2. Meteorology

2. Meteorology

Dr. Muhammad Ali [email protected]

• The science related to atmosphere and its phenomena.

• From book by Aristotle (340 B.C.) called Meteorologicawhich explored everything known about weather andp y gclimate at that time (as well as chemistry, astronomyand geography).

• At that time, all substances that fell from the sky werecalledmeteorscalledmeteors

• Falling objects from outside the atmosphereFalling objects from outside the atmosphere(meteoroids) and water and ice particles falling fromclouds within the atmosphere (hydrometeors)

• Important for estimation of precipitation and itsformation, required for regional climate processes ando at o , equ ed o eg o a c ate p ocesses a ddesign.

AtmosphereAtmosphere• The gaseous envelope around the earth It consists of dry• The gaseous envelope around the earth. It consists of dry

air, water vapor and various kinds of salts and dusts.

• 99% of the atmosphere lies within 19 miles (30 km) ofearth’s surface (piece of paper covering a ball)earth s surface (piece of paper covering a ball).

Shi ld f i h bi f d di• Shields surface inhabitants from dangerous radiantenergy (e.g. ultraviolet from the sun).

• Becomes thinner with increasing altitude, eventuallymerging with outer space

TroposphereTroposphere

• It is the zone of atmosphere adjacent to earth.It extends approximately up to seven milespp y pabove sea level.

• Almost 100 % of the total moisture contentsof the atmosphere are present in this zoneand there is comparatively high temperatureand there is comparatively high temperaturegradient in this part of atmosphere.

Vapor PressureVapor Pressure• It is the pressure exerted by the amount of waterIt is the pressure exerted by the amount of watervapors present in the atmosphere. It is usually denotedby “e or ea” and expressed in millibars, pascals (Pa) ory a p pKilo‐Pascals (KPa). (1 millibar =100 pascals)

• Since the temperature of water vapor is the same asthat of the air in the atmosphere, the maximumamount of water vapor may be said to depend on theair temperature.

• The higher the temperature, the more vapor can thet h i i h ldatmospheric air hold

Saturation Vapor PressureSaturation Vapor Pressure

Th i l “h ld” t i b f t• The air can only “hold” a certain number of water vapormolecules before they become so crowded together thatthey start sticking together to form liquid water droplets

• When the air reaches this point it is said to be “saturated”

• The amount of water vapor molecules (vapor pressure) thatthe air can “hold” or the air’s water vapor capacity is calledthe air can hold or the air s water vapor capacity is calledthe saturation vapor pressure (SVP)

d d b ‘ ’• It is denoted by ‘es’.

• In warm air, the molecules are moving fast, so, g ,they collide and readily bounce off each other,making it difficult for them to stick together.

• In cold air the molecules are moving slowIn cold air, the molecules are moving slow,making it easier for them to stick together whenthey collide.they collide.

Th f t ti i• Therefore, saturation vapor pressure isdependent upon the air temperature, with coldair being able to hold less water vapor beforeair being able to hold less water vapor beforebecoming saturated (lower saturation vaporpressure) than warm airpressure) than warm air

IsobarsIsobars

• These are the lines joining points of sameatmospheric pressure at a given elevation.p p g

Th h i l di ib i f i• The horizontal distribution of pressure isgenerally shown on weather charts by isobars.

Th i b t i b i f• The spacing between isobars is a measure ofthe pressure gradient.

Relative HumidityRelative Humidity

l i h idi d h l• Relative humidity does not measure the actualamount of water vapor in the atmosphere.

• It is a measure of how close the air is to beinggsaturated.

• Relative Humidity (RH) is the ratio of the amountof water vapor actually in the air to theof water vapor actually in the air to themaximum amount of water vapor required forsaturation at that particular temperaturesaturation at that particular temperature

• In other words, RH is a ratio of air’s waterIn other words, RH is a ratio of air s watervapor content to its capacity:

• RH = (ea/es)x 100H ( a s)

• Air with 50% RH contains only half the amountof water vapor necessary for saturation andair with 100% RH is fully saturated

Dew PointDew Point

• When the air is cooled at a constantatmospheric pressure, the temperature atp p pwhich air becomes saturated is called Dewpointpoint.

• The dew point is the temperature at whichwater vapor present in the atmosphere willwater vapor present in the atmosphere willcondense if the air is cooled.

Measurement of Relative HumidityMeasurement of Relative Humidity

• The instruments used for measurements of relative humidity are:y– Psychrometer

Hair hygrometer– Hair hygrometer

– Hygrograph

– Thermo‐hygrograph

Measurement of Relative HumidityMeasurement of Relative Humidity• Psychrometer consists of two thermometers ‐ Dry bulbthermometer and Wet bulb thermometer.

• The mercury bulb of wet bulb thermometer is coveredby a jacket of clean muslin cloth saturated with water.

• This is done by putting a beaker with distilled watery p gunderneath so that the bulb is not submerged in waterbut only the cloth.

• Then water rises due to capillary action.p y

• The thermometers areventilated by whirling ory gby use of a fan.

• As a result of evaporationcooling takes place.Readings are taken onboth the thermometerssimultaneouslysimultaneously


h d b lb di i d d d• The dry bulb reading is denoted as Ta and wetbulb reading as Tw. The difference of thesetwo temperatures is called the wet bulbdepression.

i e T ‐T =Wet bulb depressioni.e. Ta Tw=Wet bulb depression

• Using these readings, the relative humiditycan be found from the psychrometer tables.


• The value of ‘ea’ for air temperature ‘Ta’ may beobtained by the relation:

( es – ea ) = γ ( Ta ‐ Tw )

oror

ea = es – γ (Ta – Tw )

• Where γ = psychrometer constant and its value is0.660 when ‘ea’ is measured in millibar units and0.485 when it is measured in units of ‘mm of Hg’.


Given the air temperature as Ta = 24o C and the wet bulb temperature Tw = 21o C. Find p wrelative humidity and dew point.


S l tiSolutionWet bulb depression = 24 ‐ 21 = 3o C.From table A‐1 Appendix‐A we getFrom table A‐1, Appendix‐A, we get,For Ta = 24 oC, es = 22.27 mm of HgFor Tw = 21 oC ew = 18.65 mm of Hgw g

ea = 18.65‐0.485(3) = 17.20 mm of HgR l i h idi [ 17 20 / 22 27 ] 100 77 %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 ’ whichTable A‐1 of Appendix‐A corresponding to ea which is 19.7oC.


• The hair hygrometer consists of a frame in which• The hair hygrometer consists of a frame in whicha strand of hair is kept at approximately constanttension.

• Changes in length of the hair corresponding toh i l i h idi i dchanges in relative humidity are transmitted to apointer.

• This instrument is seldom used formeteorological purposes, but it is an inexpensiveg p p , phumidity indicator and is often found in homesand offices.


Th h i h h i ti ll h i• The hair hygrograph is essentially a hairhygrometer, but is automatic recordinginstrumentinstrument.

• The movement of hair activates a pen which• The movement of hair activates a pen, whichrecords on a rotating drum.

• The hygrothermograph combines the registrationof both relative humidity and temperature onof both relative humidity and temperature onone record sheet.

Solar RadiationSolar Radiation

• The only source of heat energy for earthsystem is the sun. Whatever heat is receivedyby earth or reflected is the solar energy.

• Depending upon the shape, rotation, angle ofinclination of earth, it is visualized that solarenergy received by earth is changing fromenergy received by earth is changing fromtime to time and point to point.



• Solar energy received by sun is the maximum at the equator and decreases pole‐wards. The q pvariations in solar energy are the following:

– Diurnal Variation

– Seasonal Variation

– Regional Variation.g

Solar RadiationSolar Radiation• The units for radiation flux per unit area are eitherThe units for radiation flux per unit area are eitherjoules per square centimeters or milliwatt hours persquare centimeter.square centimeter.

I t i l i ti t i• In some countries calories per square centimeters isdesignated as Langley and the corresponding unit ofdi ti fl i L l i tradiation flux is Langley per minute.

• Pyrheliometer and Pyranometer is used formeasurement.

Temperaturep

• The degree of hotness or coldness is called theThe degree of hotness or coldness is called thetemperature. Its units of measurement areDegree Centigrade ( °C ) or Degree FahrenheitDegree Centigrade ( C ) or Degree Fahrenheit( °F ).

• Temperature Variation– There are three types of temperature variations.

• Daily Variation of Temperature• Seasonal Variation• Regional Variation of Temperature

• Daily Variation of TemperatureThe daily variation of temperature is dependent on the– The daily variation of temperature is dependent on theelevation of the sun, the cloud cover and the wind speed.

– The variation of temperature is large in low latitudes andsmall in high latitudes, with the result that the dailyvariation decreases from equator towards poles.variation decreases from equator towards poles.

– The clouds reduce the radiations coming down during thed d i di i d i h i h O l dday and outgoing radiations during the night. On a cloudyday, the maximum temperature is lower and the minimumtemperature higher than on normal bright days.

– On windy days the temperature on ground surface is lowerthan on calm days because the greater mobility of airthan on calm days, because the greater mobility of airalong the vertical axis results in greater heat exchange withthe upper atmospheric layers.

• Seasonal Variation– The seasonal variation in rainfall and wind also affectthe temperature.

– During the rainy season, the cloud cover is large withthe result that less radiation is received by the earth.y

– Annual migration of vast masses of air also bringsb h l h h d h ff habout horizontal heat exchange and thus affects theannual range of temperature variation.

• Regional Variation of Temperature– Since the amount of net radiation decreases withSince the amount of net radiation decreases withincreasing latitude, the temperature tends to behighest at the equator and decreases towards the

lpoles.

• The temperature is measured with the help of thermometers.There are two types of thermometers ‐ MaximumThere are two types of thermometers ‐ Maximumthermometer (Mercury Type thermometer) and MinimumThermometer (Alcoholic type thermometer).

• In order to measure the air temperature properly,th t t b l d h i i l ti i l ti lthermometers must be placed where air circulation is relativelyunobstructed and yet they must be protected from the directsunrays and from precipitation.y p p

• Therefore thermometers are placed in white, louvered,wooden boxes, called instrument shelters. These shelters areset about 4.5 feet above the ground.

• Four commonly used terms of temperature are:– Mean Daily TemperatureMean Daily Temperature

– Normal Daily Temperature

– Mean Monthly TemperatureMean Monthly Temperature

– Mean Annual Temperature

• Mean Daily Temperaturey p– It is the average of maximum and minimumtemperatures during the past 24 hourstemperatures during the past 24 hours.

• Normal Daily Temperature– It is the average daily mean temperature for agiven day over the past 30‐years period i. e. it isgiven day over the past 30 years period i. e. it isthe mean temperature for a specific day

• Mean Monthly Temperature– It is the average of the mean monthly maximum and minimum temperatures or it is the mean ptemperature of the mean daily temperatures during the month.g

• Mean Annual Temperature– It is the mean temperature of 12 months.

Lapse Ratep• The lapse rate or vertical temperature gradient isdefined as the change in temperature per unitdefined as the change in temperature per unitdistance in the vertical direction from the Earthsurface.

• The average value of the lapse rate is 3.6oF per 1000ft.

• The greatest variation in lapse rate is found in thel f i j b h l d f Th llayer of air just above the land surface. The lapserates are of three types depending upon the type ofwater vaporswater vapors.– Dry Adiabatic Lapse Rate– Wet (Saturated) Adiabatic Lapse Rate– Pseudo‐Adiabatic Lapse Rate

• Dry Adiabatic Lapse Rate– It is the rate of change of temperature when air isnot fully saturated with water vapors.y p

– The average value of this is 17.71oF per km.

• Wet (Saturated) Adiabatic Lapse Rate– When air is fully saturated, then rate of change oftemperature is called wet adiabatic, lapse rate.p p

– Its average value is 9.84o F per km.

• Pseudo‐Adiabatic Lapse Rate– After condensed particles have fallen down fully, p y,as after a rainfall, then the rate of change of temperature is called pseudo‐adiabatic lapse rate.p p p

Its average value is also 9 84o F per 1 000m– Its average value is also 9.84o F per 1,000m.

Lapse RateLapse Rate

A parcel of air has a temperature of 50o F onsurface of the earth. At a height of 2,000 m the airbecomes saturated. Rainfall occurs and air againbecomes dry on the leeward side of a mountain.Find out temperature of this parcel of air, at analtitude of 2,500 m on leeward side of the hill.Height of hill is 3,000 m.

Lapse RateLapse Rate

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

leeward side at 2,500 m = 4.72+ (9.84 x500/1000)

= 9.64oF

Monsoon System in Pakistan

• In summer, when the land warms much faster thanthe oceans an area of low pressures develops overland and high pressure at sea.

• The reverse is the case during winter season.Therefore, winds move from sea to land duringsummer and from land to sea during winter. Windshi h d l h f di i i hiwhich undergo seasonal changes of direction in this

way are called monsoon winds.

• These winds, and for that matter any other winddifi d id bl h h isystem are modified considerably when there exist

extensive mountain ranges.

• In winter over Indo‐Pak Sub‐continent andcentral part of Asia (slightly towards easterncentral part of Asia (slightly towards easternshores of Asia) a ‘High’ is established.

• North easterly winds blow over most of theyeastern shores of Asia and India.

• These are called the North ‐ East Monsoons.

• These winds are cold and dry and thus giveese ds a e co d a d d y a d t us g efair weather to the area over which theseblow.blow.

• In summer a ‘Low’ is established over North‐Westpart of Indo‐Pak Sub continent due to heating of thearid land‐tracts while the air in the southernhemisphere coolshemisphere cools.

d d k h d bl f• Over India and Pakistan, the wind blows fromsouthwest and is known as South ‐West Monsoon.

• This is a very damp air as it originates from Indianll f h hOcean. It actually starts from the southern

hemisphere and after crossing the equator reachesthe Sub continent as a southwesterly windthe Sub‐continent as a southwesterly wind.

• In Pakistan the summer monsoon sets in byJune but its effect over the central areas andJune, but its effect over the central areas andthe sub‐mountain districts becomes well

d b th thi d k f J lpronounced by the third week of July.

• The monsoon remains effective over the areatill the end of September sometime extendingtill the end of September, sometime extendingto the second week of October.

• Prior to the onset of monsoon, Pakistan is very dryand the maximum temperature crosses 110oF overthe plains and the dew point temperature starts

h d krising as soon as the monsoon invades Pakistan.

• By the beginning of June, a “low” establishes overHyderabad, Khairpur, Multan and Sibi Divisions,hi h i ibl f h fwhich to some extent, is responsible for the onset of

South‐West monsoon over the sub‐continent.

• The south‐westerlies are predominant over theh f P kisouthern areas of Pakistan.

Th i i lif i f 3 4• The monsoon air requires lifting of 3 to 4thousands feet to cause thunder showers.There being no barrier of such a height forthese south‐westerlies, the southern area ofthese south westerlies, the southern area ofPakistan goes dry, though the monsooncurrent remains active during June tocurrent remains active during June toSeptember.

Western Disturbances

• The western disturbances are important air masses,p ,for most of the winter season in Pakistan andNorthern India.

• Since these disturbances approach from the west, itis for this reason that these are locally called westernis for this reason that these are locally called westerndisturbances.

• These originate from the Mediterranean Sea andmove in an easterly or north ‐ easterly direction.y yThey have warm and cold fronts but by the timethese reach Pakistan, these are so diffused togetherthat it is difficult to distinguish them on surfacethat it is difficult to distinguish them on surfacesynoptic charts. However they can be identified onthe upper‐air charts.pp

• During the winter these move at lower latitudes andreach northern part of Pakistan, but in summer thesereach northern part of Pakistan, but in summer thesemove in higher latitudes through Russian Turkistan.

S i h h• Some times these accentuate the monsoondepressions in the summer and cause heavy rainfallin the sub‐mountain districts, such as Sialkot, Jhelum, ,and Rawalpindi. (as in 2010)

Th ff t f th di t b i ll d• The effect of these disturbances is well pronouncedfrom December to March. Normally light rain ordrizzle is associated with them, but in case of activedi b h d h h ddisturbances, thundershower over northern andcentral divisions of Pakistan may occur.

• A western disturbances appearing over Turkey takesabout 7‐8 days to reach Pakistan.

• The western disturbances also produce secondaryp ydisturbance moving over the Persian Gulf andover the Arabian Sea.

• The secondary wave also moves to Pakistan andcauses lot of weather over the entire Pakistancauses lot of weather over the entire Pakistan.During Western Disturbances over Pakistan, thewinds are normally Westerlies or North‐yWesterlies.

• Sometimes the secondary wave continues movingin an easterly direction along the coast while theprimary is affecting Peshawar and D I Khanprimary is affecting Peshawar and D.I. KhanDivisions.

Wind Measurement

• Wind speed is measured with an instrument calledAnemometerAnemometer.

• This instrument gives continuous record over some graphcalled Anemograph.

• A very well known Anemometer is Dynes Apparatus. Itgives reading in miles of total wind movement in24 hours.

• Wind has both speed and direction. Wind direction is thedirection from which wind is blowing.g

• Wind speed is usually given in miles per hour, meters persecond or knots.

1m/sec = 2.2 mph

• The equation of the curve is,The equation of the curve is,

V/Vo = ( Ζ / Zo )1/7

• Where ‘V’ is wind speed at height ‘Z’ fromground and ‘Vo’ is wind speed measured bythe anemometer at height ‘Z0’.0

Velocity Ratio Against Elevation Ratio

678

34

56

(Z/Z

0)

0123(

00.0 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)

The speed of air at a height of 15 meter abovep gground was measured as 10 m/s. Find thespeed at 2 m level.speed at e e

S l tiSolutionV2 / V15 = ( Z2 / Z15 )1/7 orV ( Z / Z )1/7 VV2 = ( Z2 / Z15 )1/7 x V15

V2 = ( 2 / 15)1/7 x 10 = 7.5 m/s

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2. Meteorology

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