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METEOROLOGY
INTRODUCTION
Meteorology is the science related to atmosphere. Its knowledge is required for
estimation of probable maximum precipitation and optimum snowmelt conditions.
This is required for design of various hydraulic structures. A hydrologist needs to
have some knowledge of regional climate & meteorological process. Definitions
of a few terms used in this chapter are given below.
Atmosphere
Atmosphere is the gaseous envelope around the earth. It consists of dry air, water
vapor and various kinds of salts and dusts. The total amount of dry air and water
vapors would be over 5,600 billion tons and about 146 billion tons, respectively. If
all the water vapors present in the atmosphere are imagined to condense and fall
uniformly over the earth, it will accumulate to a depth of 25 mm.
Troposphere
It is the zone of atmosphere adjacent to earth. It extends approximately up to
seven miles above sea level. Almost 100 % of the total moisture contents of the
atmosphere are present in this zone and there is comparatively high temperature
gradient in this part of atmosphere.
Vapor Pressure
It is the pressure exerted by the amount of water vapors present in the atmosphere.
It is usually denoted by “e” and expressed in millibars. A Millibar is defined as
the pressure exerted by a force of one thousand dynes on one square cm surface.
1.33 millibar = 1 mm of Hg.
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Since the temperature of water vapor is the same as that of the air in the
atmosphere, the maximum amount of water vapor may be said to depend on the
air temperature. The higher the temperature, the more vapor can the atmospheric
air hold.
Saturation Vapor Pressure
When a sample of air holds the maximum quantity of water vapors at a particular
temperature it is said to be saturated. The pressure exerted by water vapors, when the
air is fully saturated with water vapors, is known as the saturation vapor pressure.
It is denoted by ‘es’. The variation of ‘es’ with temperature can be seen from any
standard handbook on Hydrology.
Isobars
These are the lines joining points of same atmospheric pressure at a given elevation.
The horizontal distribution of pressure is generally shown on weather charts by
isobars. The spacing between isobars is a measure of the pressure gradient.
RELATIVE HUMIDITY AND DEW POINT
The relation between temperature and saturation vapor pressure is shown in
Figure 2.1. Suppose that a parcel of air has a temperature ‘T’ and vapor pressure
‘e’ indicated by point ‘D’ in the Fig. If more vapor were added at constant
temperature ‘T’ point ‘D’ would move vertically upward towards ‘B’ and the air
would be saturated when ‘B’ is reached and the corresponding saturation vapor
pressure would be ‘es’.
The difference, es - e = Saturation deficit and the relation
(e / es) x 100 = Relative humidity - - - - - - - - - - - - - - - - - 2.1
When the air is cooled at a constant atmospheric pressure, the temperature at
which air becomes saturated is called Dew Point i.e. the point ‘D’ will come to
point ‘C’ in figure 2.1.
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Fig 2.1 Saturation Vapor Pressure of Water in Air
The relative humidity is the ratio of actual vapor pressure at certain temperature
and the saturation vapor pressure. Thus the relative humidity is 100 percent when
the air is saturated. If the parcel of air is cooled at constant pressure ‘e’ and
without addition of vapor, the point ‘D’ would move horizontally towards ‘C’.
The air would be saturated when ‘C’ is reached and the corresponding
temperature ‘Ta’ is the Dew point. The dew point is thus the temperature at which
water vapor present in the atmosphere will condense if the air is cooled.
Measurement of Relative Humidity
The instruments used for measurements of relative humidity are:
(i)
Psychrometer
(ii)
Hair hygrometer
(iii)
Hygrograph
(iv)
Thermo-hygrograph
Psychrometer
Psychrometer consists of two thermometers - Dry bulb thermometer and
Wet bulb thermometer. The mercury bulb of wet bulb thermometer is covered by
a jacket of clean muslin cloth saturated with water. This is done by putting a
beaker with distilled water underneath so that the bulb is not submerged in water
but only the cloth. Then water rises due to capillary action. The thermometers
are ventilated by whirling or by use of a fan. As a result of evaporation cooling
takes place. Readings are taken on both the thermometers simultaneously. The
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dry bulb reading is denoted as Td and wet bulb reading as Tw. The difference of
these two temperatures is called the wet bulb depression.
i.e. Td - Tw = Wet bulb depression - - - - - - - - - - - - - - - - - - - 2.2
Using these readings, the relative humidity can be found from the psychrometer
tables.
The value of ‘e’ for air temperature‘t’ may be obtained by the relation:
( es – e ) = γ ( T - Tw ) - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2.3
or
e = es – γ ( T - Tw ) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2.4
Where γ = psychrometer constant and its value is 0.660 when ‘e’ is measured in
millibar units and 0.485 when it is measured in units of ‘mm of Hg’.
Example 2.1
Given the air temperature as Ta = 24o C and the wet bulb temperature Tw = 21o C.
Find relative humidity and dew point.
Solution
From Equation 2.2
Wet bulb depression = 24o - 21o = 3o C.
From table A-1, Appendix-A, we get,
For Ta = 24 oC, es = 22.27 mm of Hg
For Tw = 21 oC ew = 18.65 mm of Hg
Therefore Equation 2.4 yields,
e = 18.65-0.485(3) = 17.20 mm of Hg
Relative humidity = [ 17.20 / 22.27 ] x 100 = 77 %
And the dew point is the temperature read from Table A-1 of Appendix-A
corresponding to ‘e’ which is 19.7 oC.
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The hair hygrometer consists of a frame in which a strand of hair is kept at
approximately constant tension. Changes in length of the hair corresponding to
changes in relative humidity are transmitted to a pointer. This instrument is
seldom used for meteorological purposes, but it is an inexpensive humidity
indicator and is often found in homes and offices.
The hair hygrograph is essentially a hair hygrometer, but is automatic recording
instrument. The movement of hair activates a pen, which records on a rotating
drum. The hygrothermograph combines the registration of both relative
humidity and temperature on one record sheet.
SOLAR RADIATION
The only source of heat energy for earth system is the sun. Whatever heat is
received by earth or reflected is the solar energy. Depending upon the shape,
rotation, angle of inclination of earth, it is visualized that solar energy received by
earth is changing from time to time and point to point.
The solar radiation comes to the earth in the form of a high temperature radiation.
A part of this radiation may be intercepted by the clouds but most of it is reflected
back into space. The fraction of the total incoming radiation which is reflected
back by the earth is called albedo. Under average conditions, the albedo of the
earth as a whole is about 40%, of the remaining 60% a small part is absorbed in
the atmosphere but the bulk is absorbed in the earth surface.
Solar energy received by sun is the maximum at the equator and decreases polewards. The variations in solar energy are the following:
a. Diurnal Variation
b. Seasonal Variation
c. Regional Variation.
The units for radiation flux per unit area are either joules per square centimeters or
milliwatt hours per square centimeter. In some countries calories per square
centimeters is designated as Langley and the corresponding unit of radiation flux
is Langley per minute. The intensity of direct solar radiation at normal incidence
is measured with an instrument called a Pyrheliometer.
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The principal exposure requirements of this instrument are that it should be free
from obstructions to the solar beam at all times of day and seasons of the year.
The site should be chosen so that the incidence of fog, smoke and air-borne
pollution is as less as possible in that area.
The Bi-metallic Actinograph is used for measurement of global radiation from
sun and sky on a horizontal surface. In this instrument a mechanical linkage is
used to record the temperature difference between a black coated Bi-metallic strip
exposed to solar radiation and two similar bimetallic strips painted white.
Because of the large mass of the bimetallic strips, the response time of the
instrument is large and the instrument is only suitable for the purpose of obtaining
estimates of daily totals.
TEMPERATURE
The degree of hotness or coldness is called the temperature. Its units of
measurement are Degree Centigrade ( °C ) or Degree Fahrenheit ( °F ).
Temperature Variation
There are three types of temperature variations.
i. Daily Variation of Temperature
The daily variation of temperature is dependent on the elevation of the sun, the
cloud cover and the wind speed. The variation of temperature is large in low
latitudes and small in high latitudes, with the result that the daily variation
decreases from equator towards poles.
The clouds reduce the radiations coming down during the day and outgoing
radiations during the night. On a cloudy day, the maximum temperature is lower
and the minimum temperature higher than on normal bright days. On windy days
the temperature on ground surface is lower than on calm days, because the greater
mobility of air along the vertical axis results in greater heat exchange with the
upper atmospheric layers.
ii. Seasonal Variation
The seasonal variation in rainfall and wind also affect the temperature. During the
rainy season, the cloud cover is large with the result that less radiation is received
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by the earth. Annual migration of vast masses of air also brings about horizontal
heat exchange and thus affects the annual range of temperature variation.
iii. Regional Variation of Temperature
Since the amount of net radiation decreases with increasing latitude, the
temperature tends to be highest at the equator and decreases towards the poles.
Measurement of Temperature
The temperature is measured with the help of thermometers. There are two types
of thermometers - Maximum thermometer (Mercury Type thermometer) and
Minimum Thermometer (Alcoholic type thermometer).
In order to measure the air temperature properly, thermometers must be placed
where air circulation is relatively unobstructed and yet they must be protected
from the direct sunrays and from precipitation. Therefore thermometers are
placed in white, louvered, wooden boxes, called instrument shelters. These
shelters are set about 4.5 feet above the ground. The readings are taken at 8.00
a.m. and 5.00 p.m.
The thermograph, with either a bi-metallic strip or a metallic tube filled with
alcohol or mercury for its thermometric element, makes an autograph record for
the past 24 hours, week or one month on a ruled chart wrapped around a clock
driven cylinder.
Four commonly used terms of temperature are:
i. Mean Daily Temperature
It is the average of maximum and minimum temperatures during the past 24
hours. Daily minimum and maximum temperatures are recorded at 9 am.
ii. Normal Daily Temperature
It is the average daily mean temperature for a given day over the past 30-years
period i. e. it is the mean temperature for a specific day
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iii. Mean Monthly Temperature
It is the average of the mean monthly maximum and minimum temperatures or it
is the mean temperature of the mean daily temperatures during the month.
iv. Mean Annual Temperature
It is the mean temperature of 12 months.
LAPSE RATE
The lapse rate or vertical temperature gradient is defined as the change in
temperature per unit distance in the vertical direction from the Earth surface. The
average value of the lapse rate is 3.6o F per 1,000 ft ( 304.8 m ). The greatest
variation in lapse rate is found in the layer of air just above the land surface. The
lapse rates are of three types depending upon the type of water vapors.
i. Dry Adiabatic Lapse Rate
It is the rate of change of temperature when air is not fully saturated with water
vapors. The average value of this is 17.71oF per 1,000 m.
ii. Wet (Saturated) Adiabatic Lapse Rate
When air is fully saturated, then rate of change of temperature is called wet
adiabatic, lapse rate. Its average value is 9.84o F per 1,000 m.
iii. Pseudo-Adiabatic Lapse Rate
After condensed particles have fallen down fully, as after a rainfall, then the rate
of change of temperature is called pseudo-adiabatic lapse rate. Its average value is
also 9.84o F per 1,000 m.
Example 2.2
A parcel of air has a temperature of 50o F on surface of the earth. At a height of
2,000 m the air becomes saturated. Rainfall occurs and air again becomes dry on
the leeward side of a mountain. Find out temperature of this parcel of air, at an
altitude of 2,500 m on leeward side of the hill. Height of hill is 3,000 m.
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Solution
Temperature at 2,000 m
= 50 - ( 17.72x2000/1000)
= 50 – 35.44
= 14.56 oF
Temperature at 3,000 m
= 14.56 - ( 9.84x1000/1000 )
= 4.72 oF
Temperature on the
leeward side at 2,500 m
= 4.72+ (9.84 x500/1000)
= 9.64 oF
Monsoon System in Pakistan
In summer, when the land warms much faster than the oceans an area of low
pressures develops over land and high pressure at sea. The reverse is the case
during winter season. Therefore, winds move from sea to land during summer
and from land to sea during winter. Winds which undergo seasonal changes of
direction in this way are called monsoon winds. These winds, and for that matter
any other wind system are modified considerably when there exist extensive
mountain ranges.
In winter over Indo-Pak Sub-continent and central part of Asia (slightly towards
eastern shores of Asia) a ‘High’ is established. North easterly winds blow over
most of the eastern shores of Asia and India. These are called the North - East
Monsoons. These winds are cold and dry and thus give fair weather to the area
over which these blow.
In summer a ‘Low’ is established over North-West part of Indo-Pak Sub continent
due to heating of the arid land-tracts while the air in the southern hemisphere
cools. Over India and Pakistan, the wind blows from southwest and is known as
South - West Monsoon. This is a very damp air as it originates from Indian
Ocean. It actually starts from the southern hemisphere and after crossing the
equator reaches the Sub-continent as a southwesterly wind.
In Pakistan the summer monsoon sets in by June, but its effect over the central
areas and the sub-mountain districts becomes well pronounced by the third week
of July. The monsoon remains effective over the area till the end of September,
sometime extending to the second week of October.
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Prior to the onset of monsoon, Pakistan is very dry and the maximum temperature
crosses 110 o F over the plains and the dew point temperature starts rising as soon
as the monsoon invades Pakistan. By the beginning of June, a “low” establishes
over Hyderabad, Khairpur, Multan and Sibi Divisions, which to some extent, is
responsible for the onset of South-West monsoon over the sub-continent. The
south-westerlies are predominant over the southern areas of Pakistan.
The monsoon air requires lifting of 3 to 4 thousands feet to cause thunder
showers. There being no barrier of such a height for these south-westerlies, the
southern area of Pakistan goes dry, though the monsoon current remains active
during June to September.
WESTERN DISTURBANCES
The western disturbances are important air masses, for most of the winter season
in Pakistan and Northern India. Since these disturbances approach from the west,
it is for this reason that these are locally called western disturbances. These
originate from the Mediterranean Sea and move in an easterly or north - easterly
direction. They have warm and cold fronts but by the time these reach Pakistan,
these are so diffused together that it is difficult to distinguish them on surface
synoptic charts. However they can be identified on the upper-air charts.
During the winter these move at lower latitudes and reach northern part of
Pakistan, but in summer these move in higher latitudes through Russian Turkistan.
Some times these accentuate the monsoon depressions in the summer and cause
heavy rainfall in the sub- mountain districts, such as Sialkot, Jhelum and
Rawalpindi. The effect of these disturbances is well pronounced from December
to March. Normally light rain or drizzle is associated with them, but in case of
active disturbances, thundershower over northern and central divisions of Pakistan
may occur.
The western disturbances also produce secondary disturbance moving over the
Persian Gulf and over the Arabian Sea. The secondary wave also moves to
Pakistan and causes lot of weather over the entire Pakistan. During Western
Disturbances over Pakistan, the winds are normally Westerlies or NorthWesterlies. Sometimes the secondary wave continues moving in an easterly
direction along the coast while the primary is affecting Peshawar and D.I. Khan
Divisions.
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The following conclusions have been drawn by the Pakistan Meteorology
Department on the basis of detailed study of synoptic situations responsible for
winter rainfall over Pakistan.
a. A western disturbances appearing over Turkey takes about 7-8 days to
reach Pakistan.
b. A weak western disturbance does not cause any rainfall over Pakistan. A weak
western disturbance has been defined as the one which has no closed ‘Low’
appearing over Turkey. For a well-marked ‘Low’ there are generally two closed
isobars and the lowest pressure reported is generally less than 1,010 Mb during
December to February.
c. The well-marked western disturbance causes fairly widespread rain and also
thunderstorms along the frontline over Turkey and Iraq. This can be seen about 5
to 7 days ahead. This type of circulation of air mass is a forewarning for an
approaching wet spell over the northern divisions of Pakistan
d. The winds over the Gulf of Oman become southerly while over north Persian
Gulf area northerlies will be blowing. This is a strong indication of secondary
development over the Gulf of Oman. Due to the peculiar orography of northwest
Pakistan, the cyclonic circulation further increases. This results in moderately
heavy to heavy showers over the central and northern divisions of Pakistan.
MONSOON AND WESTERN DISTURBANCES AS CAUSE OF RAINFALL
By the beginning of July, South Easterlies begin to penetrate the central areas and
sub-mountain districts of Pakistan. The South-Easterlies are nothing, but the
deflected South-Westerlies, which take a turn westwards parallel to the Himalayas
from the Bay of Bengal.
A depression is intensified when a warm pool exists above it. It is also intensified
by the interaction between the disturbances of low latitudes and high latitudes and
also between disturbances of the two hemispheres. Bay of Bengal tropical
depressions (monsoon depressions) form as a result of the confluence of Southern
air mass from the equatorial region and South-Westerlies crossing the subcontinent from the Arabian Sea. The South West monsoon air mass behaves as
two different types with the consequence that their interrelation results in
frontogenesis. There also exists a trough of low pressure, generally called
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monsoon trough, which extends roughly from Punjab in Pakistan, through Agra,
Allahabad in India to the Bay of Bengal. When the axis of the monsoon trough
moves towards the Himalayas and lies there, the rain on the plains decreases while
that on the hills increases. When the axis of the monsoon trough shifts to the
South, the entire of Pakistan comes under the influences of homogeneous
monsoon resulting in a continuous type of rain drizzle. But occasionally heavy
thundershowers do occur over the sub-mountain districts and adjoining areas of
Kashmir due to orographic effect (mountain barriers).
The monsoon depressions travel Westwards/North Westwards. Most of these
depressions dissipate within a few hundred miles of their journey over the subcontinent. Those which are strong reach as far as Rajisthan and then start moving
northwards. This change in the direction of movement of the depression is called
“recurvature”. When the storm of depression recurves itself, it receives good
moisture supply from Arabian Sea resulting in heavy rainfall in Pakistan. Some
depressions which do not recurve, emerge into the Arabian Sea and do not cause
any rainfall. It must be noted that depressions cause lifting of air mass and
therefore are the main causative factors for precipitation to occur. Higher and
continuous lifting and hence continuous precipitation occur when a depression is
strengthened by another depression. The three types of depressions in the IndoPak Subcontinent are:
a. Seasonal as a result of high summer temperature corresponding to low
temperature,
b. Monsoon depression traveling from the Bay of Bengal, and
c. Depression due to Western disturbance. When all the depressions combine
and there is a continuous supply of moisture from the Arabian Sea there
results very heavy rainfall in the northern part of Pakistan.
WIND MEASUREMENT
Wind speed is measured with an instrument called Anemometer. This instrument
gives continuous record over some graph called Anemograph. A very well
known Anemometer is Dynes Apparatus. It gives reading in miles of total wind
movement in 24 hours.
Wind has both speed and direction. Wind direction is the direction from which
wind is blowing. Wind speed is usually given in miles per hour, meters per second
or knots.
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1m/sec = 2.2 mph, and,
1 knot = 1-1/7 mph
The graph between wind speed and elevation is shown in figure 2.2.
The equation of the curve is,
V/Vo = (  / Zo )1/7 - - - - - - - - - - - - - - - - - - - - 2.5
Where ‘V’ is wind speed at height ‘Z’ from ground and ‘Vo’ is wind speed
measured by the anemometer at height ‘Z0’.
Figure 2.2 Variation of Wind Velocity with Depth
Example 2.3
The speed of air at a height of 15 meter above ground was measured as 10 m/s.
Find the speed at 2 m level.
Solution
From Equation 2.5
V2 / V15 = ( Z2 / Z15 )1/7 or
V2 = ( Z2 / Z15 )1/7 x V15
V2 = ( 2 / 15)1/7 x 10 = 7.5 m/s
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QUESTIONS
1. Define Meteorology. Why is it studied with subject of Hydrology?
2. How Relative Humidity and Dew Point are related. Describe instrument
used for measurement of relative humidity.
3. What is saturation vapor pressure? Can air saturate itself up to 100%.
4. Define temperature. Explain variation in temperature with respect to time.
5. What is Lapse Rate? How does temperature changes with altitude?
6. What are monsoons, explain the mechanism of monsoon rainfalls in
Pakistan?
7. Explain, what are the Western Disturbances?
8. Why psychrometer constant is different when ‘e’ is measured in different
units.
9. Why wind speed is measured. What is effect of wind on climate changes
in an area? Explain variation of wind velocity with respect to altitude.
EXCERCISE
1. A mass of air is having temperature of 26.9 oC. The Relative humidity of this
mass of air is 71 %. Determine Saturation Vapor pressure, Saturation deficit,
Actual Vapor Pressure, Dew Point and Wet-bulb temperature.
2. The dry bulb temperature and wet bulb temperature are respectively 25 oC and
15 oC at a certain location. Determine Dew-point Temperature, Relative
Humidity, Saturation Vapor Pressure and Actual Vapor pressure.
3. A mass of air is at temperature of 30 oC and is having relative humidity of 75
%. Determine the dew-point temperature.
4. The wind has a speed of 9 m/s at elevation of 10 m above ground and 10 m/s at
elevation of 15 m above ground. Determine the speed at height of 2 m.
5. A parcel of moist air at 45°F, initially at 152 m mean sea level is forced to pass
over a mountain ridge of 2,134 m mean sea level and then descends to its original
level. Assuming that a lift of 762 m produces saturation and precipitation, what is
the final temperature of the parcel?
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