Class lectures on Hydrology by Rabindra Ranjan Saha Lecture 2

1
Presentation -2
Relative Humidity
Relative Humidity (f) is the amount of moisture in the air to the
amount needed to saturate the air at the same temperature:
Relative Humidity (f) = (ed / ea ) x 100
Example:1:
The actual air temp. 300 and the dew point temp. 200 C. Find out
the relative humidity and saturation deficit.
Solution:
Given, Ta = 300 C and Td = 200 C
From the graph
at temp.Ta = 300C, ea =42.5 mb, at temTd = 200C , ed =23 mb
f = (ed /ea ) x 100 = (23 / 42.50 ) x 100 = 54 %
Saturation deficit = (ea — ed) = (42.5 – 23) = 19.50 mb

2
Presentation-2(contd.)
-10 0 10
ea
ed
ew
ei
5040302010
Vaporpressure(mb)
0 10 Td 20 Ta 30 40
Temperature
(mb)8070605040302010

3
1.8.5 Absolute Humidity
Absolute Humidity (ρw ) : The ratio between the mass of water vapor
per unit volume of air at a given temperature and is equivalent to the
vapor density is called absolute humidity.
Thus if a volume V (m3)of air contains mw (g) of water vapor ,
then
Absolute Humidity (ρw ) = {Mass of water vapor (g) }/{Volume of air (m3)}
ρw = mw / V (gm-3)

4
Specific Humidity
Specific Humidity (q): The ratio of the mass of water vapor
(mwg) to the mass of moist air (in kg) in a given volume is
called specific humidity.
This is the same as relating the absolute humidity (gm-3) to the
density of the same volume of unsaturated air (ρ in kg m-3):
q = mw (g)/ ( mw + md ) (kg) = ρw /ρ (g kg-1)
where,
md is the mass of the dry air in kg
mw is the mass of water vapor in g

5
Precipitable water
Precipitable water: The total amount of water vapor in a
column of air expressed as the depth of liquid water in
millimeters over the base area of the column Figure 1. The
precipitable water gives an estimate of maximum possible
rainfall
In a column of unit cross sectional area , a small
thickness, dz, of moist air contains a mass of water
given by :
d mw = ρw x dz

6
Thus , in a column of air from heights
z1 to z2 corresponds pressure p1 to p2
z2
the total mass of water mw = ∫ ρw dz
z1
Also, dp = - ρg dz
dz = - dp / pg
Thus : p2
the total mass of water , mw = ∫ ρw dz
p1
z1
z2P2
p1
Columnofair(mw)
Figure-1

7
Thus : p2
the total mass of water , mw = ∫ ρw dz
p1
p2 p2
mw = ∫ ρw / ρg (- dρ) = - ∫ (ρw /ρg) dp
p1 p1
p1
= (1/g) ∫(q )dp
p2
Converting mass water (mw) into equivalent depth over a unit
cross sectional area, the precipitable water is given by
P1
W (mm) = (0.1/ g) ∫ q dp
p2
where, p is in mb, q = ρw /ρg in g kg -1 and g = 9.81 ms-2

8
In practice it is not possible to integrate until q is not
known as function of p.
However a value of W is obtained by summing up the
contribution for a sequence of layers in the troposphere
from a series of measurement of the specific humidity (q)
of air at different heights and using the average specific
humidity q over each layer with the appropriate pressure
difference:
p1
W(mm) = (0.1/ g) ∑ q ∆p
p2

9
Radiosonde
Radiosonde is a battery-powered telemetry instrument carried into the
atmosphere usually by a weather balloon that measures various
atmospheric parameters and transmits them by radio to a ground receiver.
Modern radiosondes measure or calculate the following variables: altitude,
pressure, temperature, relative humidity, wind (both wind speed and wind
direction), cosmic ray (cosmic rays are high–energy radiation, mainly
originating outside the Solar System and even from distant galaxies.
Upon impact with the Earth’s atmosphere, cosmic rays can produce
showers of secondary particles that sometimes reach the surface)
readings at high altitude and geographical position (latitude/ longitude).
Radiosondes measuring ozone concentration are known as
ozonesondes

10
Example:1-2 The following aerological observations shown
in Data table below were taken from a radiosonde ascent. If
60% of the precipitable water is produced from the air up to the
600 mb level to form precipitation at ground level, what would
be the depth of the rainfall?
Pressure(p)-
( mb)
1006 920 800 740 700 660 600 500 400
Specific
humidity
(gkg-1)
14.0 13.4 10.2 9.4 7.2 6.6 5.6 4.0 1.8
Data Table

11
We know total precipitable water, W
p1
W(mm) = (0.1 /g) ∑ q ∆p
p2
Solution
Given
Pressure and specific humidity Data in the table.
Precibitable water is produced from the air up to the 600 mb
level to form precipitation at ground level,
To be calculated depth of rainfall if 60% of the precipitable
water forms rainfall.
Pressure calculation at different height
∆p1 = 1006-920 = 86 mb
∆p2 = 920-800 = 120 mb
and so on.......

12
Pressure(p)-
( mb)
1006 920 800 740 700 660 600 500 400
Specific
humidity
(gkg-1)
14.0 13.4 10.2 9.4 7.2 6.6 5.6 4.0 1.8
∆p 86 120 60 40 40 60 100 100 -
q (mean) 13.70 11.80 9.80 8.3 6.90 6.10 4.80 2.90 -
q(mean)*∆p 1178.2 1416 588 332.0 276.0 366 480.0 290 -
Similarly, mean specific humidity
q1 (mean) = (14 + 13.4)/2 = 13.70 gkg-1
q2 (mean) = (13.4+10.2)/2 = 11.80 gkg-1) and so on
Putting the respective values in the calculation table.
Calculation table

13
Total q (mean) * ∆p
p1
∑ q ∆p = 1178.2+1416.0+588.0 + 332.0 + 276.0 + 366.0
p2
= 4156.2
Total the precipitable water up to 600 mb level
p1
W(mm) = (0.1/ g) ∑ q (mean)* ∆p = 4156.2 * 0.1) / g = 42.36 mm
p2
As given 60% of the precipitable water only will produce
rainfall, therefore, depth of rainfall will be = 42.36 mm * 60%
= 25.4 mm .

14
Precipitation
All forms of water that reaches the earth from the atmosphere.
The characteristics for the formation of precipitation are :
1. the atmosphere must have moisture
2. there must be sufficient nuclei present to aid condensation
3. weather conditions must be favorable for condensation of
water vapor to take place.
4. the products of condensation must reach the earth .
Presentation 2 (contd.)
1.rainfall
2. snowfall
5.hail
6.sleet
3.drizzle
4.glaze
The usual forms of precipitation

15
1.Rainfall:
The precipitation in the form of water drops of sizes larger
than 0.50 mm per unit time is rainfall .
The maximum size of rain drop is about 6 mm. On the
basis of its intensity rainfall is classified as the following:
Sl.No Type Intensity
1 Light rain trace to 2.50 mm/hr
2 Moderate rain 2.5 mm /hr to 7.5 mm/hr
3 Heavy rain > 7.5 mm/hr

16
2. Snow: Another form of precipitation
consists of ice crystals which usually
combine to form flake (small piece of
something). When new, snow has an
initial density varying from 0.06 to 0.15
g/cm3 and it is usual to assume an
average density of 0.1 g/cm3
3. Drizzle
A fine sprinkle of numerous water
droplets of size has less than 0.50
mm and intensity less than 1 mm /hr
is known as drizzle. The rain drops
are so small that they float in the air.

17
4. Glaze
When rain or drizzle
comes in contact with cold
ground at around 00 C, the
water drops freeze to form
an ice coating called glaze
or freezing rain.
5. Sleet
It is frozen rain drops of
transparent grains which form
when rain falls through air at
sub freezing temperature. In
Britain sleet means
precipitation of snow and rain
simultaneously.
6. Hail
It is a showery precipitation in the
form of irregular pellets or lumps
of ice as size more than 6 mm.
Hails occur in violent thunder
storms in which vertical currents
are very strong.

18
WEATHER SYSTEM
Precipitation
The moist air masses cool to form condensation and form
nuclei after that fall on the surface of the earth is called
precipitation.
Types of precipitation
5types
(1) Frontal
(2) Cyclone (4) Convective
(3)Anti Cyclone
(5) Orographic
(i) Tropical
(ii)Extra tropical

19
(1) Frontal : A front is the interface between two distinct
air masses. Under certain favorable conditions when an air
mass and cold air mass meet, the warmer air mass is lifted
over the colder one with the formation of a front. The
ascending warmer air cools with the consequent formation
of clouds and precipitation.
(2) Cyclone : A cyclone is a large low
pressure region with circular wind motion.
Cyclone is of two types:

20
(ii) Extra tropical cyclones:
These cyclones are formed in
locations outside the tropical
zone. They posses a strong
counter clockwise wind
circulation in the northern
hemisphere. The magnitude of
the precipitation and wind
velocities are relatively lower
than those of tropical cyclone.
(i) Tropical cyclone – Tropical
cyclone is a wind system with
an intensely strong depression
with MSL pressure sometimes
below 915 mbars. A tropical
cyclone is also called cyclone in
India and Bangladesh, hurricane
in USA and typhoon in South
Asia.
Types of cyclone- Two types

21
(3) Anti cyclone: These are
regions of high pressure,
usually of large areal extent.
The weather is usually calm
at the centre. Anticyclones
cause clockwise wind
circulations in the northern
hemisphere. Winds are of
moderate
(5) Orographic precipitation : The moist air mass
may get lifted up to higher due to the presence of
mountain barriers and consequently undergo
cooling, condensation and precipitation. Such a
precipitation is called orographic precipitation.
4.Convective precipitation: It is
caused by the rising of warmer,
lighter air in colder, denser
surroundings. The difference in
temperature may result from
unequal heating at the surface,
unequal cooling at the top of the
air layer , or mechanical lifting
when the air is forced to pass
over denser, colder air mass or
over a mountain barrier.

22
Measurement of precipitation
Precipitation is expressed in terms of the depth to which
rainfall water would stand on an area if all the rain were
collected on it. Thus 1 cm of rainfall over a catchment area
of 1 km2 represents a volume of water equal to 104 m3.
The precipitation is collected and measured in a rain gauge
– Such as
(1) Pluviometer
(2) Ombrometer
(3) hyetometer

23
Rain gauge
A rain gauge consists of a cylindrical- vessel assembly kept
in the open to collect rain. The rainfall catch of the rain gauge
is affected by its exposure conditions.
The important conditions for setting a rain gauge
are :
(i) the ground must be level and in the open and the
instrument must present a horizontal catch surface
(ii) the gauge must be set as near the ground as
possible to reduce wind effects but it must be a
sufficiently high to prevent splashing, flooding etc.

24
Rain gauge
Obstroucle
h
30m or 2* h
which one is greater
5.5 m * 5.5 m
(iii) the instrument must be surrounded by an open faced
area of at least 5.5m x 5.5 m.
(iv) No object should be nearest to the instrument than 30 m
or twice the height of the obstruction.

25
Rain gauge classification:
Broadly Rain gauge is of two types:
(a) Non recording rain gauges
(b) Recording Rain gauges
(a) Non recording Rain gauge:
Extensively used is ‘ Symons’ gauge :
It consists of a circular collecting area of 12.70 cm(5 inch)
diameter connected to a funnel. The rim of the collector is set
in a horizontal plane at a height of 30.50 cm above ground
level. The funnel discharges the rainfall catch into a receiving
vessel. The funnel and receiving vessel are housed in a
metallic container.

26
Concrete block
600 mm * 600mm * 600 mm
30.5cm
12.70 cm 2.54 cm
Funnel
Collecting bottle
Metal outer container
GL
20.3cm
Figure : Non recording rain gauge
The water contained in the recording vessel is measured by a
suitable graduated measuring glass with a n accuracy up to
0.10mm. The rainfall is measured every day at 8.30 am and is
recorded as the rainfall of the day.

Class lectures on Hydrology by Rabindra Ranjan Saha Lecture 2

More Related Content

What's hot (20)

Similar to Class lectures on Hydrology by Rabindra Ranjan Saha Lecture 2 (20)

More from World University of Bangladesh (7)

Recently uploaded (20)

Class lectures on Hydrology by Rabindra Ranjan Saha Lecture 2