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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 vaporswould 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 upto 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.
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, whentheair 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.
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 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 Cand the wet bulb temperature Tw = 21oC. 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]x100 = 77 %
And the dew point is the temperature read from Table A-1 of Appendix-A corresponding to ‘e’ which is 19.7 oC.
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 pole-wards.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.
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 thatarea.
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 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 4.5feet 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 at9 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 themean temperature for a specific day
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 3.6oF 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 9.84oF 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 also9.84oF per 1,000 m.
Example 2.2
A parcel of air has a temperature of 50oF 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 3,000m.
Solution
Temperature at 2,000 m = 50 - (17.72x2000/1000)
= 50 –35.44
= 14.56oF
Temperature at 3,000 m = 14.56 - (9.84x1000/1000)
= 4.72oF
Temperature on the
leewardside at 2,500 m= 4.72+ (9.84 x500/1000)
= 9.64oF
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.
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 North-Westerlies. Sometimes the secondary wave continues moving in an easterly direction along the coast while the primary is affecting Peshawar and D.I. Khan Divisions.
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 7-8days 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.