Ch#2 meteorology lect 2

Chapter 2
MeteorologyMeteorology

MeteorologyMeteorology
 The science related to atmosphere.The science related to atmosphere.
Why ????? Its Knowledge is necessaryWhy ????? Its Knowledge is necessary
 Estimation of Probable maximum precipitationEstimation of Probable maximum precipitation
 Optimum Snowmelt ConditionsOptimum Snowmelt Conditions
 Design of various hydraulic structures.Design of various hydraulic structures.
Hydrologist should knows!!!!!!!!!!!!Hydrologist should knows!!!!!!!!!!!!
 Regional climate.Regional climate.
 Meteorological process.Meteorological process.

Definitions of some common terms!!!!!!Definitions of some common terms!!!!!!
1. Atmosphere1. Atmosphere
 The gaseous envelope around the earth.The gaseous envelope around the earth.
Atmosphere Contains?????Atmosphere Contains?????
 Dry air.Dry air.
 Water vapor.Water vapor.
 Various kinds of salts and dusts.Various kinds of salts and dusts.

The total amount of dry air and water vaporsThe total amount of dry air and water vapors
would be over 5,600 billion tons and about 146would be over 5,600 billion tons and about 146
billion tons, respectively.billion tons, respectively.
If all the water vapors present in the atmosphereIf all the water vapors present in the atmosphere
are imagined to condense and fall uniformly overare imagined to condense and fall uniformly over
the earth, it will accumulate to a depth of 25 mm.the earth, it will accumulate to a depth of 25 mm.
Is it Possible!!!!!!Is it Possible!!!!!!

 It is the zone of atmosphere adjacent to earth.It is the zone of atmosphere adjacent to earth.
 It extends approximately up to seven milesIt extends approximately up to seven miles
above sea level.above sea level.
 Almost 100 % of the total moisture contents ofAlmost 100 % of the total moisture contents of
the atmosphere are present in this zone.the atmosphere are present in this zone.
 there is comparatively high temperature gradientthere is comparatively high temperature gradient
in this part of atmosphere.in this part of atmosphere.
2. Troposphere2. Troposphere

 TheThe SunSun, in its, in its seasonalseasonal movement through the sky,movement through the sky,
passes directly over the equator twice eachpasses directly over the equator twice each yearyear, on the, on the
March and SeptemberMarch and September equinoxesequinoxes. At the equator, the. At the equator, the
rays of the sun arerays of the sun are perpendicularperpendicular to the surface of theto the surface of the
earth on these dates.earth on these dates.
 Places on the equator experience the quickest rates ofPlaces on the equator experience the quickest rates of
sunrisesunrise andand sunsetsunset in the world. Such places also have ain the world. Such places also have a
theoretical constant 12 hours of day and nighttheoretical constant 12 hours of day and night
throughout the yearthroughout the year

 TheThe equatorequator (sometimes referred to colloquially(sometimes referred to colloquially
asas "the Line""the Line") is the intersection of the) is the intersection of the EarthEarth's's
surface with thesurface with the planeplane perpendicular to theperpendicular to the
Earth'sEarth's axis of rotationaxis of rotation and containing theand containing the
Earth'sEarth's center of masscenter of mass..

Vapor PressureVapor Pressure
 It is the pressure exerted by the amount of waterIt is the pressure exerted by the amount of water
vapors present in the atmosphere.vapors present in the atmosphere.
 It is usually denoted by “e” and expressed inIt is usually denoted by “e” and expressed in
millibars.millibars.
 AA MillibarMillibar is defined as the pressure exerted byis defined as the pressure exerted by
a force of one thousand dynes on one square cma force of one thousand dynes on one square cm
surface.surface.
1.33 millibar = 1 mm of Hg.1.33 millibar = 1 mm of Hg.
Continued…

 TheThe barbar (symbol(symbol barbar),), decibardecibar (symbol(symbol dbardbar))
and theand the millibarmillibar (symbol(symbol mbarmbar, also mb) are, also mb) are
units ofunits of pressurepressure. They are not. They are not SISI units, nor areunits, nor are
theythey cgscgs units, but they are accepted for use withunits, but they are accepted for use with
thethe SISI. The bar is widely used in descriptions of. The bar is widely used in descriptions of
pressure because it is about the same aspressure because it is about the same as
atmospheric pressure, and is legally recognizedatmospheric pressure, and is legally recognized
in countries of thein countries of the European UnionEuropean Union

 The bar, decibar and millibar are defined as:The bar, decibar and millibar are defined as:
 1 bar = 100 kPa (kilopascals) = 1,000,0001 bar = 100 kPa (kilopascals) = 1,000,000 dynesdynes
per squareper square centimetercentimeter ((baryesbaryes))
 1 dbar = 0.1 bar = 10 kPa = 100,000 dyn/cm²1 dbar = 0.1 bar = 10 kPa = 100,000 dyn/cm²
 1 mbar = 0.001 bar = 0.1 kPa = 1 hPa = 1,0001 mbar = 0.001 bar = 0.1 kPa = 1 hPa = 1,000
dyn/cm²dyn/cm²
 (A(A pascalpascal is oneis one newtonnewton per square meter.)per square meter.)

Vapor PressureVapor Pressure
 Since the temperature of water vapor is the sameSince the temperature of water vapor is the same
as that of the air in the atmosphere, theas that of the air in the atmosphere, the
maximum amount of water vapor may be said tomaximum amount of water vapor may be said to
depend on the air temperature.depend on the air temperature.
 The higher the temperature, the more vapor canThe higher the temperature, the more vapor can
the atmospheric air hold. (Lifting Mechanism)the atmospheric air hold. (Lifting Mechanism)

Saturation Vapor PressureSaturation Vapor Pressure
 When a sample of air holds the maximumWhen a sample of air holds the maximum
quantity of water vapors at a particularquantity of water vapors at a particular
temperature it is said to be saturated.temperature it is said to be saturated.
 The pressure exerted by water vapors, when theThe pressure exerted by water vapors, when the
air is fully saturated with water vapors, is knownair is fully saturated with water vapors, is known
as the saturation vapor pressure.as the saturation vapor pressure.
 It is denoted by ‘eIt is denoted by ‘ess’. The variation of ‘e’. The variation of ‘ess’ with’ with
temperaturetemperature is given in Table A-1 of Appendix-is given in Table A-1 of Appendix-
A at end of this bookA at end of this book..

 InIn physicsphysics, the, the dynedyne (symbol "dyn", from Greek(symbol "dyn", from Greek
δύναμις (δύναμις (dynamisdynamis) meaning power, force) is a) meaning power, force) is a unitunit ofof
force specified in theforce specified in the centimetre-gram-secondcentimetre-gram-second (CGS) system(CGS) system
of units, a predecessor of the modern SI. One dyne isof units, a predecessor of the modern SI. One dyne is
equal to exactly 10 micronewtons. Further, the dyne canequal to exactly 10 micronewtons. Further, the dyne can
be defined as "the force required to accelerate a mass ofbe defined as "the force required to accelerate a mass of
one gram at a rate of one centimetre per secondone gram at a rate of one centimetre per second
squared".squared".
 1 dyn = 1 g·cm/s² = 10−5 kg·m/s² = 10 µN1 dyn = 1 g·cm/s² = 10−5 kg·m/s² = 10 µN

IsobarsIsobars
 These are the lines joining points of sameThese are the lines joining points of same
atmospheric pressure at a given elevation.atmospheric pressure at a given elevation.
 The horizontal distribution of pressure isThe horizontal distribution of pressure is
generally shown on weather charts by isobars.generally shown on weather charts by isobars.
 The spacing between isobars is a measure of theThe spacing between isobars is a measure of the
pressure gradient.pressure gradient.

 In atmospheric sciences (meteorology,In atmospheric sciences (meteorology,
climatology and related fields), theclimatology and related fields), the pressurepressure
gradientgradient (typically of air, more generally of any(typically of air, more generally of any
fluid) is a physical quantity that describes influid) is a physical quantity that describes in
which direction and at what rate the pressurewhich direction and at what rate the pressure
changes the most rapidly around a particularchanges the most rapidly around a particular
location. The pressure gradient is a dimensionallocation. The pressure gradient is a dimensional
quantity expressed in units of pressure per unitquantity expressed in units of pressure per unit
length. The SI unit is pascal per metre (Pa/m).length. The SI unit is pascal per metre (Pa/m).

Relative Humidity & Dew PointRelative Humidity & Dew Point
 The relation between temperature and saturation vaporThe relation between temperature and saturation vapor
pressure is shown in Figure 2.1. Suppose that a parcelpressure is shown in Figure 2.1. Suppose that a parcel
of air has a temperature ‘T’ and vapor pressure ‘e’of air has a temperature ‘T’ and vapor pressure ‘e’
indicated by point ‘D’ in the Fig.indicated by point ‘D’ in the Fig.
 If more vapor were added at constant temperature ‘T’If more vapor were added at constant temperature ‘T’
point ‘D’ would move vertically upward towards ‘B’point ‘D’ would move vertically upward towards ‘B’
and the air would be saturated when ‘B’ is reached andand the air would be saturated when ‘B’ is reached and
the corresponding saturation vapor pressure would bethe corresponding saturation vapor pressure would be
‘es’.‘es’.
Continued…

 The difference, es - e = Saturation deficit andThe difference, es - e = Saturation deficit and
the relationthe relation
 (e / es) x 100=Relative humidity- - - Eq. 2.1(e / es) x 100=Relative humidity- - - Eq. 2.1
 When the air is cooled at a constant atmosphericWhen the air is cooled at a constant atmospheric
pressure, the temperature at which air becomespressure, the temperature at which air becomes
saturated is called Dew Point i.e. the point ‘D’saturated is called Dew Point i.e. the point ‘D’
will come to point ‘C’ in figure 2.1.will come to point ‘C’ in figure 2.1.
Continued…

Fig 2.1 Saturation Vapor Pressure of Water in AirFig 2.1 Saturation Vapor Pressure of Water in Air
0
5
10
15
20
25
30
35
-10 0 10 20 30 40
Temperature °C
SaturationVaporPressure(mmHg)
B
C
. D(t, e)
Continued…
D

 The relative humidity is the ratio of actualThe relative humidity is the ratio of actual
vapor pressure at certain temperature and thevapor pressure at certain temperature and the
saturation vapor pressure. Thus the relativesaturation vapor pressure. Thus the relative
humidity is 100 percent when the air ishumidity is 100 percent when the air is
saturated.saturated.
 If the parcel of air is cooled at constantIf the parcel of air is cooled at constant
pressure ‘e’ and without addition of vapor,pressure ‘e’ and without addition of vapor,
the point ‘D’ would move horizontallythe point ‘D’ would move horizontally
towards ‘C’.towards ‘C’.
Continued…

 The air would be saturated when ‘C’ is reachedThe air would be saturated when ‘C’ is reached
and the corresponding temperature ‘Ta’ is theand the corresponding temperature ‘Ta’ is the
Dew point.Dew point.
 The dew point is thus the temperature at whichThe dew point is thus the temperature at which
water vapor present in the atmosphere willwater vapor present in the atmosphere will
condense if the air is cooled.condense if the air is cooled.

Measurement of Relative HumidityMeasurement of Relative Humidity
 The instruments used for measurements ofThe instruments used for measurements of
relative humidity are:relative humidity are:
 PsychrometerPsychrometer
 Hair hygrometerHair hygrometer
 HygrographHygrograph
 Thermo-hygrographThermo-hygrograph
Continued…

 PsychrometerPsychrometer consists of two thermometers - Dry bulbconsists of two thermometers - Dry bulb
thermometer and Wet bulb thermometer. The mercurythermometer and Wet bulb thermometer. The mercury
bulb of wet bulb thermometer is covered by a jacket ofbulb of wet bulb thermometer is covered by a jacket of
clean muslin cloth saturated with water.clean muslin cloth saturated with water.
 This is done by putting a beaker with distilled waterThis is done by putting a beaker with distilled water
underneath so that the bulb is not submerged in waterunderneath so that the bulb is not submerged in water
but only the cloth.but only the cloth.
 Then water rises due to capillary action. TheThen water rises due to capillary action. The
thermometers are ventilated by whirling or by use of athermometers are ventilated by whirling or by use of a
fan. As a result of evaporation cooling takes place.fan. As a result of evaporation cooling takes place.
Readings are taken on both the thermometersReadings are taken on both the thermometers
simultaneously.simultaneously.
Continued…

 The dry bulb reading is denoted as Td and wetThe dry bulb reading is denoted as Td and wet
bulb reading as Tw. The difference of these twobulb reading as Tw. The difference of these two
temperatures is called thetemperatures is called the wet bulb depressionwet bulb depression..
i.e. Td-Tw=Wet bulb depression- - - Eq. 2.2i.e. Td-Tw=Wet bulb depression- - - Eq. 2.2
 Using these readings, the relative humidity canUsing these readings, the relative humidity can
be found from the psychrometer tables.be found from the psychrometer tables.
Continued…

 The value of ‘e’ for air temperature‘t’ may be obtainedThe value of ‘e’ for air temperature‘t’ may be obtained
by the relation:by the relation:
( es – e ) = γ ( T - Tw ) - - - - - - - - - - - - -Eq. 2.3( es – e ) = γ ( T - Tw ) - - - - - - - - - - - - -Eq. 2.3
oror
e = es – γ ( T - Tw ) - - - - - - - - - - - - - - Eq. 2.4e = es – γ ( T - Tw ) - - - - - - - - - - - - - - Eq. 2.4
 Where γ = psychrometer constant and its value is 0.660Where γ = psychrometer constant and its value is 0.660
when ‘e’ is measured in millibar units and 0.485 when itwhen ‘e’ is measured in millibar units and 0.485 when it
is measured in units of ‘mm of Hg’.is measured in units of ‘mm of Hg’.
Continued…

 Example 2.1Example 2.1 ::
Given the air temperature as Ta = 24o C and theGiven the air temperature as Ta = 24o C and the
wet bulb temperature Tw = 21o C. Find relativewet bulb temperature Tw = 21o C. Find relative
humidity and dew point.humidity and dew point.
Continued…

SolutionSolution
From Equation 2.2From Equation 2.2
Wet bulb depression = 24o - 21o = 3o C.Wet bulb depression = 24o - 21o = 3o C.
From table A-1, Appendix-A, we get,From table A-1, Appendix-A, we get,
For Ta = 24 oC, es = 22.27 mm of HgFor Ta = 24 oC, es = 22.27 mm of Hg
For Tw = 21 oC ew = 18.65 mm of HgFor Tw = 21 oC ew = 18.65 mm of Hg
Therefore Equation 2.4 yields,Therefore Equation 2.4 yields,
e = 18.65-0.485(3) = 17.20 mm of Hge = 18.65-0.485(3) = 17.20 mm of Hg
Relative humidity = [ 17.20 / 22.27 ] x 100 = 77 %Relative humidity = [ 17.20 / 22.27 ] x 100 = 77 %
 And the dew point is the temperature read fromAnd the dew point is the temperature read from TableTable
A-1 of Appendix-AA-1 of Appendix-A corresponding to ‘e’ which is 19.7corresponding to ‘e’ which is 19.7
oC.oC.
Continued…

 TheThe hair hygrometerhair hygrometer consists of a frame inconsists of a frame in
which a strand of hair is kept at approximatelywhich a strand of hair is kept at approximately
constant tension.constant tension.
 Changes in length of the hair corresponding toChanges in length of the hair corresponding to
changes in relative humidity are transmitted to achanges in relative humidity are transmitted to a
pointer.pointer.
 This instrument is seldom used forThis instrument is seldom used for
meteorological purposes, but it is an inexpensivemeteorological purposes, but it is an inexpensive
humidity indicator and is often found in homeshumidity indicator and is often found in homes
and offices.and offices.
Continued…

 TheThe hairhair hygrographhygrograph is essentially a hairis essentially a hair
hygrometer, but is automatic recordinghygrometer, but is automatic recording
instrument.instrument.
 The movement of hair activates a pen, whichThe movement of hair activates a pen, which
records on a rotating drum.records on a rotating drum.
 TheThe hygrothermographhygrothermograph combines thecombines the
registration of both relative humidity andregistration of both relative humidity and
temperature on one record sheet.temperature on one record sheet.

Solar RadiationSolar Radiation
 The only source of heat energy for earth systemThe only source of heat energy for earth system
is the sun. Whatever heat is received by earth oris the sun. Whatever heat is received by earth or
reflected is the solar energy.reflected is the solar energy.
 Depending upon the shape, rotation, angle ofDepending upon the shape, rotation, angle of
inclination of earth, it is visualized that solarinclination of earth, it is visualized that solar
energy received by earth is changing from timeenergy received by earth is changing from time
to time and point to point.to time and point to point.
Continued…

 The solar radiation comes to the earth in the form of aThe solar radiation comes to the earth in the form of a
high temperature radiation. A part of this radiation mayhigh temperature radiation. A part of this radiation may
be intercepted by the clouds but most of it is reflectedbe intercepted by the clouds but most of it is reflected
back into space.back into space.
 The fraction of the total incoming radiation which isThe fraction of the total incoming radiation which is
reflected back by the earth is called albedo.reflected back by the earth is called albedo.
 Under average conditions, the albedo of the earth as aUnder average conditions, the albedo of the earth as a
whole is about 40%, of the remaining 60% a small partwhole is about 40%, of the remaining 60% a small part
is absorbed in the atmosphere but the bulk is absorbedis absorbed in the atmosphere but the bulk is absorbed
in the earth surface.in the earth surface.
Continued…

 Solar energy received by sun is the maximum atSolar energy received by sun is the maximum at
the equator and decreases pole-wards. Thethe equator and decreases pole-wards. The
variations in solar energy are the following:variations in solar energy are the following:
 Diurnal VariationDiurnal Variation
 Seasonal VariationSeasonal Variation
 Regional Variation.Regional Variation.
Continued…

 The units for radiation flux per unit area are eitherThe units for radiation flux per unit area are either
joules per square centimeters or milliwatt hours perjoules per square centimeters or milliwatt hours per
square centimeter.square centimeter.
 In some countries calories per square centimeters isIn some countries calories per square centimeters is
designated asdesignated as LangleyLangley and the corresponding unit ofand the corresponding unit of
radiation flux is Langley per minute.radiation flux is Langley per minute.
 The intensity of direct solar radiation at normalThe intensity of direct solar radiation at normal
incidence is measured with an instrument called aincidence is measured with an instrument called a
PyrheliometerPyrheliometer..
Continued…

 The principal exposure requirements of thisThe principal exposure requirements of this
instrument are that it should be free frominstrument are that it should be free from
obstructions to the solar beam at all times of dayobstructions to the solar beam at all times of day
and seasons of the year.and seasons of the year.
 The site should be chosen so that the incidenceThe site should be chosen so that the incidence
of fog, smoke and air-borne pollution is as lessof fog, smoke and air-borne pollution is as less
as possible in that area.as possible in that area.
Continued…

 The Bi-metallic ActinographThe Bi-metallic Actinograph is used foris used for
measurement of global radiation from sun and sky on ameasurement of global radiation from sun and sky on a
horizontal surface.horizontal surface.
 In this instrument a mechanical linkage is used toIn this instrument a mechanical linkage is used to
record the temperature difference between a blackrecord the temperature difference between a black
coated Bi-metallic strip exposed to solar radiation andcoated Bi-metallic strip exposed to solar radiation and
two similar bimetallic strips painted white.two similar bimetallic strips painted white.
 Because of the large mass of the bimetallic strips, theBecause of the large mass of the bimetallic strips, the
response time of the instrument is large and theresponse time of the instrument is large and the
instrument is only suitable for the purpose of obtaininginstrument is only suitable for the purpose of obtaining
estimates of daily totals.estimates of daily totals.

TemperatureTemperature
 The degree of hotness or coldness is called theThe degree of hotness or coldness is called the
temperature. Its units of measurement aretemperature. Its units of measurement are
Degree Centigrade ( °C ) or Degree Fahrenheit (Degree Centigrade ( °C ) or Degree Fahrenheit (
°F ).°F ).
 Temperature VariationTemperature Variation
 There are three types of temperature variations.There are three types of temperature variations.
 Daily Variation of TemperatureDaily Variation of Temperature
 Regional Variation of TemperatureRegional Variation of Temperature
Continued…

 Daily Variation of TemperatureDaily Variation of Temperature
 The daily variation of temperature is dependent on theThe daily variation of temperature is dependent on the
elevation of the sun, the cloud cover and the wind speed.elevation of the sun, the cloud cover and the wind speed.
 The variation of temperature is large in low latitudes andThe variation of temperature is large in low latitudes and
small in high latitudes, with the result that the daily variationsmall in high latitudes, with the result that the daily variation
decreases from equator towards poles.decreases from equator towards poles.
 The clouds reduce the radiations coming down during theThe clouds reduce the radiations coming down during the
day and outgoing radiations during the night. On a cloudyday and outgoing radiations during the night. On a cloudy
day, the maximum temperature is lower and the minimumday, the maximum temperature is lower and the minimum
temperature higher than on normal bright days.temperature higher than on normal bright days.
 On windy days the temperature on ground surface is lowerOn windy days the temperature on ground surface is lower
than on calm days, because the greater mobility of air alongthan on calm days, because the greater mobility of air along
the vertical axis results in greater heat exchange with thethe vertical axis results in greater heat exchange with the
upper atmospheric layers.upper atmospheric layers.
Continued…

 The seasonal variation in rainfall and wind alsoThe seasonal variation in rainfall and wind also
affect the temperature.affect the temperature.
 During the rainy season, the cloud cover is largeDuring the rainy season, the cloud cover is large
with the result that less radiation is received bywith the result that less radiation is received by
the earth.the earth.
 Annual migration of vast masses of air alsoAnnual migration of vast masses of air also
brings about horizontal heat exchange and thusbrings about horizontal heat exchange and thus
affects the annual range of temperature variation.affects the annual range of temperature variation.
Continued…

 Regional Variation of TemperatureRegional Variation of Temperature
 Since the amount of net radiation decreases withSince the amount of net radiation decreases with
increasing latitude, the temperature tends to beincreasing latitude, the temperature tends to be
highest at the equator and decreases towards thehighest at the equator and decreases towards the
poles.poles.

Measurement of TemperatureMeasurement of Temperature
 The temperature is measured with the help of thermometers.The temperature is measured with the help of thermometers.
There are two types of thermometers - MaximumThere are two types of thermometers - Maximum
thermometer (Mercury Type thermometer) and Minimumthermometer (Mercury Type thermometer) and Minimum
Thermometer (Alcoholic type thermometer).Thermometer (Alcoholic type thermometer).
 In order to measure the air temperature properly,In order to measure the air temperature properly,
thermometers must be placed where air circulation isthermometers must be placed where air circulation is
relatively unobstructed and yet they must be protected fromrelatively unobstructed and yet they must be protected from
the direct sunrays and from precipitation.the direct sunrays and from precipitation.
 Therefore thermometers are placed in white, louvered,Therefore thermometers are placed in white, louvered,
wooden boxes, called instrument shelters. These shelters arewooden boxes, called instrument shelters. These shelters are
set about 4.5 feet above the ground. The readings are takenset about 4.5 feet above the ground. The readings are taken
at 8.00 a.m. and 5.00 p.m.at 8.00 a.m. and 5.00 p.m.
Continued…

 The thermograph, with either a bi-metallic strip or aThe thermograph, with either a bi-metallic strip or a
metallic tube filled with alcohol or mercury for itsmetallic tube filled with alcohol or mercury for its
thermometric element, makes an autograph record forthermometric element, makes an autograph record for
the past 24 hours, week or one month on a ruled chartthe past 24 hours, week or one month on a ruled chart
wrapped around a clock driven cylinder.wrapped around a clock driven cylinder.
 Four commonly used terms of temperature are:Four commonly used terms of temperature are:
 Mean Daily TemperatureMean Daily Temperature
 Normal Daily TemperatureNormal Daily Temperature
 Mean Monthly TemperatureMean Monthly Temperature
 Mean Annual TemperatureMean Annual Temperature
Continued…

 Mean Daily TemperatureMean Daily Temperature
 It is the average of maximum and minimumIt is the average of maximum and minimum
temperatures during the past 24 hours.temperatures during the past 24 hours.
 Normal Daily TemperatureNormal Daily Temperature
 It is the average daily mean temperature for a givenIt is the average daily mean temperature for a given
day over the past 30-years period i. e. it is the meanday over the past 30-years period i. e. it is the mean
temperature for a specific daytemperature for a specific day
Continued…

 Mean Monthly TemperatureMean Monthly Temperature
 It is the average of the mean monthly maximum andIt is the average of the mean monthly maximum and
minimum temperatures or it is the mean temperatureminimum temperatures or it is the mean temperature
of the mean daily temperatures during the month.of the mean daily temperatures during the month.
 Mean Annual TemperatureMean Annual Temperature
 It is the mean temperature of 12 months.It is the mean temperature of 12 months.

Lapse RateLapse Rate
 The lapse rate or vertical temperature gradient isThe lapse rate or vertical temperature gradient is
defined as the change in temperature per unit distancedefined as the change in temperature per unit distance
in the vertical direction from the Earth surface.in the vertical direction from the Earth surface.
 The average value of the lapse rate is 3.6The average value of the lapse rate is 3.6oo
F per 1,000ftF per 1,000ft
(304.8 m).(304.8 m).
 The greatest variation in lapse rate is found in the layerThe greatest variation in lapse rate is found in the layer
of air just above the land surface. The lapse rates are ofof air just above the land surface. The lapse rates are of
three types depending upon the type of water vapors.three types depending upon the type of water vapors.
 Dry Adiabatic Lapse RateDry Adiabatic Lapse Rate
 Wet (Saturated) Adiabatic Lapse RateWet (Saturated) Adiabatic Lapse Rate
 Pseudo-Adiabatic Lapse RatePseudo-Adiabatic Lapse Rate
Continued…

 Dry Adiabatic Lapse RateDry Adiabatic Lapse Rate
 It is the rate of change of temperature when air isIt is the rate of change of temperature when air is
not fully saturated with water vapors.not fully saturated with water vapors.
 The average value of this is 17.71The average value of this is 17.71oo
F per 1,000m.F per 1,000m.
Continued…

 Wet (Saturated) Adiabatic Lapse RateWet (Saturated) Adiabatic Lapse Rate
 When air is fully saturated, then rate of change ofWhen air is fully saturated, then rate of change of
temperature is called wet adiabatic, lapse rate.temperature is called wet adiabatic, lapse rate.
 Its average value is 9.84Its average value is 9.84oo
F per 1,000 m.F per 1,000 m.
Continued…

 Pseudo-Adiabatic Lapse RatePseudo-Adiabatic Lapse Rate
 After condensed particles have fallen down fully, asAfter condensed particles have fallen down fully, as
after a rainfall, then the rate of change ofafter a rainfall, then the rate of change of
temperature is called pseudo-adiabatic lapse rate.temperature is called pseudo-adiabatic lapse rate.
 Its average value is also 9.84Its average value is also 9.84oo
F per 1,000m.F per 1,000m.
Continued…

 Example 2.2Example 2.2
 A parcel of air has a temperature of 50o F on surfaceA parcel of air has a temperature of 50o F on surface
of the earth. At a height of 2,000 m the air becomesof the earth. At a height of 2,000 m the air becomes
saturated. Rainfall occurs and air again becomes drysaturated. Rainfall occurs and air again becomes dry
on the leeward side of a mountain. Find outon the leeward side of a mountain. Find out
temperature of this parcel of air, at an altitude oftemperature of this parcel of air, at an altitude of
2,500 m on leeward side of the hill. Height of hill is2,500 m on leeward side of the hill. Height of hill is
3,000 m.3,000 m.
Continued…

Solution:Solution:
Temperature at 2,000 mTemperature at 2,000 m = 50 - ( 17.72x2000/1000)= 50 - ( 17.72x2000/1000)
= 50 – 35.44= 50 – 35.44
= 14.56 oF= 14.56 oF
Temperature at 3,000 mTemperature at 3,000 m = 14.56 - ( 9.84x1000/1000 )= 14.56 - ( 9.84x1000/1000 )
= 4.72 oF= 4.72 oF
Temperature on theTemperature on the
leeward side at 2,500 mleeward side at 2,500 m = 4.72+ (9.84 x500/1000)= 4.72+ (9.84 x500/1000)
= 9.64 oF= 9.64 oF

Monsoon System in PakistanMonsoon System in Pakistan
 In summer, when the land warms much faster than theIn summer, when the land warms much faster than the
oceans an area of low pressures develops over land andoceans an area of low pressures develops over land and
high pressure at sea.high pressure at sea.
 The reverse is the case during winter season. Therefore,The reverse is the case during winter season. Therefore,
winds move from sea to land during summer and fromwinds move from sea to land during summer and from
land to sea during winter. Winds which undergoland to sea during winter. Winds which undergo
seasonal changes of direction in this way are calledseasonal changes of direction in this way are called
monsoon winds.monsoon winds.
 These winds, and for that matter any other wind systemThese winds, and for that matter any other wind system
are modified considerably when there exist extensiveare modified considerably when there exist extensive
mountain ranges.mountain ranges.
Continued…

 In Winter over Indo-Pak Sub-continent andIn Winter over Indo-Pak Sub-continent and
central part of Asia (slightly towards easterncentral part of Asia (slightly towards eastern
shores of Asia) a ‘High’ is established.shores of Asia) a ‘High’ is established.
 North easterly winds blow over most of theNorth easterly winds blow over most of the
eastern shores of Asia and India.eastern shores of Asia and India.
 These are called the North - West Monsoons.These are called the North - West Monsoons.
These winds are cold and dry and thus give fairThese winds are cold and dry and thus give fair
weather to the area over which these blow.weather to the area over which these blow.
Continued…

 In summer a ‘Low’ is established over North-West partIn summer a ‘Low’ is established over North-West part
of Indo-Pak Sub continent due to heating of the aridof Indo-Pak Sub continent due to heating of the arid
land-tracts while the air in the southern hemisphereland-tracts while the air in the southern hemisphere
cools.cools.
 Over India and Pakistan, the wind blows fromOver India and Pakistan, the wind blows from
southwest and is known as South - West Monsoon.southwest and is known as South - West Monsoon.
 This is a very damp air as it originates from IndianThis is a very damp air as it originates from Indian
Ocean. It actually starts from the southern hemisphereOcean. It actually starts from the southern hemisphere
and after crossing the equator reaches the Sub-and after crossing the equator reaches the Sub-
continent as a southwesterly wind.continent as a southwesterly wind.
Continued…

 In Pakistan the summer monsoon sets in byIn Pakistan the summer monsoon sets in by
June, but its effect over the central areas and theJune, but its effect over the central areas and the
sub-mountain districts becomes wellsub-mountain districts becomes well
pronounced by the third week of July.pronounced by the third week of July.
 The monsoon remains effective over the area tillThe monsoon remains effective over the area till
the end of September, sometime extending tothe end of September, sometime extending to
the second week of October.the second week of October.
Continued…

 Prior to the onset of monsoon, Pakistan is very dry andPrior to the onset of monsoon, Pakistan is very dry and
the maximum temperature crosses 110the maximum temperature crosses 110oo
F over the plainsF over the plains
and the dew point temperature starts rising as soon asand the dew point temperature starts rising as soon as
the monsoon invades Pakistan.the monsoon invades Pakistan.
 By the beginning of June, a “low” establishes overBy the beginning of June, a “low” establishes over
Hyderabad, Khairpur, Multan and Sibi Divisions, whichHyderabad, Khairpur, Multan and Sibi Divisions, which
to some extent, is responsible for the onset of South-to some extent, is responsible for the onset of South-
West monsoon over the sub-continent.West monsoon over the sub-continent.
 The south-westerlies are predominant over theThe south-westerlies are predominant over the
southern areas of Pakistan.southern areas of Pakistan.
Continued…

 The monsoon air requires lifting of 3 to 4The monsoon air requires lifting of 3 to 4
thousands feet to cause thunder showers. Therethousands feet to cause thunder showers. There
being no barrier of such a height for thesebeing no barrier of such a height for these
south-westerlies, the southern area of Pakistansouth-westerlies, the southern area of Pakistan
goes dry, though the monsoon current remainsgoes dry, though the monsoon current remains
active during June to September.active during June to September.

Western DisturbancesWestern Disturbances
 The western disturbances are important air masses, forThe western disturbances are important air masses, for
most of the winter season in Pakistan and Northernmost of the winter season in Pakistan and Northern
India.India.
 Since these disturbances approach from the west, it isSince these disturbances approach from the west, it is
for this reason that these are locally called westernfor this reason that these are locally called western
disturbances.disturbances.
 These originate from the Mediterranean Sea and moveThese originate from the Mediterranean Sea and move
in an easterly or north - easterly direction. They havein an easterly or north - easterly direction. They have
warm and cold fronts but by the time these reachwarm and cold fronts but by the time these reach
Pakistan, these are so diffused together that it isPakistan, these are so diffused together that it is
difficult to distinguish them on surface synoptic charts.difficult to distinguish them on surface synoptic charts.
However they can be identified on the upper-air charts.However they can be identified on the upper-air charts.
Continued…

 During the winter these move at lower latitudes andDuring the winter these move at lower latitudes and
reach northern part of Pakistan, but in summer thesereach northern part of Pakistan, but in summer these
move in higher latitudes through Russian Turkistan.move in higher latitudes through Russian Turkistan.
 Some times these accentuate the monsoon depressionsSome times these accentuate the monsoon depressions
in the summer and cause heavy rainfall in the sub-in the summer and cause heavy rainfall in the sub-
mountain districts, such as Sialkot, Jhelum andmountain districts, such as Sialkot, Jhelum and
Rawalpindi.Rawalpindi.
 The effect of these disturbances is well pronouncedThe effect of these disturbances is well pronounced
from December to March. Normally light rain orfrom December to March. Normally light rain or
drizzle is associated with them, but in case of activedrizzle is associated with them, but in case of active
disturbances, thundershower over northern and centraldisturbances, thundershower over northern and central
divisions of Pakistan may occur.divisions of Pakistan may occur.
Continued…

 The western disturbances also produce secondaryThe western disturbances also produce secondary
disturbance moving over the Persian Gulf and overdisturbance moving over the Persian Gulf and over
the Arabian Sea.the Arabian Sea.
 The secondary wave also moves to Pakistan andThe secondary wave also moves to Pakistan and
causes lot of weather over the entire Pakistan.causes lot of weather over the entire Pakistan.
During Western Disturbances over Pakistan, theDuring Western Disturbances over Pakistan, the
winds are normally Westerlies or North-Westerlies.winds are normally Westerlies or North-Westerlies.
 Sometimes the secondary wave continues moving inSometimes the secondary wave continues moving in
an easterly direction along the coast while thean easterly direction along the coast while the
primary is affecting Peshawar and D.I. Khanprimary is affecting Peshawar and D.I. Khan
Divisions.Divisions.
Continued…

 The following conclusions have been drawn byThe following conclusions have been drawn by
the Pakistan Meteorology Department on thethe Pakistan Meteorology Department on the
basis of detailed study of synoptic situationsbasis of detailed study of synoptic situations
responsible for winter rainfall over Pakistan.responsible for winter rainfall over Pakistan.
 A western disturbances appearing over Turkey takesA western disturbances appearing over Turkey takes
about 7-8 days to reach Pakistan.about 7-8 days to reach Pakistan.
Continued…

 A weak western disturbance does not cause anyA weak western disturbance does not cause any
rainfall over Pakistan. A weak western disturbancerainfall over Pakistan. A weak western disturbance
has been defined as the one which has no closedhas been defined as the one which has no closed
‘Low’ appearing over Turkey. For a well-marked‘Low’ appearing over Turkey. For a well-marked
‘Low’ there are generally two closed isobars and the‘Low’ there are generally two closed isobars and the
lowest pressure reported is generally less than 1,010lowest pressure reported is generally less than 1,010
Mb during December to February.Mb during December to February.
Continued…

 The well-marked western disturbance causes fairlyThe well-marked western disturbance causes fairly
widespread rain and also thunderstorms along thewidespread rain and also thunderstorms along the
frontline over Turkey and Iraq. This can be seenfrontline over Turkey and Iraq. This can be seen
about 5 to 7 days ahead. This type of circulation ofabout 5 to 7 days ahead. This type of circulation of
air mass is a forewarning for an approaching wetair mass is a forewarning for an approaching wet
spell over the northern divisions of Pakistanspell over the northern divisions of Pakistan
Continued…

 The winds over the Gulf of Oman become southerlyThe winds over the Gulf of Oman become southerly
while over north Persian Gulf area northerlies will bewhile over north Persian Gulf area northerlies will be
blowing. This is a strong indication of secondaryblowing. This is a strong indication of secondary
development over the Gulf of Oman. Due to thedevelopment over the Gulf of Oman. Due to the
peculiar orography of northwest Pakistan, thepeculiar orography of northwest Pakistan, the
cyclonic circulation further increases. This results incyclonic circulation further increases. This results in
moderately heavy to heavy showers over the centralmoderately heavy to heavy showers over the central
and northern divisions of Pakistan.and northern divisions of Pakistan.

Monsoon & Western Disturbances AsMonsoon & Western Disturbances As
Cause Of RainfallCause Of Rainfall
 By the beginning of July, South Easterlies begin toBy the beginning of July, South Easterlies begin to
penetrate the central areas and sub-mountain districtspenetrate the central areas and sub-mountain districts
of Pakistan.of Pakistan.
 The South-Easterlies are nothing, but the deflectedThe South-Easterlies are nothing, but the deflected
South-Westerlies, which take a turn westwards parallelSouth-Westerlies, which take a turn westwards parallel
to the Himalayas from the Bay of Bengal.to the Himalayas from the Bay of Bengal.
 A depression is intensified when a warm pool existsA depression is intensified when a warm pool exists
above it. It is also intensified by the interaction betweenabove it. It is also intensified by the interaction between
the disturbances of low latitudes and high latitudes andthe disturbances of low latitudes and high latitudes and
also between disturbances of the two hemispheres.also between disturbances of the two hemispheres.
Continued…

 Bay of Bengal tropical depressions (monsoonBay of Bengal tropical depressions (monsoon
depressions) form as a result of the confluence ofdepressions) form as a result of the confluence of
Southern air mass from the equatorial region andSouthern air mass from the equatorial region and
South-Westerlies crossing the sub-continent from theSouth-Westerlies crossing the sub-continent from the
Arabian Sea.Arabian Sea.
 The South West monsoon air mass behaves as twoThe South West monsoon air mass behaves as two
different types with the consequence that theirdifferent types with the consequence that their
interrelation results in frontogenesis.interrelation results in frontogenesis.
 There also exists a trough of low pressure, generallyThere also exists a trough of low pressure, generally
called monsoon trough, which extends roughly fromcalled monsoon trough, which extends roughly from
Punjab in Pakistan, through Agra, Allahbad in India toPunjab in Pakistan, through Agra, Allahbad in India to
the Bay of Bengal.the Bay of Bengal.
Continued…

 When the axis of the monsoon trough movesWhen the axis of the monsoon trough moves
towards the Himalayas and lies there, the rain on thetowards the Himalayas and lies there, the rain on the
plains decreases while that on the hills increases.plains decreases while that on the hills increases.
 When the axis of the monsoon trough shifts to theWhen the axis of the monsoon trough shifts to the
South, the entire of Pakistan comes under theSouth, the entire of Pakistan comes under the
influences of homogeneous monsoon resulting in ainfluences of homogeneous monsoon resulting in a
continuous type of rain drizzle.continuous type of rain drizzle.
 But occasionally heavy thundershowers do occurBut occasionally heavy thundershowers do occur
over the sub-mountain districts and adjoining areasover the sub-mountain districts and adjoining areas
of Kashmir due to orographic effect (mountainof Kashmir due to orographic effect (mountain
barriers)barriers)
Continued…

 The monsoon depressions travelThe monsoon depressions travel
Westwards/North Westwards. Most of theseWestwards/North Westwards. Most of these
depressions dissipate within a few hundred milesdepressions dissipate within a few hundred miles
of their journey over the sub-continent.of their journey over the sub-continent.
 Those which are strong, reach as far as RajisthanThose which are strong, reach as far as Rajisthan
and then start moving northwards. This changeand then start moving northwards. This change
in the direction of movement of the depressionin the direction of movement of the depression
is called “recurvature”.is called “recurvature”.
Continued…

 When the storm of depression recurves itself, it receivesWhen the storm of depression recurves itself, it receives
good moisture supply from Arabian sea resulting ingood moisture supply from Arabian sea resulting in
heavy rainfall in Pakistan. Some depressions which doheavy rainfall in Pakistan. Some depressions which do
not recurve, emerge into the Arabian Sea and do notnot recurve, emerge into the Arabian Sea and do not
cause any rainfall.cause any rainfall.
 It must be noted that depressions cause lifting of airIt must be noted that depressions cause lifting of air
mass and therefore are the main causative factors formass and therefore are the main causative factors for
precipitation to occur.precipitation to occur.
 Higher and continuous lifting and hence continuousHigher and continuous lifting and hence continuous
precipitation occur when a depression is strengthenedprecipitation occur when a depression is strengthened
by another depression.by another depression.
Continued…

 The three types of depressions in the Indo-PakThe three types of depressions in the Indo-Pak
Subcontinent are:Subcontinent are:
 Seasonal as a result of high summer temperatureSeasonal as a result of high summer temperature
corresponding to low temperature,corresponding to low temperature,
 Monsoon depression traveling from the Bay of Bengal, andMonsoon depression traveling from the Bay of Bengal, and
 Depression due to Western disturbance. When all theDepression due to Western disturbance. When all the
depressions combine and there is a continuous supply ofdepressions combine and there is a continuous supply of
moisture from the Arabian Sea there results very heavymoisture from the Arabian Sea there results very heavy
rainfall in the northern part of Pakistanrainfall in the northern part of Pakistan

Wind MeasurementWind Measurement
 Wind speed is measured with an instrument calledWind speed is measured with an instrument called
AnemometerAnemometer. This instrument gives continuous record over. This instrument gives continuous record over
some graph calledsome graph called AnemographAnemograph..
 A very well known Anemometer is Dynes Apparatus. It givesA very well known Anemometer is Dynes Apparatus. It gives
reading in miles of total wind movement inreading in miles of total wind movement in
24 hours.24 hours.
 Wind has both speed and direction. Wind direction is theWind has both speed and direction. Wind direction is the
direction from which wind is blowing.direction from which wind is blowing.
 Wind speed is usually given in miles per hour, meters perWind speed is usually given in miles per hour, meters per
second or knots.second or knots.
1m/sec = 2.2 mph, and,1m/sec = 2.2 mph, and,
1 knot = 1-1/7 mph1 knot = 1-1/7 mph
Continued…

 The graph between wind speed and elevation isThe graph between wind speed and elevation is
shown in figure 2.1.shown in figure 2.1.
 The equation of the curve is,The equation of the curve is,
V/VV/Voo = (= ( ΖΖ / Z/ Zoo )1/7 - - - - - - - - - - - - - 2.5)1/7 - - - - - - - - - - - - - 2.5
 Where ‘V’ is wind speed at height ‘Z’ fromWhere ‘V’ is wind speed at height ‘Z’ from
ground and ‘Vground and ‘Voo’ is wind speed measured by the’ is wind speed measured by the
anemometer at height ‘Zanemometer at height ‘Z00’.’.
Continued…

Figure 2.1 Variation of Wind Velocity with DepthFigure 2.1 Variation of Wind Velocity with Depth
Velocity Ratio Against Elevation Ratio
0
1
2
3
4
5
6
7
8
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
(V/V0)
(Z/Z0)
Continued…

 Example 2.3Example 2.3
 The speed of air at a height of 15 meter aboveThe speed of air at a height of 15 meter above
ground was measured as 10 m/s. Find the speedground was measured as 10 m/s. Find the speed
at 2 m level.at 2 m level.
SolutionSolution
From Equation 2.5From Equation 2.5
VV22 / V/ V1515 = ( Z= ( Z22 / Z/ Z1515 ))1/71/7
oror
VV22 = ( Z= ( Z22 / Z/ Z1515 ))1/71/7
x Vx V1515
VV22 = ( 2 / 15)= ( 2 / 15)1/71/7
x 10 = 7.5 m/sx 10 = 7.5 m/s
Continued…

QuestionsQuestions
 Define Meteorology. Why is it studied with subject of Hydrology?Define Meteorology. Why is it studied with subject of Hydrology?
 How Relative Humidity and Dew Point are related. DescribeHow Relative Humidity and Dew Point are related. Describe
instrument used for measurement of relative humidity.instrument used for measurement of relative humidity.
 What is saturation vapor pressure? Can air saturate itself up toWhat is saturation vapor pressure? Can air saturate itself up to
100%.100%.
 Define temperature. Explain variation in temperature with respectDefine temperature. Explain variation in temperature with respect
to time.to time.
 What is Lapse Rate? How does temperature changes with altitude?What is Lapse Rate? How does temperature changes with altitude?
 What are monsoons, explain the mechanism of monsoon rainfalls inWhat are monsoons, explain the mechanism of monsoon rainfalls in
Pakistan?Pakistan?
 Explain, what are the Western Disturbances?Explain, what are the Western Disturbances?
 Why psychrometer constant is different when ‘e’ is measured inWhy psychrometer constant is different when ‘e’ is measured in
different units.different units.
 Why wind speed is measured. What is effect of wind on climateWhy wind speed is measured. What is effect of wind on climate
changes in an area? Explain variation of wind velocity with respectchanges in an area? Explain variation of wind velocity with respect
to altitude.to altitude.

ExerciseExercise
 A mass of air is having temperature of 26.9A mass of air is having temperature of 26.9oo
C. TheC. The
Relative humidity of this mass of air is 71 %. DetermineRelative humidity of this mass of air is 71 %. Determine
Saturation Vapor pressure, Saturation deficit, ActualSaturation Vapor pressure, Saturation deficit, Actual
Vapor Pressure, Dew Point and Wet-bulb temperature.Vapor Pressure, Dew Point and Wet-bulb temperature.
 The dry bulb temperature and wet bulb temperature areThe dry bulb temperature and wet bulb temperature are
respectively 25respectively 25oo
C and 15C and 15oo
C at a certain location.C at a certain location.
Determine Dew-point Temperature, Relative Humidity,Determine Dew-point Temperature, Relative Humidity,
Saturation Vapor Pressure and Actual Vapor pressure.Saturation Vapor Pressure and Actual Vapor pressure.
 A mass of air is at temperature of 30A mass of air is at temperature of 30oo
C and is havingC and is having
relative humidity of 75 %. Determine the dew-pointrelative humidity of 75 %. Determine the dew-point
temperature.temperature.
Continued…

ExerciseExercise
 The wind has a speed of 9m/s at elevation of 10mThe wind has a speed of 9m/s at elevation of 10m
above ground and 10m/s at elevation of 15m aboveabove ground and 10m/s at elevation of 15m above
ground. Determine the speed at height of 2m.ground. Determine the speed at height of 2m.
 A parcel of moist air at 45°F, initially at 152 m meanA parcel of moist air at 45°F, initially at 152 m mean
sea level is forced to pass over a mountain ridge ofsea level is forced to pass over a mountain ridge of
2,134 m mean sea level and then descends to its2,134 m mean sea level and then descends to its
original level. Assuming that a lift of 762 m producesoriginal level. Assuming that a lift of 762 m produces
saturation and precipitation, what is the finalsaturation and precipitation, what is the final
temperature of the parcel?temperature of the parcel?

Ch#2 meteorology lect 2

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