simple site-specific models do not account for time
dependence, modified singular models can be used (Vali, 1994; Vali, 2008). If two iden-
tical sets of droplets (identical meaning that the two sets contain the same surface area of
nucleator) are cooled at different rates a greater fraction of the droplets that are cooled
more slowly will be frozen at a given relevant temperature. This is because time depen-
dence of ice nucleation will mean that every droplet has a greater probability of freezing
in the longer time interval allowed to it by the slower cooling rate, compared to the faster
cooling rate. Modified singular models incorporate a factor that accounts for shifts
induced by differing cooling rates into typical site-specific expressions for ice nucleation.
The Framework for Reconciling Observable Stochastic Time-dependence (FROST)
derived by Herbert et al. (2014) is similar to the modified singular approach, which allows
ice nucleation data obtained from experiments conducted at different ramp rates, or in
isothermal conditions to be reconciled. The shift in freezing temperature between two
experiments conducted at cooling rates r1 and r2 can be calculated using:
ΔTf ¼ β ¼
1
λ
ln
r1
r2

(21)
where β is the shift in freezing temperature caused by the change in cooling rate and λ
is the slope, dln(J)/dT, of the individual components in the MCSM of Broadley et al.
(2012) and Herbert et al. (2014). This equation can be used to calculate λ from
experimental fraction frozen data. A similar quantity, ω, is defined as the gradient
–dln(R/A)/dT. Herbert et al. (2014) showed using computer simulations that when
ω ¼ λ a single component stochastic model can be applied. When ω 6¼ λ there is vari-
ation in the nucleating ability of droplets in the experiment and a MCSM must be used to
account for data. λ can be regarded as a fundamental property of a nucleator.
FROST can be used to reconcile ns(T) from experiments conducted at different ramp
rates by substituting fraction frozen values calculated using Eq. (21) into Eq. (20). If a
standard r1 value of 1°C min1
it can be shown that:
NF T, r
ð Þ
N0
¼ 1 exp ns T
lnr
λ

A

(22)

where NF(T,r) is the number of droplets frozen at temperature T for an experiment con-
ducted at ramp rate r. This equation is compatible with the modified singular model of
Vali (1994). Typical modified singular approaches use an empirical shift from experimen-
tal data in temperature instead of λ.
By performing multiple experiments Herbert et al. (2014) showed that FROST could
account for experimental data. Four sets of experiments conducted at four different
ramp rates could be reconciled with single λ value for two different nucleators. For
KGa-1b kaolinite this λ was equal to its ω value while for BCS 376 microcline this
was not the case, meaning that a single-component model could be used to describe
ice nucleation by KGa-1b, but not BCS 376.
3.2.6 Comparison and Summary of Models of Heterogeneous Nucleation
Heterogeneous ice nucleation is, in the majority of cases, a phenomenon with both
site-specific and time dependent characteristics. For most freezing experiments it is likely
that individual droplets contain many sites which nucleate ice more efficiently than the
majority of the nucleator surface area, one of which may nucleate ice more efficiently that
all others sites in the droplet, as assumed by the site-specific model. Ice nucleation at sites
is likely to be stochastic, and may be well described by a single component stochastic
model, possibly by classical nucleation theory with a suitably reduced free energy barrier
height. As the specific mechanism of heterogeneous ice nucleation is not known it cannot
be said that this is the case.
There is little reason to suppose that classical nucleation theory as applied to hetero-
geneous nucleation is valid for the nucleation of ice. It is generally acknowledged that the
contact angle used in Eqs. (14) and (15) has no physical meaning and serves as a proxy for
lowering the height of the free energy barrier calculated by CNT at a given temperature.
Clearly, site-specific models are also unphysical insofar as ice nucleation is to some extent
stochastic. No experiment has found that droplets repeatedly freeze at the exact same
temperature.
Site-specific models account for the strong temperature dependence observed in
nucleation by most nucleators while single component stochastic models account for
the time dependence. They ignore time dependence and droplet to droplet variability
in nucleation efficiency respectively. The various multiple component stochastic models
and time dependent site-specific models seek to add the facet of the problem that the
simple models do not account for.
Ultimately, none of these models of ice nucleation offer real insight into the under-
lying mechanism of ice nucleation (Vali, 2014). Multiple component stochastic models
generally provide the best fit to experimental data, which is not surprising as they have the
most degrees of freedom. They are, in a sense, fitting routines. That said, they are prob-
ably also the most physically realistic models of ice nucleation. Generally, it is convenient
to use site-specific models as temperature dependence is the overriding determinant of
25

freezing rate. Many recent studies have tended to determine ns as a means of comparing
ice nucleating species. Agreement is not universal however. For instance, efforts have
been made to explain the variation in freezing rate between the individual droplets in
experiments as a product of variations in the amount of material between different
droplets (Alpert and Knopf, 2016).
4. PROPERTIES OF GOOD HETEROGENEOUS ICE NUCLEATORS
Ideally, it would be possible to predict the efficiency of a heterogeneous ice nucleator
from knowledge of its physical and chemical properties. For comparison, it is possible
to describe the activity of CCN with a single hygroscopicity parameter (Petters and
Kreidenweis, 2007). At current this sort of description is not possible for ice nucleation.
Indeed, it seems unlikely that it will be so straightforward. In the case of deposition mode
ice nucleation there is a growing body of evidence that pore-condensation freezing is
responsible for ice nucleation in many cases (Campbell et al., 2016; Marcolli, 2014),
which has the potential to simplify the problem there. For the immersion mode ice
nucleation relevant to mixed-phase clouds no consistent theory exists. The difficulty
of understanding what makes a good INP stems from the small size of the ice critical
nucleus (see Fig. 2) and the small spatial extent of the nucleation event. According to
CNT, critical nuclei range in size from a 1 nm radius at 38°C to 10 nm at 4°C. These
critical nuclei are spatially rare. Whatever volume of droplet is frozen, there will usually
only be a single critical nucleus present. Droplets are typically at least picometers across.
No current technique is capable of locating and usefully measuring the physical properties
of an event this small and rare. As a result, properties of ice nucleation have usually been
inferred from experimental data.
4.1 The Traditional View of Heterogeneous Ice Nucleation
Historically, five properties were thought to be important for heterogeneous ice
nucleation. These were listed and discussed by Pruppacher and Klett (1997). While these
have never been regarded as hard and fast rules, discussion of the reasons for them and
where they fall down are instructive. They are:
(1) The insolubility requirement: nucleators must provide an interface with water.
Dissolved substances do not provide an interface and so do not nucleate ice.
(2) The size requirement: observations in the atmosphere indicate that INPs tend to be
large. This requirement is somewhat vague, although it stems from the observation
that larger particles in the atmosphere tend to be the ones that nucleate ice. It is also
assumed that an INP must be larger than a critical nucleus.
(3) The chemical bond requirement: a nucleator must be able to bind to water in order
to cause nucleation. Stronger bonding is likely to improve nucleation efficiency.

(4) The crystallographic requirement: the classic lattice matching idea first put forward
by Vonnegut (1947). Substances with a similar lattice structure and spacing to ice will
provide a template for a critical nucleus.
(5) The active site requirement: based on a combination of the observation that site-
specific descriptions often give the best account of ice nucleation and the fact that
deposition mode ice nucleation tends to occur repeatedly on specific locations on
crystals. It seems likely that this is more related to vapor condensation than ice nucle-
ation (Marcolli, 2014).
The next sections looks at how these requirements have been challenged and revised in
recent years and what is known about the mechanism of heterogeneous ice nucleation
from experimental studies. Computational studies of ice nucleation are then examined
and the outcomes of the two approaches discussed.
4.1.1 Size and Solubility of Heterogeneous INPs
While INPs have traditionally been regarded as large and insoluble (Pruppacher and
Klett, 1997) a number of counter examples are known. In recent times biological mac-
romolecules associated with pollen that have been claimed as soluble have been shown to
nucleate ice efficiently (Pummer et al., 2012, 2015). These molecules weigh from 100 to
860 kDa, which equates to a radius of less than 10 nm. This is only slightly larger than the
critical nuclei they nucleate. They are perhaps 10 times smaller than the particles that
Pruppacher and Klett (1997) envisaged as too small to efficiently nucleate ice on the basis
of older work. Similarly, Ogawa et al. (2009) showed that solutions of poly-vinyl alcohol
could nucleate ice, although only at a few degrees above homogenous nucleation
temperatures.
4.1.2 Lattice Matching
The best known example of inference of ice nucleation properties is the lattice matching
concept of Vonnegut (1947). The idea is that substances that have a similar crystal struc-
ture to ice, with similar lattice constants will pattern the first layer of ice. The amount of
lattice mismatch, or lattice disregistry (Pruppacher and Klett, 1997; Turnbull and
Vonnegut, 1952) can be readily calculated from knowledge of the crystal structure.
On this basis Vonnegut (1947) identified AgI as a potentially excellent nucleator and
all subsequent experimentation has shown that he was correct.
The role of lattice matching in ice nucleation by AgI has been questioned for some
time. Zettlemoyer et al. (1961) argued that water likely adhered to specific sites on the
surface of AgI, which may have been oxidized, rather than forming a layer over the crystal
on the basis of the difference in adsorption of water and nitrogen. More recently,
Finnegan and Chai (2003) postulated an alternative mechanism for ice nucleation by
AgI where clustering of surface charge controls ice nucleation. There is no universal
agreement on the mechanism of ice nucleation by AgI from experimentalists.
27

Experimental studies on BaF2, which also has a good lattice match to ice, did not support
the lattice matching argument (Conrad et al., 2005).
4.1.3 Bonding of Water to INPs
Intuitively, it seems obvious that water must be able to bind to a nucleator to induce ice
nucleation and that hydrophilic surfaces will nucleate ice more efficiently than hydro-
phobic surfaces. There is little relevant experimental work, as studies where surfaces
of differing but well understood hydrophilicity have been tested in the same system have
not been conducted. It is also generally difficult to choose nucleators and systems such
that all other possible variables are constrained.
Li et al. (2012) compared the ice nucleating ability of hydrophobic and hydrophilic
surfaces and obtained the somewhat surprising result that the hydrophobic surface nucle-
ated ice more efficiently. While their technique—coating of a silicon wafer with a
fluoroalkylsilane—may introduce other variables, this is evidence that hydrophilic
surfaces do not necessarily nucleate ice more efficiently than hydrophobic surfaces. It
has been reported in the past that hydrophilic soots nucleate ice more efficiently than
hydrophobic soots, although the method used leaves doubt as to the mode of ice nucle-
ation observed (Gorbunov et al., 2001).
4.1.4 Active Sites and Topographical Effects
As discussed in the section on descriptions of heterogeneous ice nucleation, it has been
argued that the surfaces of most immersion mode nucleators must have active sites on the
basis of interpretations of droplet freezing experiments and repeated freezing droplets
(Vali, 2008, 2014), although there are exceptions (Herbert et al., 2014). These experi-
ments do not constitute direct observation of the nucleation sites. It has been known
for some time that apparently depositional ice nucleation tends to occur on specific sites
on both organic substances (Fukuta and Mason, 1963) and inorganic substances (Bryant
et al., 1960). More recently it has been shown that nucleation from vapor of organic mol-
ecules probably follows a two-step process where small amounts of liquid condense prior
to freezing (Campbell et al., 2013) and proposed that ice nucleation from vapor probably
follows a similar route in many situations (Marcolli, 2014).
It is far easier to locate a depositional nucleation site than an immersed nucleation site
as crystals grow out from point of depositional nucleation relatively slowly, whereas they
grow very quickly through liquid water droplets. That said, it is possible to locate nucle-
ation points in surfaces covered by water using a high-speed camera (Gurganus et al.,
2011, 2013, 2014). Gurganus et al. (2011) conducted freezing experiments of droplets
on silicon wafers and saw no tendency for nucleation to occur repeatedly on the same
site, suggesting that no specific sites on their substrate nucleated ice more efficiently than
any others. Their study was aimed at determining whether nucleation tended to take
place at the air-water-substrate interface (contact mode, see above) or elsewhere and

showed no preference for nucleation at the interface. More recent work from Gurganus
et al. (2014) has shown that a ‘nano-textured’ silicon strand (features on a scale smaller
than 100 nm) nucleates ice much more effectively than a ‘micro-textured’ (etched
features in silicon with depths from 300 to 900 nm) silicon substrate or a smooth silicon
substrate. The difference was particularly pronounced at the point of contact between the
silicon strand and the droplet surface. This interesting result shows that topographical
differences between chemically very similar surfaces can cause differences in ice nucle-
ation behavior. The nature of topographical features implies that nucleation processes
involving them must be to some extent “site specific.”
Campbell et al. (2015) attempted to change the ice nucleating properties of silicon,
glass, and mica substrates by scratching them with diamond powders ranging from 10 nm
to 40–60 μm. They found that the scratching process made no significant difference to
the ice nucleating efficiency of the surfaces. It might be expected that the 10 nm diamond
powder would produce features on a similar scale those that Gurganus et al. (2014)
observed enhancing the ice nucleating efficiency of silicon. This was not the case. There
are many other differences between the two systems so it is difficult to suggest reasons
for this. Other studies have also reported no impact on ice nucleation efficiency from
topography on a micrometer scale (Heydari et al., 2013).
At present, it is impossible to observe ice nucleation events directly on a scale that is
useful for understanding the underlying mechanisms. An approach to defining the scale
problem is to use classical nucleation theory and assume that it gives a reasonable estimate
for the size of a critical nucleus required for heterogeneous nucleation at temperatures
above which homogenous nucleation can be observed.
The role of lattice matching in heterogeneous ice nucleation has been studies com-
putationally. Recent molecular dynamics (MD) results have suggested that a lattice match
is not the sole explanation for the efficiency of AgI and that a mechanism involving an
ordering of water above the AgI surface, which then causes ice nucleation is more likely
(Reinhardt and Doye, 2014; Zielke et al., 2015). Similarly, kaolinite had been thought of
as good ice nucleator, and this efficiency had been attributed to a good lattice match of
hydroxyl functional groups (dOH) on the basal face of kaolinite to hexagonal ice
(Pruppacher and Klett, 1997). It is now seems likely that the apparent ice nucleation
activity of kaolinite observed in older studies was largely due to contamination by feldspar
minerals (Atkinson et al., 2013). Density Functional Theory (DFT) calculations previ-
ously questioned the validity of the lattice matching mechanism for kaolinite, instead
attributing the activity to the amphoterism of the dOH groups on the surface of
kaolinite which allows them to both accept and donate hydrogen bonds, favoring the
formation of an overlayer of water molecules (Hu and Michaelides, 2007). Indeed, there
is now a significant body of computational evidence suggesting that a simplistic lattice
matching view of ice nucleation may be misleading (e.g., Cox et al., 2012, 2013;
Fitzner et al., 2015).
29

Another variable that has been examined computationally is surface hydrophilicity.
Lupi and Molinero (2014) found that simulated graphite surfaces nucleated ice less well
when decorated with –OH groups to increase hydrophilicity. Recently, Fitzner et al.
(2015) conducted a comprehensive, systematic MD study of the impact of crystallo-
graphic match and hydrophilicity on ice nucleation. By testing four different idealized
crystal surfaces with varied lattice parameters and water interaction strengths they found
three different mechanisms by which heterogeneous ice nucleation could be promoted.
They name these “In-Plane Template of the First Overlayer,” “Buckling of the First
Overlayer,” and “High Adsorption-Energy Nucleation on Compact Surfaces”. It is
interesting that even in this simplified system they found complex dependency on lattice
parameters and interaction strength. Bi et al. (2016) also found complex dependencies of
nucleation rate on the interaction between hydrophilicity and crystallinity. Computa-
tional studies have found that surface roughness on a fine scale (from several angstroms
to several nanometers), with roughness of some specific periodicities found to promote
nucleation better than others (Fitzner et al., 2015; Zhang et al., 2014).
Very recently, it has been shown computationally that the (100) face of the alkali
feldspar microcline nucleates ice efficiently and, in the same study, that ice tends to grow
out of cracks at the same angle to the exposed face of feldspar as this plane, suggesting that
ice is indeed being templated by the (100) face (Kiselev et al., 2017). This breakthrough
study points to more combined experimental/computational studies in the future, which
eventually unpick the problem of understanding heterogeneous ice nucleation.
Overall, the picture is a complex one. It can be said with some certainty that active sites
are important for ice nucleation by many nucleators. The exact properties of these active
sites are much less certain. Other nucleators do not appear to have active sites. Lattice
matching as a concept is well established and widely applied but increasingly
questioned by both laboratory and computational studies. Hydrophilicity must play some
role and simulations of ice nucleation have suggested that the relationship between hydro-
philicity and lattice match can impact ice nucleation efficiency in complicated and non-
intuitive ways. This may shed some light on why it has proved so hard to understand ice
nucleation processes in the past; relationships between physico-chemical properties and
ice nucleation efficiency are not straightforward. Immersion-mode ice nucleation
micrometer scaletopographical featureshave so far provedto play little role, although only
limited numbers of experiments have been conducted to date. There are, however, hints
that topography on a sufficiently small scale (atomic to nanometer scale) may play a role.
5. WHAT NUCLEATES ICE IN MIXED-PHASE CLOUDS?
Because we cannot directly predict what atmospheric aerosol species will nucleate ice
well from theory, we must fall back on other methods for understanding what species
might be nucleating ice in mixed-phase clouds, and how well they might be doing it.

The majority of laboratory studies of immersion-mode heterogeneous ice nucleation
have been conducted with the aim of understanding and quantifying ice nucleation
by substances that might nucleate ice in the atmosphere. Extensive reviews are available
(Hoose and M€
ohler, 2012; Murray et al., 2012). The following section very briefly details
these substances.
Large amounts of mineral dusts are emitted to into the atmosphere, mostly from arid
regions in Africa and Asia (Prospero et al., 2002). It has, for many years, been known that
snow crystals contain mineral dust residues (Kumai, 1961) and more recent work has
found that mineral dusts make up a large proportion of ice crystal residues in certain cloud
types (Murray et al., 2012; Pratt et al., 2009). There is a volume of older work on ice
nucleation by mineral dusts (Pruppacher and Klett, 1997). In many of these cases only
onset freezing temperatures are recorded and it is therefore difficult to assess the relative
efficiency of freezing. Recently, ns values for a range of natural mixed dusts have been
calculated (Connolly et al., 2009; Niemand et al., 2012) as well as for proxies of natural
dusts such as NX illite and Arizona test dust (ATD) (Broadley et al., 2012; Connolly et al.,
2009; Marcolli et al., 2007).
Until recently it had been thought that clay minerals were responsible for the ice
nucleation activity of mineral dusts (L€
u€
ond et al., 2010; Pinti et al., 2012; Pruppacher
and Klett, 1997), partially on the basis of kaolinite’s lattice match to hexagonal ice
(Pruppacher and Klett, 1997). Atkinson et al. (2013) have recently shown that feldspars
nucleate ice far more efficiently than the other major components of mineral dusts and
that they are likely to be responsible for much of the ice nucleation observed in mixed
phase clouds in various regions of the world. It now also known that different poly-
morphs of feldspar nucleate ice differently well (Harrison et al., 2016).
The ice nucleation activities of a wide range of biological entities have been inves-
tigated in the past. The starting point for much of this work was the discovery by
Schnell and Vali (1972) that decomposing leaf matter induced freezing at higher temper-
atures than any other nucleator they tested. It was discovered that the efficient nucleator
was associated with the bacterium Pseudomonas syringae, (Lindow et al., 1989; Maki et al.,
1974) a plant pathogen. It is generally thought that the efficient ice nucleation of
P. syringae allows it to ingest nutrients from plants at a temperature just below the melting
point of water where the plants would usually avoid frost damage by supercooling. Since
the discovery of the ice nucleation activity of P. syringae many other bacteria have been
shown to nucleate ice at high temperatures (Lee et al., 1995).
Other biological ice nucleators include fungi (Fr€
ohlich-Nowoisky et al., 2014;
O’Sullivan et al., 2014; Pouleur et al., 1992), pollen (Pummer et al., 2012, 2015), and
plankton (Alpert et al., 2011; Knopf et al., 2011; Schnell, 1975). Recently it has become
increasingly clear that pollen and fungi emit separable macromolecular INP of far smaller
size than the pollen and fungi themselves (O’Sullivan et al., 2015, 2016; Pummer et al.,
2012, 2015). It has recently been shown that small ice nucleating entities, most probably
31

of biological origin, are present in the sea-surface microlayer and may be emitted to the
atmosphere (Wilson et al., 2015).
Anthropogenic burning of fossil fuels and biomass contribute significantly to global
aerosol (Bond et al., 2013). Various studies of ice nucleation by soots have been con-
ducted (Demott, 1990; Diehl and Mitra, 1998; Gorbunov et al., 2001) as well as studies
of ice nucleation by various biomass products (Petters et al., 2009). In general, it would
appear that soots do not nucleate ice particularly efficiently, although the relative paucity
of data makes meaningful comparison to other species challenging.
AgI and related compounds were identified as good nucleators in the early days of ice
nucleation research (Passarelli et al., 1973; Vonnegut, 1947; Vonnegut and Chessin,
1971) and have been used for cloud seeding ever since. They are known to nucleate
ice very efficiently (DeMott, 1995). No significant amount of AgI is likely to be present
in mixed-phase clouds.
Comparison of ice nucleating species is complicated by the fact that different instru-
ments, even those of the same type, do not always give the same answer when used to test
the same INPs (Hiranuma et al., 2015). However, it is possible to say something about the
general effectiveness of the different classes of nucleators. Murray et al. (2012) calculated
ns values for a wide range of immersion mode measurements. As discussed in
Section 3.2.6 ns values are probably the best metric for comparing different nucleators.
Fig. 3 is a reproduction of the comparison figure from Murray et al. (2012). They are not
perfect, as calculating surface areas for species with varying natures is not straightforward.
A further problem is that many nucleators have only been tested using very small-
sized cloud droplets with correspondingly small amounts of nucleator surface area. As
a result, there is little data at warmer temperatures for many nucleators. Conversely, bio-
logical nucleators have mostly only been tested in larger droplets, although dilution has
allowed extension of the range of ns values tested. (e.g., Wex et al., 2015). The only
examples of non-biological ice nucleators tested at ns values below 103
cm2
in Fig. 3
are the volcanic ash tested by Fornea et al. (2009) and BCS 376 microcline (2013).
What can be seen is that P. syringae nucleates ice far more efficiently than any other
nucleator for which ns has been calculated, with similar site concentrations to other
nucleators at much warmer temperatures. BCS 376 microcline, an alkali feldspar, was
tested by Atkinson et al. (2013) and shown to nucleate ice more efficiently than the atmo-
spherically relevant minerals they tested. BCS 376 microcline also has a higher active site
density at all temperatures than all other non-biological nucleators. Other feldspar min-
erals have also been tested (Zolles et al., 2015; Niedermeier et al., 2015; Augustin-
Bauditz et al., 2014) and are of broadly similar, or somewhat lesser activity than BCS
376 microcline. Illite, kaolinite, Arizona Test Dust (ATD) and natural dusts all appear
to be rather less effective nucleators than BCS 376 microcline. It seems quite likely that
nucleation by ATD and natural dusts is dominated by their feldspar content (Augustin-
Bauditz et al., 2014; Atkinson et al., 2013).

AgI is a highly efficient nucleator (DeMott, 1995) and is better at nucleating ice than
any other non-biological nucleator that has been tested, including feldspars. Birch pollen
nucleates with similar efficiency to BCS-376 microcline and the plankton N. atomus is
rather less active. Overall, biological nucleators such as P. syringae and fungal proteins
nucleate ice more efficiently than any other species. AgI and related compounds are prob-
ably the next most efficient nucleators known but will not be present in mixed-phase
clouds, unless artificially introduced. Alkali feldspar is more efficient than other minerals,
volcanic ashes, and combustion products.
To assess the relative importance of different nucleator species in mixed-phase clouds
both the concentration of INPs and their efficiencies must be taken into account. Several
studies have attempted to do this. For instance, Hoose et al. (2010b) found that mineral
dusts dominate immersion-mode ice nucleation between 0°C and 38°C globally. Dif-
ferent studies have come to different conclusions about the relevance of biological ice
nucleators. Phillips et al. (2009) concluded that biological nucleating species may play
a key role globally, while work by Hoose and co-workers has suggested otherwise
(Hoose et al., 2008, 2010a). It seems likely that much of the discrepancy arises from dif-
fering assumptions about the nature and quantity of biological INPs present in the
Fig. 3 An adapted version of figure 18 from Murray et al. (2012) with some additional data from
subsequent studies. The figure shows ice nucleation efficiencies for a range of different nucleators.
It can be seen that bacterial ice nucleators are much more effective than non-biological ice
nucleators. BCS 376 microcline from Atkinson et al. (2013) nucleates ice more efficiently than other
non-biological nucleators, except for AgI (DeMott, 1995).
33

atmosphere. Further laboratory and field measurements may be needed to constrain these
variables. More recently, it has been suggested that alkali feldspars can account for a large
proportion of ice nucleation observed in the atmosphere (Atkinson et al., 2013) and more
recently still that a combination of feldspar and marine organic sources account still better
for observations, while leaving the possibility of the existence of an extra, terrestrial
source of INPs (Vergara Temprado et al., 2016). Overall, while the precise identity of
the most important INPs in the atmosphere is not known much progress has been made.
6. FIELD MEASUREMENTS OF ICE NUCLEATING PARTICLES
Direct measurement of the concentration of INPs in the atmosphere is important for pro-
viding inputs to and comparisons for atmospheric models, thereby contributing to under-
standing of ice nucleation in the atmosphere. The practice of measuring INP
concentrations in the atmosphere has enjoyed something of a resurgence in recent years
(DeMott et al., 2011) and the amount of data available is steadily increasing. The instru-
ments mentioned previously can and have been used for measuring INP concentrations
in the atmosphere. In the case of dry dispersion instruments, particularly CFDCs (which
are often portable), aerosol collection is relatively straightforward, although much care is
needed. Many studies of this sort have been carried out (e.g., Boose et al., 2016; DeMott
et al., 2010; Rosinski et al., 1987, 1995). For wet-dispersion methods some approach by
which atmospheric aerosol can be transferred to liquid droplets is required. Of late, this
has been accomplished by collecting aerosol onto filter membranes and either washing
this aerosol off the filter in order to conduct a droplet freezing assay (DeMott et al.,
2016) or placing pieces of the filters directly into water before freezing (Stopelli et al.,
2016). Another method is to use a micro-orifice uniform-deposit impactor (MOUDI)
to collect size-separated aerosol onto glass slides. Droplet freezing experiments can then
be conducted on these glass slides (Mason et al., 2016).
7. SUMMARY
Immersion mode ice nucleation must play a key role in the evolution of mixed-phase
clouds. The precise nature of this role and how ice nucleation interacts with other micro-
physical and dynamical processes in mixed-phase clouds remains to be established. Sev-
eral different methods of testing the ice nucleating efficiency of nucleators and the INP
concentration in the atmosphere are available although the comparability of these
methods is not entirely established at this point. What can be said is that knowledge
of which aerosol species are likely to cause ice nucleation in mixed-phase clouds is
improving rapidly, as is understanding of why certain substances nucleate ice efficiently.
Various mathematical descriptions of ice nucleation are available and the number of mea-
surements of INP concentrations relevant to mixed-phase clouds in the atmosphere is
increasing rapidly.

ACKNOWLEDGMENTS
I would like to thank Prof. Benjamin Murray for reading a version of this chapter and providing helpful
comments and Dr. Constantin Andronache for giving me the opportunity to contribute this chapter. This
article has been adapted from a chapter of my PhD thesis.
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41

CHAPTER 3
Detection of Mixed-Phase Clouds From
Shortwave and Thermal Infrared
Satellite Observations
Yoo-Jeong Noh, Steven D. Miller
Contents
1. Introduction 43
2. Cloud Phase Determination From Passive Satellite Radiometers 46
3. Determination of Liquid-Top Mixed-Phase (LTMP) 48
3.1 Theoretical Basis 49
3.2 Algorithm Description 52
4. Application to Current Satellite Observations 57
4.1 S-NPP VIIRS Case Study 57
4.2 Himawari-8 AHI Case Study 59
5. Discussion and Conclusion 61
References 64
1. INTRODUCTION
Clouds may contain liquid water droplets, ice particles, or a mixture of both below freez-
ing down to -40°C (Rauber and Tokay, 1991; Cober et al., 2001). The latter, referred to
as mixed-phase clouds, occur commonly in the Earth’s atmosphere (e.g., Pinto, 1998;
Verlinde et al., 2007; Wang et al., 2004), since water droplets often exist in the form
of supercooled liquid below the freezing point. In situ measurements during field cam-
paigns at middle and high latitudes (e.g., Fleishauer et al., 2002; Hogan et al., 2003;
Zuidema et al., 2005; Niu et al., 2008; Carey et al., 2008; Shupe et al., 2008a,b; Noh
et al., 2011, 2013) reveal that a large number of mid-level non-precipitating clouds
contain predominately supercooled liquid droplets and a small number of ice crystals
at or near cloud top, and precipitating ice virga in the middle and lower portions of
the cloud. Fig. 1 shows examples from aircraft measurements of such mixed-phase clouds
as observed during the Canadian CloudSat/CALIPSO Validation Programme and
CLEX-10 joint field campaign (see Noh et al., 2011, 2013 for more details).
Aircraft flying through mixed-phase clouds with the presence of large supercooled
liquid droplets can cause rapid ice accretion of ice (icing) on the wings and frames
43

Fig. 1 Sample aircraft in situ measurements (vertical profiles of Liquid Water Content (LWC)/Ice Water
Content (IWC) and temperatures) for mixed-phase clouds collected during the C3VP/CLEX-10 field
campaign (Noh et al., 2013).

(Smith et al., 2012), posing a direct and serious in-flight hazard. Hence, a detailed under-
standing of the characteristics and microphysical properties of mixed-phase clouds is
essential to improving aviation safety and reducing risk to both military operations
and the civilian aviation community alike. An improved understanding of mixed-phase
cloud morphology is also important for the parameterization of these ubiquitous clouds in
climate and weather prediction models (e.g., Randall et al., 2007). These clouds have
significant impacts on the atmospheric radiative heating profile, which feeds back to cir-
culations occurring on both weather and climate spatial/temporal scales. In turn, changes
to the local radiative heating profiles affects cloud formation and the overall radiation
budget of Earth (Larson et al., 2006; Fleishauer et al., 2002).
The impact of these mixed-phase clouds is non-uniformly distributed. Hu et al.
(2010) show large amounts of supercooled liquid water clouds at high latitudes, especially
over the relatively pristine maritime air mass of the Southern Ocean. The Southern
Ocean has the highest observed mixed-phase cloud fraction in the world (Mace et al.,
2007). Trenberth and Fasullo (2010) suggest that biases in top-of-atmosphere net radi-
ative forcing simulations in the Southern Ocean are tied to problems in cloud represen-
tation. Recent studies indicate that 40%–60% of clouds in the temperature range between
0°C and 30°C are mixed-phase and 30%–60% are supercooled liquid water clouds
(e.g., Korolev et al., 2003; Mazin, 2006; Shupe et al., 2006; Zhang et al., 2010). The
longevity and areal extent of these supercooled-liquid and mixed-phase clouds have a
significant impact on the radiative balance (Sun and Shine, 1995; DeMott et al., 2010).
Despite their recognized importance, the properties of mixed-phase clouds are rela-
tively unknown and remain a source of uncertainty in retrievals from satellites and
numerical weather/climate models (e.g., Sun and Shine, 1994; Fowler et al., 1996;
Beesley and Moritz, 1999; Harrington et al., 1999; Klein et al., 2009). Many numerical
models and satellite retrieval algorithms still use simple approaches to partitioning cloud
phases and microphysical distributions in temperature space, with an adjusting fraction of
liquid/ice between freezing (0°C) and the homogeneous freezing point (near 40°C),
with a dependence on droplet size, chemical composition, and ambient vertical velocity
(Heymsfield and Miloshevich, 1993; Heymsfield et al., 2005; Swanson, 2009). Many
numerical models use temperature limits (empirically based) to discriminate between
liquid and ice, for instance, specifying thresholds of 23°C by Tiedtke (1993),
15°C by Smith (1990) and Boucher et al. (1995), and 9°C by Gregory and Morris
(1996), as reviewed by Shupe et al. (2008a). Similar temperature thresholds are also found
in various remote sensing retrieval algorithms, such as the CloudSat water content
algorithm (Noh et al., 2011).
Overall, the physical mechanisms responsible for mixed-phase clouds and the fre-
quency/scale/distribution of their occurrence globally are not well understood. Whereas
in situ measurements can provide useful data points for these clouds, such observations are
costly and spatially limited. Satellite remote-sensing based methods are best suited to the
45
Shortwave and Thermal Infrared Satellite Observations

continuous monitoring and characterization of these mixed phase clouds. Active satellite
sensor measurements such as CloudSat (Stephens et al., 2002) and CALIPSO (Winker
et al., 2009) have provided a unique view of cloud vertical structures including
mixed-phase clouds, but such sensors are applicable only to a very limited domain along
their curtain observations. A strategy based on passive imaging radiometry would best
leverage the satellite platform for observing the distribution of mixed-phase clouds
and the further detailed structures such as liquid-top mixed-phase (hereafter, LTMP)
clouds, globally. We propose one such approach in the discussion to follow that attempts
to add a new dimension to current state of understanding.
2. CLOUD PHASE DETERMINATION FROM PASSIVE SATELLITE
RADIOMETERS
Multispectral band measurements in the optical spectrum (ranging from 0.4 to 14 μm)
from satellite-based radiometers have been used widely to globally determine cloud
occurrence, classify cloud type, and retrieve cloud top height/pressure, integrated liq-
uid/ice water content, cloud emissivity, and cloud top microphysics (e.g., Nakajima
and King, 1990; Inoue and Ackerman, 2002; Platnick et al., 2003). These passive satellite
radiometer observations also provide information about cloud top phase. Cloud phase
information is often available via a combination of some infrared (IR) channels such
as 8.5, 11, and 12 μm bands (Strabala et al., 1994; Baum et al., 2000; Pavolonis,
2010a). Due to the different spectral sensitivity to cloud phase (liquid vs. ice) around
8.5 μm and 11–13.5 μm, the difference in measured radiation (or brightness temperature)
between an 8.5 μm channel and an 11 μm channel (or 12 μm or 13.3 μm channel) can be
used for phase determination, accounting for the background conditions (e.g., surface
temperature, surface emissivity, atmospheric temperature, and atmospheric water vapor)
of a given cloudy scene (Pavolonis, 2010b). A key advantage of thermal-infrared-only
techniques is the ability to apply the algorithms to both daytime and nighttime observa-
tions. Due to strong absorption of both liquid and ice water at these thermal infrared
wavelengths (e.g., Hu and Stamnes, 1993), the phase information generally corresponds
to cloud top conditions (visible optical thickness of 1.0 into the cloud, or typically the
first few hundred meters) (Pavolonis et al., 2005). Hence, the results of such retrievals are
typically referred to as cloud top properties.
When using the thermal bands, thresholds used to determine cloud phase are rela-
tively simple and based on in situ measurements. For example, Korolev et al. (2003)
found that at temperatures below 238.0 K, the ice phase is dominant, but the relationship
becomes more complicated for mixed-phase clouds. Examples from the MODerate-
resolution Imaging Spectroradiometer (MODIS) (Platnick et al., 2003) IR cloud phase
retrieval product (5 km5 km) of Collection-5 MYD06 Level-2 data (Menzel et al.,
2010) are shown together with cloud top temperatures in Fig. 2. MODIS is a 36-channel

scanning spectroradiometer with visible, near-infrared and infrared channels with a swath
width of 2330 km (King et al., 2003). Brightness temperature differences between the 8.5
and 11 μm channels are compared with brightness temperatures from 11 μm to deter-
mine dominant cloud phase (liquid, ice, or mixed-phase), exploiting differences in
absorption by liquid water and ice at these wavelengths. “Mixed-phase” cloud in the sat-
ellite retrieval usually means high probability of containing both liquid water and ice near
cloud top (Pavolonis, 2010b). In Fig. 2, many cloudy pixels which have sub-freezing top
temperatures are often classified as “uncertain.” The red circles show where in situ aircraft
measurement of mixed-phase clouds were collected (Noh et al., 2011). The aircraft data
revealed a supercooled liquid water topped mixed-phase cloud structure (the in situ
examples of these mixed phase clouds are shown in Fig. 1), and CALIPSO (not shown
here) confirmed the presence of supercooled liquid water at cloud top.
For detection of supercooled liquid water clouds during the daytime, differential scat-
tering/absorption properties between liquid and ice in the mid-wave infrared window
(e.g., 3.9 μm) have been coupled with a measurement of thermal infrared window
Cloud_phase_infrared Cloud_top_temperature (K)
Clear
292
286
279
273
266
260
253
247
240
234
227
221
214
208
Water
Ice
Mixed
Uncer
Cloud_phase_infrared Cloud_top_temperature (K)
Clear
286
280
275
270
264
259
254
248
243
237
232
227
221
216
Water
Ice
Mixed
Uncer
Fig. 2 MODIS Level-2 IR Cloud Phase and Cloud Top Temperature products (MYD06) for Oct. 31, 2006
and Nov. 5, 2006 with field experiment regions indicated by the red circles (where mixed-phase clouds
were observed from the C3VP/CLEX-10 aircraft measurement).
47
Shortwave and Thermal Infrared Satellite Observations

(e.g., 11.0 μm) (e.g., Ellrod, 1996; Lee et al., 1997; Ellrod and Bailey, 2007). The mid-
wave IR reflectance of sunlight is higher for liquid cloud droplets, due to a higher
imaginary-part of the complex index of refraction (proportional to absorption/emission)
for ice at this wavelength. Thresholds imposed on this reflectance, determined conser-
vatively from radiative transfer simulations of liquid and ice-topped clouds, are used to
assign cloud top phase. Under the assumption of an optically thick cloud emitting as a
blackbody in the thermal infrared band (reasonable for most liquid-phase clouds), the
thermal infrared brightness temperature is a good approximation of the cloud top tem-
perature. If the cloud top phase was determined as liquid based on the mid-wave infrared
reflectance thresholds and the temperature is less than 0°C, a super-cooled liquid water
classification is assigned. The classification is often referred to as mixed phase (both liquid
and ice) due to uncertainties in the thresholds assumed.
3. DETERMINATION OF LIQUID-TOP MIXED-PHASE (LTMP)
Here we propose a daytime multispectral algorithm that attempts to profile and identify
LTMP clouds from passive satellite radiometer observations. The previous techniques
utilizing the mid-wave and thermal infrared bands (e.g., Ellrod, 1996; Lee et al.,
1997; Ellrod and Bailey, 2007) would potentially classify some LTMP clouds simply
as “supercooled liquid.” The objective of the current algorithm (hereafter, referred to
as the LTMP algorithm) is to enlist additional bands in the shortwave infrared (SIR)
part of the spectrum that are capable of probing below cloud top (i.e., to deeper levels
within the cloud) to identify a subset of these liquid-topped clouds that may contain an
ice-dominated phase below cloud top, akin to the structures shown in Fig. 1. A full
description and physical basis for the LTMP algorithm is provided by Miller et al. (2014).
The LTMP algorithm makes use of reflected sunlight in narrow SIR bands at 1.6
and 2.25 μm to optically probe below liquid-topped clouds, using the unique phase-
dependent behavior of these bands to infer the phase. Detection is basically predicated
ondifferentialabsorptionpropertiesbetweenliquidandiceparticlesforvaryingsun/sensor
geometry and cloud optical properties. The algorithm is applied to the subset of clouds in
the scene that were determined a priori, based on conventional passive radiometer phase-
determinationtechniquesdescribedabove,tobesupercooledliquidtop.Itusesdifferential
absorption features between liquid and ice in different atmospheric window bands in
the shortwave-infrared (SIR; 1–3 μm where thermal emission signals from terrestrial
and atmospheric sources are small), using reflectance measurements whose weighting
functions peak below the cloud top. Comparing these measurements to those that would
be expected for an entirely liquid-phase cloud (based on radiative transfer simulations),
conservative thresholds are used to identify cases where a sub-cloud top ice/mixed phase
layer is likely to be present.

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I occasionally saw strangers at the station, which is a mile from the
village, inquiring their way to the churchyard; but I was told there
had been a notable falling off of the pilgrims and visitors of late.
During the first few months after his burial, they nearly denuded the
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number of silly geese that came there to crop the grass was much
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A most agreeable walk I took one day down to Annan. Irving's name
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Irving's grave, in the crypt of the cathedral, a most dismal place, and
was touched to see the bronze tablet that marked its site in the
pavement bright and shining, while those about it, of Sir this or Lady
that, were dull and tarnished. Did some devoted hand keep it
scoured, or was the polishing done by the many feet that paused
thoughtfully above this name? Irving would long since have been
forgotten by the world had it not been for his connection with
Carlyle, and it was probably the lustre of the latter's memory that I
saw reflected in the metal that bore Irving's name. The two men
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written word has no projectile force. It makes a vast difference
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Some men are like nails, easily drawn; others are like rivets, not
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give way, and be forgotten soon. People who differed from him in
opinion have stigmatized him as an actor, a mountebank, a
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played with the force of gravity. Behold how he toiled! He says, One

monster there is in the world,—the idle man. He did not merely
preach the gospel of work; he was it,—an indomitable worker from
first to last. How he delved! How he searched for a sure foundation,
like a master builder, fighting his way through rubbish and
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nine months, the French Revolution three years, Cromwell four
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turbulent water beneath it, than these books convey the reader over
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reader on his way, not to beguile or amuse him, was always his
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lightness and frivolousness that hard, serious workers always have.
He was impatient of poetry and art; they savored too much of play
and levity. His own work was not done lightly and easily, but with
labor throes and pains, as of planting his piers in a weltering flood
and chaos. The spirit of struggling and wrestling which he had
inherited was always uppermost. It seems as if the travail and
yearning of his mother had passed upon him as a birthmark. The
universe was madly rushing about him, seeking to engulf him.
Things assumed threatening and spectral shapes. There was little
joy or serenity for him. Every task he proposed to himself was a
struggle with chaos and darkness, real or imaginary. He speaks of
Frederick as a nightmare; the Cromwell business as toiling amid
mountains of dust. I know of no other man in literature with whom
the sense of labor is so tangible and terrible. That vast, grim,
struggling, silent, inarticulate array of ancestral force that lay in him,
when the burden of written speech was laid upon it, half rebelled,
and would not cease to struggle and be inarticulate. There was a
plethora of power: a channel, as through rocks, had to be made for
it, and there was an incipient cataclysm whenever a book was to be
written. What brings joy and buoyancy to other men, namely, a

genial task, brought despair and convulsions to him. It is not the
effort of composition,—he was a rapid and copious writer and
speaker,—but the pressure of purpose, the friction of power and
velocity, the sense of overcoming the demons and mud-gods and
frozen torpidity he so often refers to. Hence no writing extant is so
little like writing, and gives so vividly the sense of something done.
He may praise silence and glorify work. The unspeakable is ever
present with him; it is the core of every sentence: the inarticulate is
round about him; a solitude like that of space encompasseth him.
His books are not easy reading; they are a kind of wrestling to most
persons. His style is like a road made of rocks: when it is good, there
is nothing like it; and when it is bad, there is nothing like it!
In Past and Present Carlyle has unconsciously painted his own life
and character in truer colors than has any one else: Not a May-
game is this man's life, but a battle and a march, a warfare with
principalities and powers; no idle promenade through fragrant
orange groves and green, flowery spaces, waited on by the choral
Muses and the rosy Hours: it is a stern pilgrimage through burning,
sandy solitudes, through regions of thick-ribbed ice. He walks among
men; loves men with inexpressible soft pity, as they cannot love him:
but his soul dwells in solitude, in the uttermost parts of Creation. In
green oases by the palm-tree wells, he rests a space; but anon he
has to journey forward, escorted by the Terrors and the Splendors,
the Archdemons and Archangels. All heaven, all pandemonium, are
his escort. Part of the world will doubtless persist in thinking that
pandemonium furnished his chief counsel and guide; but there are
enough who think otherwise, and their numbers are bound to
increase in the future.

IV
A HUNT FOR THE NIGHTINGALE
While I lingered away the latter half of May in Scotland, and the first
half of June in northern England, and finally in London, intent on
seeing the land leisurely and as the mood suited, the thought never
occurred to me that I was in danger of missing one of the chief
pleasures I had promised myself in crossing the Atlantic, namely, the
hearing of the song of the nightingale. Hence, when on the 17th of
June I found myself down among the copses near Hazlemere, on the
borders of Surrey and Sussex, and was told by the old farmer, to
whose house I had been recommended by friends in London, that I
was too late, that the season of the nightingale was over, I was a
good deal disturbed.
I think she be done singing now, sir; I ain't heered her in some
time, sir, said my farmer, as we sat down to get acquainted over a
mug of the hardest cider I ever attempted to drink.
Too late! I said in deep chagrin, and I might have been here
weeks ago.
Yeas, sir, she be done now; May is the time to hear her. The cuckoo
is done too, sir; and you don't hear the nightingale after the cuckoo
is gone, sir.
(The country people in this part of England sir one at the end of
every sentence, and talk with an indescribable drawl.)
But I had heard a cuckoo that very afternoon, and I took heart from
the fact. I afterward learned that the country people everywhere
associate these two birds in this way; you will not hear the one after
the other has ceased. But I heard the cuckoo almost daily till the
middle of July. Matthew Arnold reflects the popular opinion when in

one of his poems (Thyrsis) he makes the cuckoo say in early June,
—
The bloom is gone, and with the bloom go I!
The explanation is to be found in Shakespeare, who says,—
The cuckoo is in June
Heard, not regarded,
as the bird really does not go till August. I got out my Gilbert White,
as I should have done at an earlier day, and was still more disturbed
to find that he limited the singing of the nightingale to June 15. But
seasons differ, I thought, and it can't be possible that any class of
feathered songsters all stop on a given day. There is a tradition that
when George I. died the nightingales all ceased singing for the year
out of grief at the sad event; but his majesty did not die till June 21.
This would give me a margin of several days. Then, when I looked
further in White, and found that he says the chaffinch ceases to sing
the beginning of June, I took more courage, for I had that day heard
the chaffinch also. But it was evident I had no time to lose; I was
just on the dividing line, and any day might witness the cessation of
the last songster. For it seems that the nightingale ceases singing
the moment her brood is hatched. After that event, you hear only a
harsh chiding or anxious note. Hence the poets, who attribute her
melancholy strains to sorrow for the loss of her young, are entirely
at fault. Virgil, portraying the grief of Orpheus after the loss of
Eurydice, says:—
So Philomela, 'mid the poplar shade,
Bemoans her captive brood; the cruel hind
Saw them unplumed, and took them; but all night
Grieves she, and, sitting on a bough, runs o'er
Her wretched tale, and fills the woods with woe.
But she probably does nothing of the kind. The song of a bird is not
a reminiscence, but an anticipation, and expresses happiness or joy

only, except in those cases where the male bird, having lost its mate,
sings for a few days as if to call the lost one back. When the male
renews his powers of song, after the young brood has been
destroyed, or after it has flown away, it is a sign that a new brood is
contemplated. The song is, as it were, the magic note that calls the
brood forth. At least, this is the habit with other song-birds, and I
have no doubt the same holds good with the nightingale. Destroy
the nest or brood of the wood thrush, and if the season is not too
far advanced, after a week or ten days of silence, during which the
parent birds by their manner seem to bemoan their loss and to take
counsel together, the male breaks forth with a new song, and the
female begins to construct a new nest. The poets, therefore, in
depicting the bird on such occasions as bewailing the lost brood, are
wide of the mark; he is invoking and celebrating a new brood.
As it was mid-afternoon, I could only compose myself till nightfall. I
accompanied the farmer to the hay-field and saw the working of his
mowing-machine, a rare implement in England, as most of the grass
is still cut by hand, and raked by hand also. The disturbed skylarks
were hovering above the falling grass, full of anxiety for their nests,
as one may note the bobolinks on like occasions at home. The
weather is so uncertain in England, and it is so impossible to predict
its complexion, not only from day to day but from hour to hour, that
the farmers appear to consider it a suitable time to cut grass when it
is not actually raining. They slash away without reference to the
aspects of the sky, and when the field is down trust to luck to be
able to cure the hay, or get it ready to carry between the showers.
The clouds were lowering and the air was damp now, and it was
Saturday afternoon; but the farmer said they would never get their
hay if they minded such things. The farm had seen better days; so
had the farmer; both were slightly down at the heel. Too high rent
and too much hard cider were working their effects upon both. The
farm had been in the family many generations, but it was now about
to be sold and to pass into other hands, and my host said he was
glad of it. There was no money in farming any more; no money in

anything. I asked him what were the main sources of profit on such
a farm.
Well, he said, sometimes the wheat pops up, and the barley drops
in, and the pigs come on, and we picks up a little money, sir, but not
much, sir. Pigs is doing well naow. But they brings so much wheat
from Ameriky, and our weather is so bad that we can't get a good
sample, sir, one year in three, that there is no money made in
growing wheat, sir. And the wuts (oats) were not much better.
Theys as would buy hain't got no money, sir. Up to the top of the
nip, for top of the hill, was one of his expressions. Tennyson had a
summer residence at Blackdown, not far off. One of the Queen's
poets, I believe, sir. Yes, I often see him riding about, sir.
After an hour or two with the farmer, I walked out to take a survey
of the surrounding country. It was quite wild and irregular, full of
bushy fields and overgrown hedge-rows, and looked to me very
nightingaly. I followed for a mile or two a road that led by tangled
groves and woods and copses, with a still meadow trout stream in
the gentle valley below. I inquired for nightingales of every boy and
laboring-man I met or saw. I got but little encouragement; it was
too late. She be about done singing now, sir. A boy whom I met in
a footpath that ran through a pasture beside a copse said, after
reflecting a moment, that he had heard one in that very copse two
mornings before,—about seven o'clock, sir, while I was on my way
to my work, sir. Then I would try my luck in said copse and in the
adjoining thickets that night and the next morning. The railway ran
near, but perhaps that might serve to keep the birds awake. These
copses in this part of England look strange enough to American
eyes. What thriftless farming! the first thought is; behold the fields
grown up to bushes, as if the land had relapsed to a state of nature
again. Adjoining meadows and grain-fields, one may see an
inclosure of many acres covered with a thick growth of oak and
chestnut sprouts, six or eight or twelve feet high. These are the
copses one has so often heard about, and they are a valuable and
productive part of the farm. They are planted and preserved as
carefully as we plant an orchard or a vineyard. Once in so many

years, perhaps five or six, the copse is cut and every twig is saved; it
is a woodland harvest that in our own country is gathered in the
forest itself. The larger poles are tied up in bundles and sold for
hoop-poles; the fine branches and shoots are made into brooms in
the neighboring cottages and hamlets, or used as material for
thatching. The refuse is used as wood.
About eight o'clock in the evening I sallied forth, taking my way over
the ground I had explored a few hours before. The gloaming, which
at this season lasts till after ten o'clock, dragged its slow length
along. Nine o'clock came, and, though my ear was attuned, the
songster was tardy. I hovered about the copses and hedge-rows like
one meditating some dark deed; I lingered in a grove and about an
overgrown garden and a neglected orchard; I sat on stiles and
leaned on wickets, mentally speeding the darkness that should bring
my singer out. The weather was damp and chilly, and the tryst grew
tiresome. I had brought a rubber water-proof, but not an overcoat.
Lining the back of the rubber with a newspaper, I wrapped it about
me and sat down, determined to lay siege to my bird. A footpath
that ran along the fields and bushes on the other side of the little
valley showed every few minutes a woman or girl, or boy or laborer,
passing along it. A path near me also had its frequent figures
moving along in the dusk. In this country people travel in footpaths
as much as in highways. The paths give a private, human touch to
the landscape that the roads do not. They are sacred to the human
foot. They have the sentiment of domesticity, and suggest the way
to cottage doors and to simple, primitive times.
Presently a man with a fishing-rod, and capped, coated, and booted
for the work, came through the meadow, and began casting for trout
in the stream below me. How he gave himself to the work! how
oblivious he was of everything but the one matter in hand! I doubt if
he was conscious of the train that passed within a few rods of him.
Your born angler is like a hound that scents no game but that which
he is in pursuit of. Every sense and faculty were concentrated upon
that hovering fly. This man wooed the stream, quivering with
pleasure and expectation. Every foot of it he tickled with his decoy.

His close was evidently a short one, and he made the most of it. He
lingered over every cast, and repeated it again and again. An
American angler would have been out of sight down stream long
ago. But this fisherman was not going to bolt his preserve; his line
should taste every drop of it. His eager, stealthy movements denoted
his enjoyment and his absorption. When a trout was caught, it was
quickly rapped on the head and slipped into his basket, as if in
punishment for its tardiness in jumping. Be quicker next time, will
you? (British trout, by the way, are not so beautiful as our own.
They have more of a domesticated look. They are less brilliantly
marked, and have much coarser scales. There is no gold or vermilion
in their coloring.)
Presently there arose from a bushy corner of a near field a low,
peculiar purring or humming sound, that sent a thrill through me; of
course, I thought my bird was inflating her throat. Then the sound
increased, and was answered or repeated in various other directions.
It had a curious ventriloquial effect. I presently knew it to be the
nightjar or goatsucker, a bird that answers to our whip-poor-will.
Very soon the sound seemed to be floating all about me,—Jr-r-r-r-r
or Chr-r-r-r-r, slightly suggesting the call of our toads, but more
vague as to direction. Then as it grew darker the birds ceased; the
fisherman reeled up and left. No sound was now heard,—not even
the voice of a solitary frog anywhere. I never heard a frog in
England. About eleven o'clock I moved down by a wood, and stood
for an hour on a bridge over the railroad. No voice of bird greeted
me till the sedge-warbler struck up her curious nocturne in a hedge
near by. It was a singular medley of notes, hurried chirps, trills, calls,
warbles, snatched from the songs of other birds, with a half-chiding,
remonstrating tone or air running through it all. As there was no
other sound to be heard, and as the darkness was complete, it had
the effect of a very private and whimsical performance,—as if the
little bird had secluded herself there, and was giving vent to her
emotions in the most copious and vehement manner. I listened till
after midnight, and till the rain began to fall, and the vivacious
warbler never ceased for a moment. White says that, if it stops, a

stone tossed into the bush near it will set it going again. Its voice is
not musical; the quality of it is like that of the loquacious English
house sparrows; but its song or medley is so persistently animated,
and in such contrast to the gloom and the darkness, that the effect
is decidedly pleasing.
This and the nightjar were the only nightingales I heard that night. I
returned home, a good deal disappointed, but slept upon my arms,
as it were, and was out upon the chase again at four o'clock in the
morning. This time I passed down a lane by the neglected garden
and orchard, where I was told the birds had sung for weeks past;
then under the railroad by a cluster of laborers' cottages, and along
a road with many copses and bushy fence-corners on either hand,
for two miles, but I heard no nightingales. A boy of whom I inquired
seemed half frightened, and went into the house without answering.
After a late breakfast I sallied out again, going farther in the same
direction, and was overtaken by several showers. I heard many and
frequent bird-songs,—the lark, the wren, the thrush, the blackbird,
the whitethroat, the greenfinch, and the hoarse, guttural cooing of
the wood-pigeons,—but not the note I was in quest of. I passed up a
road that was a deep trench in the side of a hill overgrown with low
beeches. The roots of the trees formed a network on the side of the
bank, as their branches did above. In a framework of roots, within
reach of my hand, I spied a wren's nest, a round hole leading to the
interior of a large mass of soft green moss, a structure displaying
the taste and neatness of the daintiest of bird architects, and the
depth and warmth and snugness of the most ingenious mouse
habitation. While lingering here, a young countryman came along
whom I engaged in conversation. No, he had not heard the
nightingale for a few days; but the previous week he had been in
camp with the militia near Guildford, and while on picket duty had
heard her nearly all night. 'Don't she sing splendid to-night?' the
boys would say. This was tantalizing; Guildford was within easy
reach; but the previous week,—that could not be reached. However,
he encouraged me by saying he did not think they were done
singing yet, as he had often heard them during haying-time. I

inquired for the blackcap, but saw he did not know this bird, and
thought I referred to a species of tomtit, which also has a black cap.
The woodlark I was also on the lookout for, but he did not know this
bird either, and during my various rambles in England I found but
one person who did. In Scotland it was confounded with the titlark
or pipit.
I next met a man and boy, a villager with a stove-pipe hat on,—and,
as it turned out, a man of many trades, tailor, barber, painter, etc.,—
from Hazlemere. The absorbing inquiry was put to him also. No, not
that day, but a few mornings before he had. But he could easily call
one out, if there were any about, as he could imitate them. Plucking
a spear of grass, he adjusted it behind his teeth and startled me
with the shrill, rapid notes he poured forth. I at once recognized its
resemblance to the descriptions I had read of the opening part of
the nightingale song,—what is called the challenge. The boy said,
and he himself averred, that it was an exact imitation. The chew,
chew, chew, and some other parts, were very bird-like, and I had no
doubt were correct. I was astonished at the strong, piercing quality
of the strain. It echoed in the woods and copses about, but, though
oft repeated, brought forth no response. With this man I made an
engagement to take a walk that evening at eight o'clock along a
certain route where he had heard plenty of nightingales but a few
days before. He was confident he could call them out; so was I.
In the afternoon, which had gleams of warm sunshine, I made
another excursion, less in hopes of hearing my bird than of finding
some one who could direct me to the right spot. Once I thought the
game was very near. I met a boy who told me he had heard a
nightingale only fifteen minutes before, on Polecat Hill, sir, just this
side the Devil's Punch-bowl, sir! I had heard of his majesty's punch-
bowl before, and of the gibbets near it where three murderers were
executed nearly a hundred years ago, but Polecat Hill was a new
name to me. The combination did not seem a likely place for
nightingales, but I walked rapidly thitherward; I heard several
warblers, but not Philomel, and was forced to conclude that probably
I had crossed the sea to miss my bird by just fifteen minutes. I met

many other boys (is there any country where boys do not prowl
about in small bands of a Sunday?) and advertised the object of my
search freely among them, offering a reward that made their eyes
glisten for the bird in song; but nothing ever came of it. In my
desperation, I even presented a letter I had brought to the village
squire, just as, in company with his wife, he was about to leave his
door for church. He turned back, and, hearing my quest, volunteered
to take me on a long walk through the wet grass and bushes of his
fields and copses, where he knew the birds were wont to sing. Too
late, he said, and so it did appear. He showed me a fine old edition
of White's Selborne, with notes by some editor whose name I have
forgotten. This editor had extended White's date of June 15 to July
1, as the time to which the nightingale continues in song, and I felt
like thanking him for it, as it gave me renewed hope. The squire
thought there was a chance yet; and in case my man with the spear
of grass behind his teeth failed me, he gave me a card to an old
naturalist and taxidermist at Godalming, a town nine miles above,
who, he felt sure, could put me on the right track if anybody could.
At eight o'clock, the sun yet some distance above the horizon, I was
at the door of the barber in Hazlemere. He led the way along one of
those delightful footpaths with which this country is threaded,
extending to a neighboring village several miles distant. It left the
street at Hazlemere, cutting through the houses diagonally, as if the
brick walls had made way for it, passed between gardens, through
wickets, over stiles, across the highway and railroad, through
cultivated fields and a gentleman's park, and on toward its
destination,—a broad, well-kept path, that seemed to have the same
inevitable right of way as a brook. I was told that it was repaired and
looked after the same as the highway. Indeed, it was a public way,
public to pedestrians only, and no man could stop or turn it aside.
We followed it along the side of a steep hill, with copses and groves
sweeping down into the valley below us. It was as wild and
picturesque a spot as I had seen in England. The foxglove pierced
the lower foliage and wild growths everywhere with its tall spires of
purple flowers; the wild honeysuckle, with a ranker and coarser

fragrance than our cultivated species, was just opening along the
hedges. We paused here, and my guide blew his shrill call; he blew
it again and again. How it awoke the echoes, and how it awoke all
the other songsters! The valley below us and the slope beyond,
which before were silent, were soon musical. The chaffinch, the
robin, the blackbird, the thrush—the last the loudest and most
copious—seemed to vie with each other and with the loud whistler
above them. But we listened in vain for the nightingale's note. Twice
my guide struck an attitude and said, impressively, There! I believe
I 'erd 'er. But we were obliged to give it up. A shower came on, and
after it had passed we moved to another part of the landscape and
repeated our call, but got no response, and as darkness set in we
returned to the village.
The situation began to look serious. I knew there was a nightingale
somewhere whose brood had been delayed from some cause or
other, and who was therefore still in song, but I could not get a clew
to the spot. I renewed the search late that night, and again the next
morning; I inquired of every man and boy I saw.

I met many travelers,
Who the road had surely kept;
They saw not my fine revelers,—
These had crossed them while they slept;
Some had heard their fair report,
In the country or the court.
I soon learned to distrust young fellows and their girls who had
heard nightingales in the gloaming. I knew one's ears could not
always be depended upon on such occasions, nor his eyes either.
Larks are seen in buntings, and a wren's song entrances like
Philomel's. A young couple of whom I inquired in the train, on my
way to Godalming, said Yes, they had heard nightingales just a few
moments before on their way to the station, and described the spot,
so I could find it if I returned that way. They left the train at the
same point I did, and walked up the street in advance of me. I had
lost sight of them till they beckoned to me from the corner of the
street, near the church, where the prospect opens with a view of a
near meadow and a stream shaded by pollard willows. We heard
one now, just there, they said, as I came up. They passed on, and I
bent my ear eagerly in the direction. Then I walked farther on,
following one of those inevitable footpaths to where it cuts
diagonally through the cemetery behind the old church, but I heard
nothing save a few notes of the thrush. My ear was too critical and
exacting. Then I sought out the old naturalist and taxidermist to
whom I had a card from the squire. He was a short, stout man, racy
both in look and speech, and kindly. He had a fine collection of birds
and animals, in which he took great pride. He pointed out the
woodlark and the blackcap to me, and told me where he had seen
and heard them. He said I was too late for the nightingale, though I
might possibly find one yet in song. But he said she grew hoarse late
in the season, and did not sing as a few weeks earlier. He thought
our cardinal grosbeak, which he called the Virginia nightingale, as
fine a whistler as the nightingale herself. He could not go with me
that day, but he would send his boy. Summoning the lad, he gave

him minute directions where to take me,—over by Easing, around by
Shackerford church, etc., a circuit of four or five miles. Leaving the
picturesque old town, we took a road over a broad, gentle hill, lined
with great trees,—beeches, elms, oaks,—with rich cultivated fields
beyond. The air of peaceful and prosperous human occupancy which
everywhere pervades this land seemed especially pronounced
through all this section. The sentiment of parks and lawns, easy,
large, basking, indifferent of admiration, self-sufficing, and full,
everywhere prevailed. The road was like the most perfect private
carriage-way. Homeliness, in its true sense, is a word that applies to
nearly all English country scenes; homelike, redolent of affectionate
care and toil, saturated with rural and domestic contentment; beauty
without pride, order without stiffness, age without decay. This
people love the country, because it would seem as if the country
must first have loved them. In a field I saw for the first time a new
species of clover, much grown in parts of England as green fodder
for horses. The farmers call it trifolium, probably Trifolium
incarnatum. The head is two or three inches long, and as red as
blood. A field of it under the sunlight presents a most brilliant
appearance. As we walked along, I got also my first view of the
British blue jay,—a slightly larger bird than ours, with a hoarser voice
and much duller plumage. Blue, the tint of the sky, is not so
common, and is not found in any such perfection among the British
birds as among the American. My boy companion was worthy of
observation also. He was a curious specimen, ready and officious,
but, as one soon found out, full of duplicity. I questioned him about
himself. I helps he, sir; sometimes I shows people about, and
sometimes I does errands. I gets three a week, sir, and lunch and
tea. I lives with my grandmother, but I calls her mother, sir. The
master and the rector they gives me a character, says I am a good,
honest boy, and that it is well I went to school in my youth. I am
ten, sir. Last year I had the measles, sir, and I thought I should die;
but I got hold of a bottle of medicine, and it tasted like honey, and I
takes the whole of it, and it made me well, sir. I never lies, sir. It is
good to tell the truth. And yet he would slide off into a lie as if the
track in that direction was always greased. Indeed, there was a kind

of fluent, unctuous, obsequious effrontery in all he said and did. As
the day was warm for that climate, he soon grew tired of the chase.
At one point we skirted the grounds of a large house, as thickly
planted with trees and shrubs as a forest; many birds were singing
there, and for a moment my guide made me believe that among
them he recognized the notes of the nightingale. Failing in this, he
coolly assured me that the swallow that skimmed along the road in
front of us was the nightingale! We presently left the highway and
took a footpath. It led along the margin of a large plowed field, shut
in by rows of noble trees, the soil of which looked as if it might have
been a garden of untold generations. Then the path led through a
wicket, and down the side of a wooded hill to a large stream and to
the hamlet of Easing. A boy fishing said indifferently that he had
heard nightingales there that morning. He had caught a little fish
which he said was a gudgeon. Yes, said my companion in response
to a remark of mine, they's little; but you can eat they if they is
little. Then we went toward Shackerford church. The road, like most
roads in the south of England, was a deep trench. The banks on
either side rose fifteen feet, covered with ivy, moss, wild flowers,
and the roots of trees. England's best defense against an invading
foe is her sunken roads. Whole armies might be ambushed in these
trenches, while an enemy moving across the open plain would very
often find himself plunging headlong into these hidden pitfalls.
Indeed, between the subterranean character of the roads in some
places and the high-walled or high-hedged character of it in others,
the pedestrian about England is shut out from much he would like to
see. I used to envy the bicyclists, perched high upon their rolling
stilts. But the footpaths escape the barriers, and one need walk
nowhere else if he choose.
Around Shackerford church are copses, and large pine and fir woods.
The place was full of birds. My guide threw a stone at a small bird
which he declared was a nightingale; and though the missile did not
come within three yards of it, yet he said he had hit it, and
pretended to search for it on the ground. He must needs invent an
opportunity for lying. I told him here I had no further use for him,

and he turned cheerfully back, with my shilling in his pocket. I spent
the afternoon about the woods and copses near Shackerford. The
day was bright and the air balmy. I heard the cuckoo call, and the
chaffinch sing, both of which I considered good omens. The little
chiffchaff was chiffchaffing in the pine woods. The whitethroat, with
his quick, emphatic Chew-che-rick or Che-rick-a-rew, flitted and
ducked and hid among the low bushes by the roadside. A girl told
me she had heard the nightingale yesterday on her way to Sunday-
school, and pointed out the spot. It was in some bushes near a
house. I hovered about this place till I was afraid the woman, who
saw me from the window, would think I had some designs upon her
premises. But I managed to look very indifferent or abstracted when
I passed. I am quite sure I heard the chiding, guttural note of the
bird I was after. Doubtless her brood had come out that very day.
Another girl had heard a nightingale on her way to school that
morning, and directed me to the road; still another pointed out to
me the whitethroat and said that was my bird. This last was a rude
shock to my faith in the ornithology of schoolgirls. Finally, I found a
laborer breaking stone by the roadside,—a serious, honest-faced
man, who said he had heard my bird that morning on his way to
work; he heard her every morning, and nearly every night, too. He
heard her last night after the shower (just at the hour when my
barber and I were trying to awaken her near Hazlemere), and she
sang as finely as ever she did. This was a great lift. I felt that I could
trust this man. He said that after his day's work was done, that is, at
five o'clock, if I chose to accompany him on his way home, he would
show me where he had heard the bird. This I gladly agreed to; and,
remembering that I had had no dinner, I sought out the inn in the
village and asked for something to eat. The unwonted request so
startled the landlord that he came out from behind his inclosed bar
and confronted me with good-humored curiosity. These back-country
English inns, as I several times found to my discomfiture, are only
drinking places for the accommodation of local customers, mainly of
the laboring class. Instead of standing conspicuously on some street
corner, as with us, they usually stand on some byway, or some little
paved court away from the main thoroughfare. I could have plenty

of beer, said the landlord, but he had not a mouthful of meat in the
house. I urged my needs, and finally got some rye-bread and
cheese. With this and a glass of home-brewed beer I was fairly well
fortified. At the appointed time I met the cottager and went with him
on his way home. We walked two miles or more along a charming
road, full of wooded nooks and arbor-like vistas. Why do English
trees always look so sturdy, and exhibit such massive repose, so
unlike, in this latter respect, to the nervous and agitated expression
of most of our own foliage? Probably because they have been a long
time out of the woods, and have had plenty of room in which to
develop individual traits and peculiarities; then, in a deep fertile soil,
and a climate that does not hurry or overtax, they grow slow and
last long, and come to have the picturesqueness of age without its
infirmities. The oak, the elm, the beech, all have more striking
profiles than in our country.
Presently my companion pointed out to me a small wood below the
road that had a wide fringe of bushes and saplings connecting it
with a meadow, amid which stood the tree-embowered house of a
city man, where he had heard the nightingale in the morning; and
then, farther along, showed me, near his own cottage, where he had
heard one the evening before. It was now only six o'clock, and I had
two or three hours to wait before I could reasonably expect to hear
her. It gets to be into the hevening, said my new friend, when she
sings the most, you know. I whiled away the time as best I could. If
I had been an artist, I should have brought away a sketch of a
picturesque old cottage near by, that bore the date of 1688 on its
wall. I was obliged to keep moving most of the time to keep warm.
Yet the no-see-'ems, or midges, annoyed me, in a temperature
which at home would have chilled them buzzless and biteless.
Finally, I leaped the smooth masonry of the stone wall and
ambushed myself amid the tall ferns under a pine-tree, where the
nightingale had been heard in the morning. If the keeper had seen
me, he would probably have taken me for a poacher. I sat shivering
there till nine o'clock, listening to the cooing of the wood-pigeons,
watching the motions of a jay that, I suspect, had a nest near by,

and taking note of various other birds. The song-thrush and the
robins soon made such a musical uproar along the borders of a
grove, across an adjoining field, as quite put me out. It might veil
and obscure the one voice I wanted to hear. The robin continued to
sing quite into the darkness. This bird is related to the nightingale,
and looks and acts like it at a little distance; and some of its notes
are remarkably piercing and musical. When my patience was about
exhausted, I was startled by a quick, brilliant call or whistle, a few
rods from me, that at once recalled my barber with his blade of
grass, and I knew my long-sought bird was inflating her throat. How
it woke me up! It had the quality that startles; it pierced the
gathering gloom like a rocket. Then it ceased. Suspecting I was too
near the singer, I moved away cautiously, and stood in a lane beside
the wood, where a loping hare regarded me a few paces away. Then
my singer struck up again, but I could see did not let herself out;
just tuning her instrument, I thought, and getting ready to transfix
the silence and the darkness. A little later, a man and boy came up
the lane. I asked them if that was the nightingale singing; they
listened, and assured me it was none other. Now she's on, sir; now
she's on. Ah! but she don't stick. In May, sir, they makes the woods
all heccho about here. Now she's on again; that's her, sir; now she's
off; she won't stick. And stick she would not. I could hear a hoarse
wheezing and clucking sound beneath her notes, when I listened
intently. The man and boy moved away. I stood mutely invoking all
the gentle divinities to spur the bird on. Just then a bird like our
hermit thrush came quickly over the hedge a few yards below me,
swept close past my face, and back into the thicket. I had been
caught listening; the offended bird had found me taking notes of her
dry and worn-out pipe there behind the hedge, and the concert
abruptly ended; not another note; not a whisper. I waited a long
time and then moved off; then came back, implored the outraged
bird to resume; then rushed off, and slammed the door, or rather the
gate, indignantly behind me. I paused by other shrines, but not a
sound. The cottager had told me of a little village three miles
beyond, where there were three inns, and where I could probably
get lodgings for the night. I walked rapidly in that direction;

committed myself to a footpath; lost the trail, and brought up at a
little cottage in a wide expanse of field or common, and by the good
woman, with a babe in her arms, was set right again. I soon struck
the highway by the bridge, as I had been told, and a few paces
brought me to the first inn. It was ten o'clock, and the lights were
just about to be put out, as the law or custom is in country inns. The
landlady said she could not give me a bed; she had only one spare
room, and that was not in order, and she should not set about
putting it in shape at that hour; and she was short and sharp about
it, too. I hastened on to the next one. The landlady said she had no
sheets, and the bed was damp and unfit to sleep in. I protested that
I thought an inn was an inn, and for the accommodation of
travelers. But she referred me to the next house. Here were more
people, and more the look and air of a public house. But the wife
(the man does not show himself on such occasions) said her
daughter had just got married and come home, and she had much
company and could not keep me. In vain I urged my extremity;
there was no room. Could I have something to eat, then? This
seemed doubtful, and led to consultations in the kitchen; but, finally,
some bread and cold meat were produced. The nearest hotel was
Godalming, seven miles distant, and I knew all the inns would be
shut up before I could get there. So I munched my bread and meat,
consoling myself with the thought that perhaps this was just the ill
wind that would blow me the good I was in quest of. I saw no
alternative but to spend a night under the trees with the
nightingales; and I might surprise them at their revels in the small
hours of the morning. Just as I was ready to congratulate myself on
the richness of my experience, the landlady came in and said there
was a young man there going with a trap to Godalming, and he
had offered to take me in. I feared I should pass for an escaped
lunatic if I declined the offer; so I reluctantly assented, and we were
presently whirling through the darkness, along a smooth, winding
road, toward town. The young man was a drummer; was from
Lincolnshire, and said I spoke like a Lincolnshire man. I could believe
it, for I told him he talked more like an American than any native I
had met. The hotels in the larger towns close at eleven, and I was

set down in front of one just as the clock was striking that hour. I
asked to be conducted to a room at once. As I was about getting in
bed there was a rap at the door, and a waiter presented me my bill
on a tray. Gentlemen as have no luggage, etc., he explained; and
pretend to be looking for nightingales, too! Three-and-sixpence; two
shillings for the bed and one-and-six for service. I was out at five in
the morning, before any one inside was astir. After much trying of
bars and doors, I made my exit into a paved court, from which a
covered way led into the street. A man opened a window and
directed me how to undo the great door, and forth I started, still
hoping to catch my bird at her matins. I took the route of the day
before. On the edge of the beautiful plowed field, looking down
through the trees and bushes into the gleam of the river twenty rods
below, I was arrested by the note I longed to hear. It came up from
near the water, and made my ears tingle. I folded up my rubber coat
and sat down upon it, saying, Now we will take our fill. But—the bird
ceased, and, tarry though I did for an hour, not another note
reached me. The prize seemed destined to elude me each time just
as I thought it mine. Still, I treasured what little I had heard.
It was enough to convince me of the superior quality of the song,
and make me more desirous than ever to hear the complete strain. I
continued my rambles, and in the early morning once more hung
about the Shackerford copses and loitered along the highways. Two
schoolboys pointed out a tree to me in which they had heard the
nightingale, on their way for milk, two hours before. But I could only
repeat Emerson's lines:—
Right good-will my sinews strung,
But no speed of mine avails
To hunt up their shining trails.
At nine o'clock I gave over the pursuit and returned to Easing in
quest of breakfast. Bringing up in front of the large and comfortable-
looking inn, I found the mistress of the house with her daughter
engaged in washing windows. Perched upon their step-ladders, they

treated my request for breakfast very coldly; in fact, finally refused
to listen to it at all. The fires were out, and I could not be served. So
I must continue my walk back to Godalming; and, in doing so, I
found that one may walk three miles on indignation quite as easily
as upon bread.
In the afternoon I returned to my lodgings at Shotter Mill, and made
ready for a walk to Selborne, twelve miles distant, part of the way to
be accomplished that night in the gloaming, and the rest early on
the following morning, to give the nightingales a chance to make
any reparation they might feel inclined to for the neglect with which
they had treated me. There was a footpath over the hill and through
Leechmere bottom to Liphook, and to this, with the sun half an hour
high, I committed myself. The feature in this hill scenery of Surrey
and Sussex that is new to American eyes is given by the furze and
heather, broad black or dark-brown patches of which sweep over the
high rolling surfaces, like sable mantles. Tennyson's house stands
amid this dusky scenery, a few miles east of Hazlemere. The path
led through a large common, partly covered with grass and partly
grown up to furze,—another un-American feature. Doubly precious is
land in England, and yet so much of it given to parks and pleasure-
grounds, and so much of it left unreclaimed in commons! These
commons are frequently met with; about Selborne they are miles in
extent, and embrace the Hanger and other woods. No one can
inclose them, or appropriate them to his own use. The landed
proprietor of whose estates they form a part cannot; they belong to
the people, to the lease-holders. The villagers and others who own
houses on leased land pasture their cows upon them, gather the
furze, and cut the wood. In some places the commons belong to the
crown and are crown lands. These large uninclosed spaces often
give a free-and-easy air to the landscape that is very welcome. Near
the top of the hill I met a little old man nearly hidden beneath a
burden of furze. He was backing it home for fuel and other uses. He
paused obsequious, and listened to my inquiries. A dwarfish sort of
man, whose ugliness was redolent of the humblest chimney corner.
Bent beneath his bulky burden, and grinning upon me, he was a

visible embodiment of the poverty, ignorance, and, I may say, the
domesticity of the lowliest peasant home. I felt as if I had
encountered a walking superstition, fostered beside a hearth lighted
by furze fagots and by branches dropped by the nesting rooks and
ravens,—a figure half repulsive and half alluring. On the border of
Leechmere bottom I sat down above a straggling copse, aflame as
usual with the foxglove, and gave eye and ear to the scene. While
sitting here, I saw and heard for the first time the black-capped
warbler. I recognized the note at once by its brightness and
strength, and a faint suggestion in it of the nightingale's. But it was
disappointing: I had expected a nearer approach to its great rival.
The bird was very shy, but did finally show herself fairly several
times, as she did also near Selborne, where I heard the song oft
repeated and prolonged. It is a ringing, animated strain, but as a
whole seemed to me crude, not smoothly and finely modulated. I
could name several of our own birds that surpass it in pure music.
Like its congeners, the garden warbler and the whitethroat, it sings
with great emphasis and strength, but its song is silvern, not golden.
Little birds with big voices, one says to himself after having heard
most of the British songsters. My path led me an adventurous course
through the copses and bottoms and open commons, in the long
twilight. At one point I came upon three young men standing
together and watching a dog that was working a near field,—one of
them probably the squire's son, and the other two habited like
laborers. In a little thicket near by there was a brilliant chorus of bird
voices, the robin, the song-thrush, and the blackbird, all vying with
each other. To my inquiry, put to test the reliability of the young
countrymen's ears, they replied that one of the birds I heard was the
nightingale, and, after a moment's attention, singled out the robin as
the bird in question. This incident so impressed me that I paid little
attention to the report of the next man I met, who said he had
heard a nightingale just around a bend in the road, a few minutes'
walk in advance of me. At ten o'clock I reached Liphook. I expected
and half hoped the inn would turn its back upon me again, in which
case I proposed to make for Wolmer Forest, a few miles distant, but
it did not. Before going to bed, I took a short and hasty walk down a

promising-looking lane, and again met a couple who had heard
nightingales. It was a nightingale, was it not, Charley?
If all the people of whom I inquired for nightingales in England could
have been together and compared notes, they probably would not
have been long in deciding that there was at least one crazy
American abroad.
I proposed to be up and off at five o'clock in the morning, which
seemed greatly to puzzle mine host. At first he thought it could not
be done, but finally saw his way out of the dilemma, and said he
would get up and undo the door for me himself. The morning was
cloudy and misty, though the previous night had been of the fairest.
There is one thing they do not have in England that we can boast of
at home, and that is a good masculine type of weather: it is not
even feminine; it is childish and puerile, though I am told that
occasionally there is a full-grown storm. But I saw nothing but
petulant little showers and prolonged juvenile sulks. The clouds have
no reserve, no dignity; if there is a drop of water in them (and there
generally are several drops), out it comes. The prettiest little
showers march across the country in summer, scarcely bigger than a
street watering-cart; sometimes by getting over the fence one can
avoid them, but they keep the haymakers in a perpetual flurry. There
is no cloud scenery, as with us, no mass and solidity, no height nor
depth. The clouds seem low, vague, and vapory,—immature,
indefinite, inconsequential, like youth.
The walk to Selborne was through mist and light rain. Few bird
voices, save the cries of the lapwing and the curlew, were heard.
Shortly after leaving Liphook the road takes a straight cut for three
or four miles through a level, black, barren, peaty stretch of country,
with Wolmer Forest a short distance on the right. Under the low-
hanging clouds the scene was a dismal one,—a black earth beneath
and a gloomy sky above. For miles the only sign of life was a baker's
cart rattling along the smooth, white road. At the end of this solitude
I came to cultivated fields, and a little hamlet and an inn. At this inn
(for a wonder!) I got some breakfast. The family had not yet had

theirs, and I sat with them at the table, and had substantial fare.
From this point I followed a footpath a couple of miles through fields
and parks. The highways for the most part seemed so narrow and
exclusive, or inclusive, such penalties seemed to attach to a view
over the high walls and hedges that shut me in, that a footpath was
always a welcome escape to me. I opened the wicket or mounted
the stile without much concern as to whether it would further me on
my way or not. It was like turning the flank of an enemy. These well-
kept fields and lawns, these cozy nooks, these stately and exclusive
houses that had taken such pains to shut out the public gaze,—from
the footpath one had them at an advantage, and could pluck out
their mystery. On striking the highway again, I met the postmistress,
stepping briskly along with the morning mail. Her husband had died,
and she had taken his place as mail-carrier. England is so densely
populated, the country is so like a great city suburb, that your mail is
brought to your door everywhere, the same as in town. I walked a
distance with a boy driving a little old white horse with a cart-load of
brick. He lived at Hedleigh, six miles distant; he had left there at five
o'clock in the morning, and had heard a nightingale. He was sure; as
I pressed him, he described the place minutely. She was in the
large fir-tree by Tom Anthony's gate, at the south end of the village.
Then, I said, doubtless I shall find one in some of Gilbert White's
haunts; but I did not. I spent two rainy days at Selborne; I passed
many chilly and cheerless hours loitering along those wet lanes and
dells and dripping hangers, wooing both my bird and the spirit of the
gentle parson, but apparently without getting very near to either.
When I think of the place now, I see its hurrying and anxious
haymakers in the field of mown grass, and hear the cry of a child
that sat in the hay back of the old church, and cried by the hour
while its mother was busy with her rake not far off. The rain had
ceased, the hay had dried off a little, and scores of men, women,
and children, but mostly women, had flocked to the fields to rake it
up. The hay is got together inch by inch, and every inch is fought
for. They first rake it up into narrow swaths, each person taking a
strip about a yard wide. If they hold the ground thus gained, when
the hay dries an hour or two longer, they take another hitch, and

thus on till they get it into the cock or carry it from the windrow. It
is usually nearly worn out with handling before they get it into the
rick.
From Selborne I went to Alton, along a road that was one prolonged
rifle-pit, but smooth and hard as a rock; thence by train back to
London. To leave no ground for self-accusation in future, on the
score of not having made a thorough effort to hear my songster, I
the next day made a trip north toward Cambridge, leaving the train
at Hitchin, a large picturesque old town, and thought myself in just
the right place at last. I found a road between the station and the
town proper called Nightingale Lane, famous for its songsters. A
man who kept a thrifty-looking inn on the corner (where, by the
way, I was again refused both bed and board) said they sang night
and morning in the trees opposite. He had heard them the night
before, but had not noticed them that morning. He often sat at night
with his friends, with open windows, listening to the strain. He said
he had tried several times to hold his breath as long as the bird did
in uttering certain notes, but could not do it. This, I knew, was an
exaggeration; but I waited eagerly for nightfall, and, when it came,
paced the street like a patrolman, and paced other streets, and
lingered about other likely localities, but caught nothing but
neuralgic pains in my shoulder. I had no better success in the
morning, and here gave over the pursuit, saying to myself, It
matters little, after all; I have seen the country and had some object
for a walk, and that is sufficient.
Altogether I heard the bird less than five minutes, and only a few
bars of its song, but enough to satisfy me of the surprising quality of
the strain.
It had the master tone as clearly as Tennyson or any great prima
donna or famous orator has it. Indeed, it was just the same. Here is
the complete artist, of whom all these other birds are but hints and
studies. Bright, startling, assured, of great compass and power, it
easily dominates all other notes; the harsher chur-r-r-r-rg notes

serve as foil to her surpassing brilliancy. Wordsworth, among the
poets, has hit off the song nearest:—
Those notes of thine,—they pierce and pierce;
Tumultuous harmony and fierce!
I could easily understand that this bird might keep people awake at
night by singing near their houses, as I was assured it frequently
does; there is something in the strain so startling and awakening. Its
start is a vivid flash of sound. On the whole, a high-bred, courtly,
chivalrous song; a song for ladies to hear leaning from embowered
windows on moonlight nights; a song for royal parks and groves,—
and easeful but impassioned life. We have no bird-voice so piercing
and loud, with such flexibility and compass, such full-throated
harmony and long-drawn cadences; though we have songs of more
melody, tenderness, and plaintiveness. None but the nightingale
could have inspired Keats's ode,—that longing for self-forgetfulness
and for the oblivion of the world, to escape the fret and fever of life.
And with thee fade away into the forest dim.

V
ENGLISH AND AMERICAN SONG-BIRDS
The charm of the songs of birds, like that of a nation's popular airs
and hymns, is so little a question of intrinsic musical excellence, and
so largely a matter of association and suggestion, or of subjective
coloring and reminiscence, that it is perhaps entirely natural for
every people to think their own feathered songsters the best. What
music would there not be to the homesick American, in Europe, in
the simple and plaintive note of our bluebird, or the ditty of our song
sparrow, or the honest carol of our robin; and what, to the European
traveler in this country, in the burst of the blackcap, or the
redbreast, or the whistle of the merlin! The relative merit of bird-
songs can hardly be settled dogmatically; I suspect there is very
little of what we call music, or of what could be noted on the musical
scale, in even the best of them; they are parts of nature, and their
power is in the degree in which they speak to our experience.
When the Duke of Argyll, who is a lover of the birds and a good
ornithologist, was in this country, he got the impression that our
song-birds were inferior to the British, and he refers to others of his
countrymen as of like opinion. No wonder he thought our robin
inferior in power to the missel thrush, in variety to the mavis, and in
melody to the blackbird! Robin did not and could not sing to his ears
the song he sings to ours. Then it is very likely true that his grace
did not hear the robin in the most opportune moment and season,
or when the contrast of his song with the general silence and
desolation of nature is the most striking and impressive. The
nightingale needs to be heard at night, the lark at dawn rising to
meet the sun; and robin, if you would know the magic of his voice,
should be heard in early spring, when, as the sun is setting, he
carols steadily for ten or fifteen minutes from the top of some near
tree. There is perhaps no other sound in nature; patches of snow

linger here and there; the trees are naked and the earth is cold and
dead, and this contented, hopeful, reassuring, and withal musical
strain, poured out so freely and deliberately, fills the void with the
very breath and presence of the spring. It is a simple strain, well
suited to the early season; there are no intricacies in it, but its
honest cheer and directness, with its slight plaintive tinge, like that
of the sun gilding the treetops, go straight to the heart. The
compass and variety of the robin's powers are not to be despised
either. A German who has great skill in the musical education of
birds told me what I was surprised to hear, namely, that our robin
surpasses the European blackbird in capabilities of voice.
The duke does not mention by name all the birds he heard while in
this country. He was evidently influenced in his opinion of them by
the fact that our common sandpiper appeared to be a silent bird,
whereas its British cousin, the sandpiper of the lakes and streams of
the Scottish Highlands, is very loquacious, and the male bird has a
continuous and most lively song. Either the duke must have seen
our bird in one of its silent and meditative moods, or else, in the
wilds of Canada where his grace speaks of having seen it, the
sandpiper is a more taciturn bird than it is in the States. True, its
call-notes are not incessant, and it is not properly a song-bird any
more than the British species is; but it has a very pretty and pleasing
note as it flits up and down our summer streams, or runs along on
their gray, pebbly, and bowlder-strewn shallows. I often hear its
calling and piping at night during its spring migratings. Indeed, we
have no silent bird that I am aware of, though our pretty cedar-bird
has, perhaps, the least voice of any. A lady writes me that she has
heard the hummingbird sing, and says she is not to be put down,
even if I were to prove by the anatomy of the bird's vocal organs
that a song was impossible to it.
Argyll says that, though he was in the woods and fields of Canada
and of the States in the richest moment of the spring, he heard little
of that burst of song which in England comes from the blackcap, and
the garden warbler, and the whitethroat, and the reed warbler, and
the common wren, and (locally) from the nightingale. There is no

lack of a burst of song in this country (except in the remote forest
solitudes) during the richest moment of the spring, say from the 1st
to the 20th of May, and at times till near midsummer; moreover,
more bird-voices join in it, as I shall point out, than in Britain; but it
is probably more fitful and intermittent, more confined to certain
hours of the day, and probably proceeds from throats less loud and
vivacious than that with which our distinguished critic was familiar.
The ear hears best and easiest what it has heard before. Properly to
apprehend and appreciate bird-songs, especially to disentangle them
from the confused murmur of nature, requires more or less
familiarity with them. If the duke had passed a season with us in
some one place in the country, in New York or New England, he
would probably have modified his views about the silence of our
birds.
One season, early in May, I discovered an English skylark in full song
above a broad, low meadow in the midst of a landscape that
possessed features attractive to a great variety of our birds. Every
morning for many days I used to go and sit on the brow of a low hill
that commanded the field, or else upon a gentle swell in the midst of
the meadow itself, and listen to catch the song of the lark. The maze
and tangle of bird-voices and bird-choruses through which my ear
groped its way searching for the new song can be imagined when I
say that within hearing there were from fifteen to twenty different
kinds of songsters, all more or less in full tune. If their notes and
calls could have been materialized and made as palpable to the eye
as they were to the ear, I think they would have veiled the landscape
and darkened the day. There were big songs and little songs,—songs
from the trees, the bushes, the ground, the air,—warbles, trills,
chants, musical calls, and squeals, etc. Near by in the foreground
were the catbird and the brown thrasher, the former in the bushes,
the latter on the top of a hickory. These birds are related to the
mockingbird, and may be called performers; their songs are a series
of vocal feats, like the exhibition of an acrobat; they throw musical
somersaults, and turn and twist and contort themselves in a very
edifying manner, with now and then a ventriloquial touch. The

catbird is the more shrill, supple, and feminine; the thrasher the
louder, richer, and more audacious. The mate of the latter had a
nest, which I found in a field under the spreading ground-juniper.
From several points along the course of a bushy little creek there
came a song, or a melody of notes and calls, that also put me out,—
the tipsy, hodge-podge strain of the polyglot chat, a strong, olive-
backed, yellow-breasted, black-billed bird, with a voice like that of a
jay or a crow that had been to school to a robin or an oriole,—a
performer sure to arrest your ear and sure to elude your eye. There
is no bird so afraid of being seen, or fonder of being heard.
The golden voice of the wood thrush that came to me from the
border of the woods on my right was no hindrance to the ear, it was
so serene, liquid, and, as it were, transparent: the lark's song has
nothing in common with it. Neither were the songs of the many
bobolinks in the meadow at all confusing,—a brief tinkle of silver
bells in the grass, while I was listening for a sound more like the
sharp and continuous hum of silver wheels upon a pebbly beach.
Certain notes of the red-shouldered starlings in the alders and
swamp maples near by, the distant barbaric voice of the great
crested flycatcher, the jingle of the kingbird, the shrill, metallic song
of the savanna sparrow, and the piercing call of the meadowlark, all
stood more or less in the way of the strain I was listening for,
because every one had a touch of that burr or guttural hum of the
lark's song. The ear had still other notes to contend with, as the
strong, bright warble of the tanager, the richer and more melodious
strain of the rose-breasted grosbeak, the distant, brief, and emphatic
song of the chewink, the child-like contented warble of the red-eyed
vireo, the animated strain of the goldfinch, the softly ringing notes of
the bush sparrow, the rapid, circling, vivacious strain of the purple
finch, the gentle lullaby of the song sparrow, the pleasing wichery,
wichery of the yellow-throat, the clear whistle of the oriole, the
loud call of the high-hole, the squeak and chatter of swallows, etc.
But when the lark did rise in full song, it was easy to hear him
athwart all these various sounds, first, because of the sense of
altitude his strain had,—its skyward character,—and then because of

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Mixedphase Clouds Observations And Modeling Constantin Andronache

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