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Atmosphere – Weather and Climate

Weather Pertains to the condition of atmosphere at any place at a specific time or for a short period of time (Generally for hours or days)

ClimateAverage weather conditions of a specified area for a considerable time i.e. 30 – 35 years

  • Generally, climate of temperate latitudes is far more variable than that of tropics.
  • The climate of British Isle is so changeable that many people have commented that ‘Britain has no climate, only weather’.
  • Conversely, the climate of Egypt is so static that it makes a good deal of sense when people say that ‘Egypt has no weather, only climate’.
  • Death rates are normally high in tropical countries & low in deserts, because germs are not transmitted readily in regions of high temperature & low humidity.
  • It is because of the variable water content in the atmosphere that we have such great contrasts in weather & climate over different parts of the world.
  • If we were to live in a dry atmosphere, absolutely without water, there would be no weather & not much climate.

 


Elements of Weather & Climate

Rainfall

  • Measured by Rain gauge
  • An inch of rainfall means the amount of water that would cover the ground to a depth of 1 inch, provided none evaporated, percolated or drained off
  • Daily record of rainfall will be added at the end of the month to find the total rainfall for that month
  • Total for each month is again added at the end of the year to find the annual rainfall
  • The mean annual rainfall is obtained from the averages of annual rainfall taken over a long period of say 30 -35 years
  • For plotting in maps, places having the same mean annual rainfall are joined by a line called an isohyet

 


Pressure

  • Measured by barometer, invented by Galileo & Torricelli
  • Air is made up of number of mixed gases & has weight, therefore exerts pressure on the earth’s surface which varies from place to place & from time to time.
  • As shown in diagram, variation in the atmospheric pressure on the mercury surface is balanced by a column of mercury in glass tube.

barometer to measure pressure

  • Any liquid can be used for this purpose, but mercury has been chosen because it is the heaviest liquid known
  • If ordinary water was used, corresponding column for normal atmospheric pressure would be 34 feet
  • On map, places of equal pressure are joined by the lines called isobars
  • Pressure reading at different places varies with a number of factors viz. Altitude, Gravitational forces at different latitudes & Temperature; due to sensitivity of mercury to it.
  • A mercury barometer that dips in liquid mercury is inconvenient for outdoor measurements
  • Hence a more probable but less accurate type known as aneroid barometer is used
  • In airplanes, a modified type of aneroid barometer called altimeter is used & for continuous record of pressure changes a self-recording barogram is used.

 


Temperature

  • Temperature is measured by thermometer, a narrow glass tube filled with mercury or alcohol
  • A temperature taken in open daylight is very high, as it measures the direct insolation of the sun, better described as temperature in the sun, used for measuring temperature for agricultural purposes.
  • But the temperature that we are accustomed to in climatic graphs is shade temperature, i.e. temperature of the air.
  • Hence, precautions must be taken to exclude the intensity of sun’s radiant heat, which is done by placing thermometers in a standard meteorological shelter known as Stevenson screen.
  • Lines joining places of equal temperature in the map are termed as isotherms

 

Importance of Temperature

  • Temperature influences the actual amount of water vapour present in the air & thus decides the moisture carrying capacity of the air.
  • It decides the rate of evaporation & condensation, & therefore governs the degree of stability of the atmosphere.
  • As relative humidity is directly related to the temperature of the air, it affects the nature & types of cloud formation & precipitation

 


Factors influencing Temperature

Latitude
  • Due to the earths inclination, temperature reduces from equator to poles
  • Mainly due to direct & oblique sunrays falling differently on different latitudes
Altitude
  • Since the atmosphere is mainly heated by conduction from the earth
  • Hence places near to earth surface are warmer than those higher up
  • Thus, temperature decreases with increasing height above the sea level
Continentality
  • Land surfaces are heated more quickly than the water surfaces, due to higher specific heat of the water
  • Hence warmer summers & colder winters prevails in continental interiors as compared with maritime districts
Ocean currents & winds
  • Both ocean currents & winds affect temperature by transporting their heat or coldness into adjacent regions
  • For e.g. the westerlies that come to Britain & Norway tend to be cool winds in summer & warm winds in winter
Slope, Shelter & Aspect
  • A steep slope experiences more rapid change in temperature than a gentle one.
  • Mountain ranges that have an eastward alignment like the alps show a higher temperature on the south facing sunny slope than the north facing sheltered slope.
  • The greater insolation of the southern slope is better suited for vine cultivation & has a more flourishing vegetative cover, consequently more settlements
Natural vegetation
  • There is a definite difference in temperature between forested regions & open ground
  • Thick amazon forest cuts off much of incoming insolation keeping the land surface of the jungle cool & few degrees lower than the open spaces in corresponding latitudes
Soil
  • Light soils reflect more heat than the darker ones which are better absorbers of heat, which may give rise to slight variations in temperature of the region.
  • Dry soils like sand are very sensitive to temperature compared to wet clayly soils which retains moisture & warms up & cools down more slowly

Winds

  • The instrument widely used for measuring wind direction is a wind vane or weather clock
  • The speed of the wind is generally measured by anemometer

anemometer

 


Sunshine

  • In meteorological station, sunshine duration is recorded by a sun dial
  • On maps, places with equal sunshine duration are joined by isohels

 


Clouds

  • When air rises, it is cooled by expansion & after dew point has been reached, cooling leads to condensation of water vapour in atmosphere.
  • Tiny droplets of water vapour which are too small to fall as rain or snow will be suspended in the air & float as clouds
  • The amount of cloud cover in the sky is expressed in eighths or Oktas for e.g. 4/8 is half covered & 8/8 is complete overcast
  • On maps, places with an equal degree of cloudiness are joined by lines known as isonephs

Okta scale to measure clouds


Haze

  • The term is usually used connection with the reduction of visibility in regions of low humidity
  • This is caused by smoke & dust particles in industrial areas; or by unequal refraction of light in air of different densities in the lower atmosphere

 


Koeppen’s Scheme of Climate classification

  • Koeppen reorganized a closed relationship b/w vegetation & climate
  • He based his classification on the amount of precipitation & temperature
  • Divided earth in 5 major climatic group, 4 based on temp, 1 on precipitation along with cold climate of high lands
  • The Koeppen system recognizes five major climatic types; each type is designated by a capital letter
Tropical A Average temp. of coldest month > = 18*C
Dry Climates B Evaporation > Precipitation
Warm Temperate C Av. temp. of coldest month 18*C > x >  3*C (mid latitudes)
Cold snow forest climate D Average temp. of coldest month <= 3*C
Cold Climates E Average temp. of all month < 10*C
High Land Climate H Cold due to elevation

Koeppen Scheme of Climate classification

Amw Monsoon with short dry season West Coast below Goa
As Monsoon with dry summer East Coast Tamilnadu
Aw Tropical Savanna Most of the Peninsular plateaus, south of the Tropic of Cancer
Bshw Semi-arid steppe climate North-western Gujarat, some parts of western Rajasthan and Punjab
Bwhw Hot desert Extreme western Rajasthan
Cwg Monsoon with dry winter Ganga plain, eastern Rajasthan, northern Madhya Pradesh, most of North-east India
Dfc Cold humid winter with short summer Arunachal Pradesh
E Polar type Jammu and Kashmir, Himachal Pradesh and Uttarakhand

Merits of Koeppen’s Scheme of classification

  • Because of a visible association of vegetation with climatic types, Keoppen’s classification becomes all the more appealing to geographers.
  • It is possible to assign a given place to a particular climatic sub-group only on the basis of certain easily acquired statistics about an area’s temperature and precipitation.
  • This system of climatic classification is descriptive and generalized
  • It uses a shorthand code of letters for the climatic types, so that repetition of descriptive terms becomes unnecessary.
  • All the major climate groups, sub-groups, and further subdivisions are described by a combination of letters.
  • This classification is so simple and detailed that it can be easily used at different educational levels.

 

Limitations of Koeppen’s Scheme of classification

  • Koeppen based his classification on the mean monthly values of temperature and precipitation.
  • By these statistics the most potent factor of precipitation can only be estimated, rather than measured accurately.
  • It is empirical and, therefore, is based on facts and observations.
    The causative factors of climate have been totally ignored. Thus, the air masses, which form the very basis of modern climatology, could not find any place in Koppen’s classification.
  • The letter symbols used by Koeppen in his climatic classification provide international shorthand describing climatic regions that are rather difficult to characterize in words.

 


Thornthwaite Scheme of Climate classification

  • It is based on concept of water balance i.e. rate of precipitation – rate of evaporation.
  • An area is arid if there is water deficiency in all months
  • An area is humid if it has water surplus in all months

Thornthwaite Scheme of Climate classification

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