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MIXED-PHASE
CLOUDS
Observations and Modeling
Edited by
CONSTANTIN ANDRONACHE

Elsevier
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Notices
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CONTRIBUTORS
Andrew S. Ackerman
National Aeronautics and Space Administration, Goddard Institute for Space Studies, New York,
NY, United States
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
Colorado State University, Fort Collins, CO, United States
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.
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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

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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
Mixed-Phase Clouds © 2018 Elsevier Inc.
https://doi.org/10.1016/B978-0-12-810549-8.00002-7 All rights reserved.

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

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115
together beautiful. It has been handed down to us
through countless ages, a living flame of action and life,
a gypsy dance. This is beauty. This is life. I hope you
may forget me and know only this marvel of beauty and
truth, sweetness and light.”
And now, under the ruthless hand of Eve, she saw her
thing of beauty torn apart and pieced with fragments of
bold movements and discordant notes which made her
dances much more brazen.
But that was not all. “Your toes,” decreed the merciless,
dark-faced director, “are too limber; your legs are too
stiff. You must look to the brass rail for remedy.”
“The brass rail?” She did not say the words. Soon
enough she found out. In a cold back room she stood
for half an hour, gripping a long brass rail safely
anchored some three feet from the floor, twisting her
toes and bending her poor limbs until she could have
screamed with pain. It helped not a bit that a dozen
members of the chorus, who never spoke a word to her,
were going through the same painful performance.
She uttered wailing complaints to Angelo in his studio
that night. Angelo passed the complaint on to the
poker-faced manager.
“If you wish to direct your play,” this dictator decreed,
“you may do so, provided,” he prodded Angelo in the
ribs until it hurt, “provided you are able and willing also
to finance it.”
“It’s a hard life, my child,” Dan Baker said to Jeanne the
next night, as the light of the fire played on his weary
old face. “You think the brass rail is terrible. But think of
me. They have put me in a gymnasium for an hour each

116
117
day, where a Samson of a chap uses me for a dumbbell,
an Indian club and a punching bag.”
Jeanne laughed at his description and felt better.
“They’re spoiling your dance, little girl,” he said in a
more serious tone. “But never mind. Do your old dance
in the old way here in this room or in the park, just as
you were doing it when I first saw you. Keep it full of
freshness, life and beauty, stretch it to fill the time, and
when we open,” his voice died to a whisper, “on our
great first night, dance your gypsy dance just as you
learned it back there in France, and I promise you that
all will be more than well.”
Petite Jeanne caught her breath. Here was a bold
proposal. Would she dare?
Springing to her feet, she went swinging away in a wild
whirl. When she dropped back in her place before the
fire, she whispered hoarsely,
“I will!”
Her strong young hand met his in a grip that was a
pledge.
But were these things to be? Even as she lay there
blinking at the fire, some imp of darkness seemed to
whisper, “You will never do it. You never will.”
She looked at the Fire God resting at the edge of the
flames, and thought she saw him frown.

118
CHAPTER XIV
THE FLUTTER OF WINGS
Petite Jeanne was a gifted person. She was a dancer of
uncommon ability. Those who studied her closely and
who were possessed of eyes that truly saw things had
pronounced her a genius. Yet she was possessed of an
even greater gift; she knew the art of making friends.
Defeated by an ancient unwritten law, in her attempt to
be a friend to the girls of the chorus, she had found her
friends among the lowly ones of the theatre. For with all
her art she never lost the human touch.
She had not haunted the ratty old theatre long before
Mary, the woman who dusted seats, Jimmie, the
spotlight operator, Tom, the stoker who came up grimy
from the furnaces, and Dave, the aged night watchman,
one and all, were her friends.
That was why, on special occasions, these people did
exactly what she wanted. One night at the ghostly hour
of eleven she found herself, bare-footed and clad in
scanty attire, doing her dance upon the stage while
Jimmie, grinning in his perch far aloft, sent a mellow
spot of light down to encircle and caress her as a beam
of sunshine or a vapory angel might have done.

119
Dave, the watchman and her faithful guardian, was not
far away. So, for the moment, she knew no fear. The
rancorous voice of the director, the low grumble of the
manager, were absent. Now she might dance as nature
and the gypsies had taught her, with joy and abandon.
Since she had fully decided that on the night of nights,
when for the first time in months the old Blackmoore
was thronged, she would take matters into her own
hands and dance as God, the stars and all out-doors
had taught her, and feeling that only practice on the
stage itself would give her heart the courage and her
brain the assurance needed for that eventful hour, she
had bribed these friends to assist her. And here she
was.
Dance on this night she did. Jimmie watched and
marveled. Such grace and simple, joyous abandon, such
true melody of movement, such color in motion, he had
not known before.
“Ah!” he whispered. “She is possessed! The gypsies
have bewitched her! She will never be real again.”
Indeed, had she given one wild leap in the air and risen
higher and higher until she vanished into thin darkness
as a ghost or an angel, he would have experienced no
astonishment.
Surprise came to him soon enough, for all that.
Suddenly the fairy-like arms of the dancer fell to her
sides. Her lithe body became a statue. And there she
stood in that circle of light, rigid, motionless, listening.
Then, throwing her arms high in a gesture of petition,
she cried,

120
121
“Jimmie! The flutter of wings! Can you hear them? How
they frighten me!
“Jimmie,” she implored, “don’t let the spotlight leave
me! Can you hear them, Jimmie? Wings. Fluttering
wings. They mean death! Do you hear them, Jimmie?”
Leaning far forward, Jimmie heard no wings. But in that
stillness he fancied he heard the mad beating of the
little French girl’s heart, or was it his own?
So, for one tense moment, they remained in their
separate places, motionless.
Then, with a little shudder, the girl shook herself free
from the terror and called more cheerily,
“There! They are gone now, the wings. Throw on a
light, and come and take me home, Jimmie. I can dance
no more to-night.”
As she turned to move toward the spot on the floor
where her precious God of Fire stood leering at her, she
seemed to catch a sound of furtive movement among
the shadows. She could not be sure. Her heart leapt,
and was still.
Five minutes later she and Jimmie were on a brightly
lighted street.
“Wings,” the little French girl murmured once more.
“The flutter of wings!” And again, as they neared her
home, “Wings.”
“Aw, forget it!” Jimmie muttered.

122
She was not to forget. She was to hear that flutter
again, and yet again.

123
CHAPTER XV
A TOUCH IN THE DARK
During all these busy days Petite Jeanne did not entirely
lose track of her friend Merry of the smiling Irish eyes.
Being endowed with a particularly friendly nature, she
was more than glad to find friends outside the little
circle in which she moved. Besides, she was deeply
grateful to the little girl who had led her to the place
where she had, in so miraculous a manner, purchased
the priceless Fire God for only three silver coins.
“It was the beginning of all my good fortune,” she said
to Merry on one occasion. “And,” she added quickly, “all
my very hard work as well.”
So it happened more than once that she took the
elevated train to the office where the auction sale of
unclaimed, and damaged express packages was held
every Friday. There she sat in the front row beside
Merry and enjoyed two hours of relaxation. The endless
variety of goods on sale, from a baby buggy without
wheels to a black and white puppy with an enticing
bark, intrigued her more and more; particularly the
“union,” Merry’s little circle of choice friends.

124
125
To a casual observer these men would have seemed a
rough lot. Soon enough Jeanne, with her power of
looking into men’s hearts, learned that these men who
struggled daily for their bread had been endowed by
nature with hearts of gold.
Their interest in Merry was of a fatherly and sportsman-
like sort. Knowing her brother and his handicaps they
were glad to help her.
Unfortunately, at this time there was little they could do
for her. Each Friday she brought a smaller purse and
carried fewer articles away. The little basement shop,
where Tad toiled incessantly, was feeling the pinch of
hard times. Few were the visitors that came down the
cellar stairs these days, and fewer still were the
purchases they carried away. Only when the blue eyes
of the girl spied some article for which she had an
immediate sale did she venture a bid.
More than once when some particular member of the
“union” had made a fortunate purchase and met with an
immediate sale, he offered Merry a loan. Always the
answer was the same: a loyal Irish smile and, “Thanks.
You’ll be needing it next time.”
Little wonder that Petite Jeanne, sitting in the glowing
light of such glorious friendships, absorbed warmth that
carried her undaunted through rehearsals amid the cold
and forbidding circle within the old Blackmoore walls.
It was on one of these visits to the auction house that
the little French girl received an invitation to an unusual
party.
Weston, the ruddy-faced German who kept a shop near
Maxwell street, together with Kay King and a stout man

126
known by the name of John, had bid in a large number
of traveling bags and trunks. They were an unusual lot,
these bags and boxes. Many of the trunks were
plastered from end to end with foreign labels. Three of
the bags, all exactly alike, were of the sort carried only
by men of some importance who reside in the British
Isles.
“How I’d love to see what’s in them!” Jeanne exclaimed.
“Do you want to know?” Weston demanded. “Then I’ll
tell you. Junk! That’s all. I buy only junk. Inside these
are some suits. Moths eat holes in them. Silk dresses,
maybe; all mildewed.”
“Must be fun to open them, though. You never can tell
what you might find.”
“Ja, you can never tell,” Weston agreed.
“Do you want to see what’s in them?” Kay King, who
was young and good looking, leaned forward. “Come
down to Maxwell Street on Sunday. We’ll save them
until then, won’t we?” He appealed to his companions.
“Ja, sure!”
“Sure we will!”
Petite Jeanne turned to Merry. “Will you go?” she asked,
suddenly grown timid.
“Yes, I’d like to,” Merry assented quickly. “I’ve never
seen their shops. I’d love to.”
“All right,” Jeanne said with a smile. “We’ll come. And
perhaps we’ll bring some friends.”

127
“Ja, bring friends. As many as you like. Mebby we could
perhaps sell them some suitcases?”
Kay King gave Jeanne his card. And there, for the time,
the matter rested. But Jeanne did not allow it to escape
her memory. It was to be, she told herself, one of the
strangest and most interesting opening-up parties it had
been her privilege to attend.
That night Petite Jeanne once more danced alone
beneath the yellow glow of Jimmie’s spotlight. The affair
of two nights before had frightened her more than she
cared to admit. But this little French girl possessed an
indomitable spirit. She knew what she wanted; knew
quite as well why she wanted it, and was resolved that,
come what might, she should have it.
On this particular night she would gladly have taken her
strong and fearless companion, Florence, with her to
the theatre. But Florence had come upon a bit of good
fortune; she had been employed to conduct classes in a
settlement house gymnasium two hours each evening.
“That,” she had exclaimed joyously, “means bread and
butter!”
So Petite Jeanne had come alone. And why not? Was
not Jimmie over there in the balcony? And was not her
friend, the night watchman, somewhere in the building?
“What of the gypsy who would steal your god if he
might?” Florence had asked.
“Well, what of him?” Jeanne had demanded. “We
haven’t seen him prowling about, have we? Given up,
and gone south. That’s what I think. In New Orleans by
this time.”

128
129
Long ere this, as you will recall, Jeanne had resolved
what she should do on the opening night. When the
curtain rose for her first big scene, when she received
the cue to begin her dance, she would make it her
dance indeed. At that moment, before the throng of
first-nighters, she would defy the tyrannical director. She
would forget the steps they had taught her. Before the
gypsy campfire she would become a gypsy once again
and dance, as never before, that native dance to the
Fire God. Bihari, the gypsy, had taught her that dance,
and there was nothing like it in all the world, she felt
sure.
It was a daring resolve and might, she knew, result in
disaster. Yet the very daring of it inspired her. And why
not? Was she not after all, in spirit at least, a gypsy, a
free soul unhampered by the shams and fake pretenses,
the senseless conventions of a city’s life?
With this in mind, she danced in the dark theatre with
utter abandon. Forgetting all but the little Fire God
whose tiny eyes glowed at the rim of the yellow circle of
light, she danced as she had many times by the
roadsides of France.
She had reached the very zenith of the wild whirl. It
seemed to Jimmie that she would surely leave the floor
and soar aloft, when suddenly he became conscious
that all was not well. He read it in her face. She did not
stop dancing. She did not so much as speak; yet her lips
formed words and Jimmie read them:
“Wings, fluttering of wings!”
“A plague on the wings!” exclaimed Jimmie, as his
muscles stiffened in readiness for an emergency.

Wings! Did he hear them? He could not be sure. He
would see what he could see!
He touched a button and a light flashed brightly from a
white globe aloft.
His keen eyes searched the place in vain. Yet sixty
seconds had not elapsed before there came the sound
of a slight impact, followed by a terrific crash. The light
above blinked out.
In his excitement, Jimmie threw off the spotlight and
the theatre beneath him became a well of darkness.
And what of Jeanne? When the crash came her dance
ended. When the spotlight blinked out she sprang back
in terror. At that instant something touched her ankle.
With a little cry of fright, she bounded forward. Her foot
came in contact with some solid object and sent it
spinning.

130
131
“The Fire God!” she thought in consternation. “I have
kicked him across the stage.”
Then the house lights flashed on, and all was light as
day.
Flashing a quick look about the stage, the girl found
everything as it had been, except that the Fire God was
standing on his head in a corner, and half way down the
center aisle was a pile of shattered glass. This glass
had, a moment before, been the white globe aloft.
“Jimmie!” she called. “It’s all right. The globe fell, that’s
all.”
“Must have been loose,” Jimmie grumbled. “Good thing
it fell now. Might have killed somebody.”
But Jeanne was sure it had not been loose. She had not
forgotten that flutter of wings.
“Some one,” she told herself, “is trying to frighten me.
But I shan’t be frightened.”
At that she walked to the corner of the stage, took up
her Fire God, slipped on her coat and prepared to go
home.
“Jimmie,” she called, loud enough for anyone who might
be hiding in the place to hear, “that’s all for to-night. But
come again day after to-morrow. What do you say?”
“O. K.,” Jimmie shouted back.
Jeanne was to regret this rashness, if rashness it might
be called.

132
133
CHAPTER XVI
THE BATTLE OF MAXWELL
STREET
“But what is it?” Petite Jeanne stepped back, half in
terror, as she gripped Florence’s arm and stared about
her.
They had just alighted from a Halsted Street car and
had entered the maze of booths, carts, rough board
counters, and wagons. “This is Maxwell Street on a
bright Sunday afternoon in late autumn,” replied Merry
with a smile.
They were on their way, Petite Jeanne and Merry, to the
promised party at which many mysterious bags and
trunks were to be opened. Florence was with them; so,
too, was Angelo. Dan Baker also had agreed to come at
the last moment. So they were quite a party, five in all.
About these portable stores swarmed a motley throng.
Some were white, some brown, some black. All, stall
keepers and prospective purchasers alike were poor, if
one were to judge by attire.
“Don’t be afraid,” Merry smiled at the little French girl.
“These are harmless, kindly people. They are poor, to be

134
sure. But in this world, ninety out of every hundred are
poor and probably always will be.
“Some of these people have a few poor things to sell.
The others hope to purchase them at a bargain; which
indeed they often do.
“So you see,” she ended, “like other places in the world,
Maxwell Street deserves its place in the sun, for it
serves the poor of this great city. What could be
nobler?”
“Ah, yes, What could be nobler?” the little French girl
echoed.
“How strange!” she murmured as they walked along.
“There is no order here. See! There are shoes. Here are
cabbages. And here are more shoes. There are
chickens. Here are more shoes. And yonder are
stockings to go with the shoes. How very queer.”
“Yes,” Florence sighed, “there is no order in the minds
of the very poor. Perhaps that is why they are poor.”
“Come!” Merry cried impatiently. “We must find the
shops of our friends. They are on Peoria Street. Two
blocks up.”
“Lead the way.” Petite Jeanne motioned her friends to
follow.
As they wedged their way through the throng, Petite
Jeanne found her spirits drooping. “How sad it all
seems!” she thought to herself. “There is a little dried up
old lady. She must be eighty. She’s trying to sell a few
lemons. And here is a slip of a girl. How pinched her

135
136
face is! She’s watching over a few wretched stockings. If
you whistled through them they’d go into rags.
“And yet,” she was ready to smile again, “they all seem
cheerful.”
She had said this last aloud. “Yes,” Merry answered,
“cheerful and kind. Very considerate of one another. It is
as if suffering, hunger, rags, disease, brought friends
who cannot be bought with gold.”
“It is true. And such a beautiful truth. I—”
Petite Jeanne broke short off, then dodged quickly to
one side. She had barely escaped being run down by an
automobile. Coming in from behind, the driver had not
honked his horn.
The man was large. The companion at his side was
large. The bright blue car was large. The whole outfit
fairly oozed comfort, riches and self-satisfaction.
“Stand gawking around and you’ll get a leg taken off!”
The driver’s voice was harsh, unkind. He spoke to the
little French girl.
The hot fire that smouldered behind Angelo’s dark eyes
blazed forth.
“What are you doing here, anyway?” he demanded in a
fury. “Running people down! Crowding them about! You
with your big car! If you want to gaze, why don’t you
walk as we do?”
The car came to a halt. A deep flush had spread over
the driver’s face. Springing from the car, he launched a

137
blow that sent the slight Italian youth spinning into the
crowd behind him.
But what was this? Hardly had the man swayed back, a
leer of satisfaction on his face, than a whirling catapult
launched itself upon him. A circle of steel closed about
his neck. He found himself whirling through space. He
landed with a mighty clatter atop a pile of frying pans
and stew kettles.
Quickly scrambling to his feet, he glowered at the
gathering throng as he demanded,
“Who did that?”
For the count of ten, no one answered. Then a scrawny
little Irishman, who wore a Cross of Honor on his
ragged jacket, pushed Florence forward as he whispered
hoarsely,
“Tell ’im, Miss. I’m wid y’. Me, as never lost a battle yet.”
“I did!” The girl’s words were clear and quite distinct.
A hush fell over the thickening crowd. A fight on
Maxwell Street is always an occasion. But a fight
between a prosperous man and a good looking girl!
Who had seen this before?
Florence, as you will recall, was not one of those
weaklings who subsist on pickles and ice-cream in order
to develop a slender figure. She weighed one hundred
and sixty, was an athletic instructor, knew a few tricks
and was hard as a rock.
There was no fight. The man looked her up and down.
Then he called her a name. It was a nasty name,

138
seldom heard on Maxwell Street. For the people there,
though poor, are a gentle folk.
Then Maxwell Street, slow going, gentle, kindly,
poverty-stricken Maxwell Street, went mad. Who threw
the first ripe tomato that struck this prosperous insulter
squarely on the jaw? No one will ever know. Enough
that it was thrown. It was followed quickly by a bushel
more, and after that by a cart load of over-ripe fish.
When at last the irate but badly beaten man of
importance turned his car southward and fled from
Maxwell Street, his beautiful car was no longer blue. It
was tomato-pink and fish-yellow. And his costume
matched the car.
Then Maxwell Street indulged in a good laugh. In this
laugh Angelo did not join. He divided his attention
between the business of nursing his swollen jaw and
paying the poor venders of tomatoes and fish for their
missing wares.
“Some people,” he might have been heard to grumble to
himself, “talk too much.”
“The battle of Maxwell Street!” exclaimed Merry at his
elbow. Her eyes shone. “And we won!”
“I am sure of it!” Angelo agreed heartily. “However, I am
out four dollars and sixty-five cents for fish and
tomatoes.”
“But look!” Merry pointed to the battered little Irishman
with the Cross of Honor. “He is taking up a collection.
You will be paid.”

139
140
“No, no! That cannot be!” True distress was in the
Italian boy’s eyes. “Stop him.”
“No. We must not!” Merry’s tone was tense with
emotion. “You are their hero. You stood up for their
rights. Would you be so mean as to rob them of the
right to do homage to their hero?”
“Ah, me!” Angelo rubbed his eyes. “This is a very
strange world.”
In the end he departed with a heavy sack of nickels and
pennies, while the crowd shouted their approval of the
“brave little Dago.” And for once Angelo did not hate
this name they had given his people.
They had gone another block before Angelo spoke
again. What he said both puzzled and troubled the little
French girl. “That whole affair,” he said quietly, “was a
faux pas.”
“How could it be!” she exclaimed. “I thought it quite
wonderful. What right have those big, bluffing bullies to
run down poor people on Maxwell Street?”
“None at all,” Angelo replied soberly. “But after all, the
battle of Maxwell Street is not our battle. This is a large
city. Yet it is strange the way we meet the same people
again and again. If that man really comes upon me in
some other place, if he finds out what I do and where I
live, he will do his best to ruin me. That is the way of
his kind.”
Little did Angelo guess the manner in which his
prophecy was to come true, much less the manner of
vengeance that would be employed.

141
Petite Jeanne remained silent for a moment. Then she
gave Angelo’s arm an affectionate squeeze as she
answered: “I shall pray every night that he may never
see you even once again.”

142
CHAPTER XVII
TRAVELING MYSTERIES
Even to Merry, who had never before visited her friends
on Peoria Street just off Maxwell Street, the shop of
Weston was something of a shock. It was nothing more
than a hollow shell of a building with a great heap of
second-hand goods of all sorts piled in one corner. Not a
shelf, counter or table adorned this bleak interior. The
plaster was cracked, the walls threatening to fall.
“I sell all in the street,” he explained in answer to their
looks of astonishment. With a wave of his hand he
indicated rough board counters where a miscellaneous
assortment of human beings were pawing over a stock
in trade as varied as themselves.
Now and again one would hold up an article in one
hand, a coin in the other, and a bargain was speedily
made.
“I don’t see how he lives,” Petite Jeanne whispered.
“He’s been doing this for twenty years, and he’s not
bankrupt yet,” Merry whispered back.
They were led next to the shop of Kay King. This
boasted of some little magnificence. There were shelves

143
and tables and one glass showcase. Since his principal
stock was composed of second-hand books, the wall
was lined with them.
“A curious place for a book store, this Maxwell Street,”
Dan Baker mused.
“I don’t do so badly,” Kay King smiled. “The poor wish to
read. And here for a nickel, a dime, a quarter, I sell
them a lamp to their feet, a light to their pathway.”
“Truly a missionary enterprise in a city wilderness,” the
gentle old man murmured.
As for Petite Jeanne, her eyes had roamed up and down
the dusty rows of books and had come to rest at last
upon a badly hung pair of portieres at the back of the
room.
“That,” she told herself, “is where he sleeps when the
day is done, a dark and dingy hole.
“And yet,” she mused, “who can help admiring him?
Here in his dingy little world he is master of his own
destiny. While others who sell books march down each
morning to punch a clock and remain bowing and
scraping, saying ‘Yes mam’ this and ‘Yes mam’ that to
females who think themselves superior beings, he
moves happily among his own books selling when and
as he chooses.”
Her reflections were broken off by a word from Kay King
himself.
“There’s a story in every one.” He nodded toward the
row of trunks and bags they had come to inspect.

144
145
“Little does one dream as he packs his trunk for a
journey that he may never see that trunk again. Sad as
it may seem, this is often the case.
“So, all unconscious of curious prying eyes, we tuck the
very stories of our lives away in our trunks and watch
them go speeding away in a motor van.”
“How?” Petite Jeanne asked.
“How? Look at this. Here is one I purchased some time
ago.” He swung a large, strongly built wardrobe trunk
about, threw it open and produced a bundle of letters.
“This,” he explained, “is a young man. These letters are
from his mother. And these,” he produced another
packet, “are from other women. Still others are from his
pals. They tell his story. And what a story! Bright, well
educated, from a good family. But oh, such a rotter! He
betrays his employer, his sweetheart, his pals. He
deludes his trusting mother. And, how he lies to her!
“It is all written here.” He patted the letters.
“I had a letter from him yesterday,” he continued. “He
wants the trunk; says it is a treasure and an heirloom;
wants the contents, too; says sentiment makes him
treasure these things. Sentiment!” He fairly stormed.
“He knows but one emotion! He loves; ah yes, he loves
himself supremely! He has not a redeeming trait.
“He wants this trunk because he is afraid. Afraid of me!”
His laugh was bitter. “Me! I never hurt a flea. I only wish
I could; that I were hard and ruthless as some men are,
stamping their way through, trampling over others to
fortune!

146
“But he shall pay,” he went on more calmly after a
moment. “I mean to charge him twenty dollars.
“Then,” he smiled, “I shall return this one to its owners
free.” He placed a hand on a sturdy little army locker.
“This one belongs to a little family. How many trunks
do! Father, mother and the little ones, all their clothes in
one trunk! And then lost!
“There should be a society for the return of lost
baggage to poor people.
“There are many like these. People come to a strange
city for work. There is no work. They leave their trunks
in the depot. Storage piles up. They cannot pay.
“But this must bore you!”
“No, no! Please go on.”
“There is not much more to tell. See!” He lifted the lid of
the trunk. “Everything is spotlessly clean. A man’s shirts,
a woman’s house dresses, little frocks and rompers for
two tiny girls. Poor folks they are, like you and me. He
was a soldier, too. There is a sharp-shooter’s medal on a
pin cushion. There’s a child’s birth certificate, a doll with
its nose kissed white, and a small Bible. They lost all
that.
“And I—I shall send it back.”
“They will pay you,” said Petite Jeanne.
“They will not pay. They cannot. Some are always poor.
These are like that.

147
148
“But this one—” His lips curled in sudden scorn. “This
big boy who goes strutting through the world, he shall
pay, and I shall pass it on to these who need and
perhaps deserve it.
“But I am keeping you here!” he cried. “Here are the
trunks we have saved for your own eyes. You will see
that Weston has spoken truthfully. They are filled for the
most part with junk. But now and then there is a story,
a real story of some romantic life. See, this one opens
easily. I have found a key for it.”
“Wait!” On Jeanne’s face was a look almost of distress.
“You have told me so much. It seems so cruel that we
should pry into their lives. It—it’s like coming upon
people in the dark. I—I’m afraid. I—”
“Oh, come!” he laughed. “It’s not half as bad as that.
Probably we won’t come upon anything of interest at all.
Indeed that’s almost sure to be the case, and I am
inclined to repent inviting you here.” So saying, he lifted
the lid of the first of the row of trunks, and the show
began.

149
CHAPTER XVIII
BAGS OF FORBIDDEN TREASURE
Weston’s prophesy that the trunks contained “only junk”
proved to be true. As trunk after trunk was opened,
their search for hidden treasure continued to be
unrewarded. Always there was the suggestion of
pinching poverty, carelessness and neglect. These
trunks were lost to their owners because they had not
the ready money to pay the charges. One need not say
that such as these have few valuable treasures to pack
in a trunk.
The air of the small shop grew heavy with the odor of
soiled clothing, cheap, highly scented soap and spilled
talcum powder. The ladies had given up the search and
were wandering about, looking at books, when the
searching party came at last upon the three large
pigskin bags from the British Isles.
“There is something to intrigue you!” exclaimed Angelo.
“And see! They are all tightly locked.”
Kay King’s eyes shone. He had bid in these bags at a
rather high figure. He was hoping that his judgment
regarding their contents had been correct.

150
“Let me try these.” He rattled a huge bunch of keys. Not
one of them would open the bags. “Oh well,” he smiled,
“one may pick his own locks.” With skill born of ripe
experience he opened the locks with a bit of twisted
wire.
“Now!” He breathed deeply. “Now!”
They all crowded around. A wide-mouthed bag flew
open, revealing its contents. At once an exclamation
was on every lip. Not one of them all but knew on the
instant that Kay had made an exceedingly good buy.
The bag was packed to the very top with the choicest of
wearing apparel. Indeed, not one of them all had worn
such rich garments. A man’s outfit included shirts of
finest silk and softest woolens, suits of broadcloth and
shoes of rarest quality.
The second bag, though varying somewhat in its
contents, matched the first in quality.
It was the third bag that set them gasping. For in this
one the owner had packed with tender care the articles
dearest to his heart. An ivory toilet set mounted with
gold, a costly present from some dear friend; a brace of
gold-mounted pistols; fountain pens; paper knives,
elaborately carved; an astonishing collection of rare
articles. And at one side, carefully wrapped in a
swathing of silk, were three oval frames of beaten gold.
Petite Jeanne’s fingers trembled as she unwrapped them
and revealed, one after another, the portraits of a
beautiful lady, a handsome boy and a marvelous girl, all
dimples and golden hair.
“Oh!” She breathed deeply and the breath was half a
sob.

151
152
More was to come. Having taken up an unframed
picture, she studied it for a space of seconds. Then, as
her trembling fingers let the picture fall, her slender
form stiffened and her face went white as she said in
words that seemed to choke her:
“You can’t sell these things. You truly can’t.”
“Why can’t I?” Kay challenged. He had not looked into
her white face.
“Because—” She put out a hand to steady herself.
“Because they belong to a friend of mine. That is he,”
she said, holding up the picture, “and that,” pointing to
a signature at the bottom, “is his name.
“He—he came over on the boat with me. He—he was
very, very kind to me. Helped me over the hard places.
“To sell out these would be a sacrilege.
“Sell them to me!” she pleaded, laying a hand on Kay’s
arm. “I’ll pay you twice what you gave for them. Please,
please do!” She was all but in tears.
She could not know the bargain she appeared anxious
to drive. Only Weston and Kay King knew. They knew
that in all their pinched and poverty-stricken lives they
had never before made such a find; that the bags and
their contents were worth not twice but ten times what
Kay had paid for them.
And only Angelo, who had accidentally caught sight of
her bankbook, knew that for the sake of a friend she
had known only on a short voyage, she was willing to
spend her all.

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