UPSC Prelims 2020 Important Topics: Climatology, Atmosphere, Pressure & Temperature systems, Rainfall

3. TEMPERATURE DISTRIBUTION

3.1. HEAT TRANSFER

  • The earth receives almost all of its energy from the sun.
  • The earth in turn radiates back to space the energy received from the sun. As a result, the earth neither warms up nor does it get cooled over a period of time. Thus, the amount of heat received by different parts of the earth is not the same.
  • This variation causes pressure differences in the atmosphere. This leads to transfer of heat from one region to the other by winds.

3.2. INSOLATION

  • The earth’s surface receives most of its energy in short wavelengths.
  • The energy received by the earth is known as incoming solar radiation which in short is termed as insolation.
  • As the earth is a geoid resembling a sphere, the sun’s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun’s energy.
  • On an average the earth receives 1.94 calories per sq.cm per minute at the top of its atmosphere.
  • The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the sun.
  • During its revolution around the sun, the earth is farthest from the sun (152 million km on 4th July). This position of the earth is called APHELION.
  • On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelion.
  • Therefore, the annual insolation received by the earth on 3rd January is slightly more than the amount received on 4th July.
  • However, the effect of this variation in the solar output is masked by other factors like the distribution of land and sea and the atmospheric circulation.
  • Hence, this variation in the solar output does not have great effect on daily weather changes on the surface of the earth.

3.3. VARIABILITY OF INSOLATION AT THE SURFACE OF THE EARTH

  • The amount and the intensity of insolation vary during a day, in a season and in a year.
  • The major factors that cause variations in insolation are:
    (i) the rotation of earth on its axis;
    (ii) the angle of inclination of the sun’s rays;
    (iii) the length of the day;
    Other minor causes
    (i) the transparency of the atmosphere;
    (ii) the configuration of land in terms of its aspect.
  • The fact that the earth’s axis makes an angle of 66½° with the plane of its orbit round the sun has a greater influence on the amount of insolation received at different latitudes.
  • The second factor that determines the amount of insolation received is the angle of inclination of the rays. This depends on the latitude of a place.
  • The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays.
  • The area covered by vertical rays is always less than the slant rays.
  • If more area is covered, the energy gets distributed and the net energy received per unit area decreases.
  • Moreover, the slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and

MARCH 21 – VERNAL EQUINOX

diffusion

  • Sun is above the Equator.
  • Day and Night are equal throughout the world.
  • Spring season begins from vernal equinox.

JUNE 22 – SUMMER SOLSTICE

  • Sun moves towards North and is above the Tropic of Capricorn.
  • In the Northern Hemisphere, daytime is lengthier than night time.
  • Summer in Northern Hemisphere and winter in Southern Hemisphere.
  • In North Pole (Arctic) sunrays will fall 6 month during this period.

SEPTEMBER 23 – AUTUMNAL EQUINOX

  • Sun is above the Equator.
  • Day and Night is Equal throughout the world.
  • This is the beginning of Autumn Season.

DECEMBER 22 – WINTER SOLSTICE

  • Sun is above the Tropic of Capricorn.
  • Winter in Northern Hemisphere and summer in Southern Hemisphere.
  • In Antarctica 6 months sun rays (daytime) during this period. But in Arctic 6 months night time.http://iasgatewayy.com/wp-content/uploads/2019/06/Seasonal.png

THE PASSAGE OF SOLAR RADIATION THROUGH THE ATMOSPHERE

  • The atmosphere is largely transparent to short wave solar radiation.
  • The incoming solar radiation passes through the atmosphere before striking the earth’s surface.
  • Within the troposphere, water vapour, ozone and other gases absorb much of the near infrared radiation.
  • Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds colour to the sky.
  • The red colour of the rising and the setting sun and the blue colour of the sky are the result of scattering of light within the atmosphere.

SPATIAL DISTRIBUTION OF INSOLATION AT THE EARTH’S SURFACE

  • The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles.
  • Maximum insolation is received over the subtropical deserts, where the cloudiness is the least.
  • Equator receives comparatively less insolation than the tropics.
  • Generally, at the same latitude the insolation is more over the continent than over the oceans.
  • In winter, the middle and higher latitudes receive less radiation than in summer.

HEATING AND COOLING OF ATMOSPHERE

  • There are different ways of heating and cooling of the atmosphere.
  • The earth after being heated by insolation transmits the heat to the atmospheric layers near to the earth in long wave form.
  • The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated. This process is called conduction.
  • Conduction takes place when two bodies of unequal temperature are in contact with one another; there is a flow of energy from the warmer to cooler body.
  • The transfer of heat continues until both the bodies attain the same temperature or the contact is broken.

CONDUCTION & CONVECTION

  • Conduction is important in heating the lower layers of the atmosphere.
  • The air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat of the atmosphere. This process of vertical heating of the atmosphere is known as convection.
  • The convective transfer of energy is confined only to the troposphere.

ILLUSTRATION FOR HEAT BUDGET

For example out of 100 units, 35 units are reflected back to space even before reaching the earth’s surface. Of these, 27 units are reflected back from the top of the clouds and 2 units from the snow and ice-covered areas of the earth. The reflected amount of radiation is called the albedo of the earth. The remaining 65 units (out of 100) are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface. The earth radiates back 51 units in the form of terrestrial radiation. Of these, 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere (6 units absorbed directly by the atmosphere, 9 units through convection and turbulence and 19 units through latent heat of condensation). 48 units absorbed by the atmosphere (14 units from insolation +34 units from terrestrial radiation) are also radiated back into space. Thus, the total radiation returning from the earth and the atmosphere respectively is 17+48=65 units which balance the total of 65 units received from the sun. This is termed the heat budget or heat balance of the earth.

ADVECTION

  • The transfer of heat through horizontal movement of air is called advection.
  • Horizontal movement of the air is relatively more important than the vertical movement. In middle latitudes, most of diurnal (day and night) variation in daily weather is caused by advection alone.
  • In tropical regions particularly in northern India during summer season local winds called ‘loo’ is the outcome of advection process.

3.4. TERRESTRIAL RADIATION

  • The insolation received by the earth is in short waves forms and heats up its surface.
  • The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in long wave form. This energy heats up the atmosphere from below. This process is known as terrestrial radiation.
  • The long wave radiation is absorbed by the atmospheric gases particularly by carbon dioxide and the other greenhouse gases.
  • Thus, the atmosphere is indirectly heated by the earth’s radiation.

The atmosphere in turn radiates and transmits heat to the space. Finally the amount of heat received from the sun is returned to space, thereby maintaining constant temperature at the earth’s surface and in the atmosphere.

3.5. HEAT BUDGET OF THE PLANET EARTH

  • The earth as a whole does not accumulate or lose heat. It maintains its temperature.
  • This can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation.
  • While passing through the atmosphere some amount of energy is reflected, scattered and absorbed. Only the remaining part reaches the earth surface.
  • Heat budget explains, why the earth neither warms up nor cools down despite the huge transfer of heat that takes place.

3.5.1. Variation in the Net Heat Budget At The Earth’s Surface

  • As explained earlier, there are variations in the amount of radiation received at the earth’s surface.
  • Some part of the earth has surplus radiation balance while the other part has deficit.
  • There is a surplus of net radiation balance between 40 degrees north and south and the regions near the poles have a deficit.
  • The surplus heat energy from the tropics is redistributed pole wards and as a result the tropics do not get progressively heated up due to the accumulation of excess heat or the high latitudes get permanently frozen due to excess deficit.

3.6. TEMPERATURE

  • The interaction of insolation with the atmosphere and the earth’s surface creates heat which is measured in terms of temperature.

While heat represents the molecular movement of particles comprising a substance, the temperature is the measurement in degrees of how hot (or cold) a thing (or a place) is.

3.6.1. Factors Controlling Temperature Distribution

  • The temperature of air at any place is influenced by (i) the latitude of the place; (ii) the altitude of the place; (iii) distance from the sea, the air mass circulation; (iv) the presence of warm and cold ocean currents; (v) local aspects.

i. THE LATITUDE

  • The temperature of a place depends on the insolation received.
  • It has been explained earlier that the insolation varies according to the latitude hence the temperature also varies accordingly.

ii. THE ALTITUDE

  • The atmosphere is indirectly heated by terrestrial radiation from below.
  • Therefore, the places near the sea-level record higher temperature than the places situated at higher elevations. In other words, the temperature generally decreases with increasing height.
  • The rate of decrease of temperature with height is termed as the normal lapse rate. It is 6.5°C per 1,000 m.

iii. DISTANCE FROM THE SEA

  • Another factor that influences the temperature is the location of a place with respect to the sea.
  • Compared to land, the sea gets heated slowly and loses heat slowly and Land heats up and cools down quickly.
  • Therefore, the variation in temperature over the sea is less compared to land.
  • The places situated near the sea come under the moderating influence of the sea and land breezes which moderate the temperature.

iv. AIR-MASS AND OCEAN CURRENTS

  • Like the land and sea breezes, the passage of air masses also affects the temperature.
  • The places, which come under the influence of warm air-masses
  • experience higher temperature and the places that come under the influence of cold air masses experience low temperature.
  • Similarly, the places located on the coast where the warm ocean currents flow record higher temperature than the places located on the coast where the cold currents flow.

Isotherms

  • It follows the parallels of Latitudes in an east west direction.
  • There is a shift in the position of isotherms with the change of season.
  • Where horizontal temperature changes are large, Isotherms are closely spaced.
  • Where horizontal temperature differences are less, Isotherms are widely spaced.
  • Due to differential heating of land and water, temperature above the oceans and land masses varies even on the same latitude. Isotherms, therefore, bend slightly while crossing from landmasses to oceans and vice versa.

3.7. HORIZONTAL DISTRIBUTION OF TEMPERATURE

  • Normally, Temperature decreases from Equator to pole.
  • The highest temperatures are found in the tropics and sub-tropics.
  • They receive the largest amount of insolation throughout the year. On the other hand, lowest temperatures are recorded in Polar Regions, where the amount of solar energy received is very small.
  • The temperature distribution is generally shown on the map with the help of isotherms and the horizontal distribution of temperature is represented and studied with the help of isotherms.
  • The Isotherms are lines joining places having equal temperature.
  • In general, the effect of the latitude on temperature is well pronounced on the map, as the isotherms are generally parallel to the latitude.
  • In the northern hemisphere the land surface area is much larger than in the southern hemisphere. Hence, the effects of land mass and the ocean currents are well pronounced.
  • Isotherms within the tropics are widely spaced as temperature gradient is very gentle and insignificant.
  • The temperature gradient is very steep in higher latitudes as well as on the eastern margins of the continents.

3.8. STUDY OF TEMPERATURE DISTRIBUTION

  • For most places on the earth, January and July represent the seasonal extremes of temperature.
  • Therefore, the global distribution of temperature can well be understood by studying the temperature distribution in January and July.

3.8.1. Global Distribution of Temperature In January

  • The sun shines almost vertically over Tropic of Capricorn in the month of
  • It is winter in the northern hemisphere and summer in the southern hemisphere.
  • In the northern hemisphere, land mass is cooler than the oceans.
  • As a result, lowest temperature occurs in north-east Asia and Greenland. Verkhoyansk (Siberia) experiences mean January temperature of -500 C.
  • In the southern hemisphere, the conditions during this season are just the reverse.
  • Temperature is, therefore, high over the land mass in the southern hemisphere rising over 300 C in four areas – north – west Argentina, east – central Africa, Borneo and Central Australia.
  • The effect of the ocean is well pronounced in the southern hemisphere. Here the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere.

ISOTHERMS DURING JANUARY

  • In January, the isotherms deviate to the north over the ocean and to the south over the continent. This can be seen on the North Atlantic Ocean.
  • The presence of warm ocean currents like Gulf Stream and North Atlantic drift, make the Northern Atlantic Ocean warmer and the isotherms bend towards the north.
  • Over the land the temperature decreases sharply and the isotherms bend towards south in Europe. It is much pronounced in the Siberian plain.
  • As the air over the ocean is warmer than that over the landmasses in the northern hemisphere, the isotherms bend equator ward while crossing the landmasses and poleward while crossing the oceans.
  • Therefore, the isotherms bend equator ward while crossing the oceans and pole ward while crossing the landmasses.
  • Due to the presence of vast expanse of landmasses, isotherms are irregular and closely spaced in the northern hemisphere.
  • They are more regular and widely spaced in the southern hemisphere.

3.8.2. Global Distribution of Temperature in July

  • In July, the isotherms generally run parallel to the latitude.
  • At this time of the year, the sun shines almost vertically above the Tropic of Cancer in the northern hemisphere.
  • It is summer for the northern hemisphere and winter for the southern hemisphere.
  • Maximum temperature of over 300 C occurs entirely in the northern hemisphere between 100 and 400 N latitudes. The areas include the south-eastern USA, the Sahara, Arabia, Iraq, Iran, Afghanistan, large part of China and a small part of south India.

PHYSICAL GEOGRAPHY

  • However, the temperature remains below freezing point in Greenland and the mountain highlands.
  • The highest range of temperature is more than 60°C over the north-eastern part of Eurasian continent. This is due to continentality.
  • The least range of temperature 3°C, is found between 20° S and 15° N.

ISOTHERMS DURING JULY

  • In the northern hemisphere, the isotherms bend equator ward while crossing the oceans and poleward while crossing the land masses.
  • In the southern hemisphere, it is vice versa. Isotherms reveal wider spacing on the ocean than on the continents.

3.9. VERTICAL DISTRIBUTION OF TEMPERATURE

  • Temperature decreases with increasing height in the troposphere but the rate of decrease varies according to seasons, duration of sunshine and location.
  • On an average, the rate of decrease of temperature with increasing altitudes in a stationary column of air with absence of any vertical motion is 6.50 C per 1000 metres.
  • This decrease of temperature is called vertical temperature gradient or normal lapse rate. The decrease of temperature upward in the atmosphere proves the fact that the atmosphere gets heat from the earth’s surface through the processes of conduction, radiation and convection.

3.10. INVERSION OF TEMPERATURE

  • Under normal conditions, the temperature of the atmosphere falls with altitude.
  • But there are some special conditions under which the atmospheric temperature increases instead of decreasing with height.
  • This rise of temperature with height is known as inversion of temperature.
  • It is clear that in case of inversion of temperature, the air near the earth’s surface is cold while higher above it is warm.

3.10.1. Following Conditions Favour Inversion Of Temperature

1. LONG NIGHTS

  • Insolation is received during day time and it is radiated during night.
  • The earth’s surface cools down at night due to radiation.
  • The air of the lower layer touching the earth’s surface is sufficiently cooled while the air of upper layer is still warm.
  • Thus, long nights are helpful for inversion of temperature.

2. CLEAR SKY

  • Clear sky is essential for reflection of heat radiations by earth’s surface thereby cooling it.
  • Clouds obstruct this reflection and hamper the occurrence of inversion of temperature.

3. STABLE WEATHER

  • Continuous radiation of heat is possible in a stable weather.
  • This condition leads to temperature inversion. Change in weather disturbs the temperature inversion.

4. DRY AIR

  • Moist air has greater capacity to absorb heat radiation and obstructs the temperature inversion.
  • But dry air does not absorb much radiation and promotes temperature inversion.

5. ICE COVER

  • Areas covered with ice reflect most of the heat radiation and the layer of air touching it becomes cold while the upper air remains warm. This leads to temperature inversion.

AIR DRAINAGE

  • During long winter nights, the air on higher slopes cools down quickly and becomes dense.
  • It moves down the slope and settles down on the valley bottom by pushing up the comparatively warmer air.
  • Sometimes, the temperature of the air at the valley bottom falls below freezing point, whereas the air at higher altitude remains comparatively warm.
  • This is known as ‘Air Drainage Temperature Inversion’.

3.10.2. WEATHER INFLUENCE

  • Surface inversion promotes stability in the lower layers of the atmosphere.
  • Smoke and dust particles get collected beneath the inversion layer and spread horizontally to fill the lower strata of the atmosphere.
  • Dense fogs in mornings are common occurrences especially during winter season.
  • This inversion commonly lasts for few hours until the sun comes up and beings to warm the earth.
  • The inversion occurs upto the height of 30-40 feet in the low latitudes, a few hundred feet in the middle latitudes and half a mile in the high latitudes.
  • It is apparent that the duration and height of surface inversion increase poleward. This inversion promotes stability in the lower portion of the atmosphere and causes dense fogs.
  • Fog is formed due to the situation of warm air above and cold air below because the warm air is cooled from below and resultant condensation causes the formation of tiny droplets around suspended dust particles and smokes during winter nights.
  • The smokes coming out of houses and chimneys intensify fogs and become responsible for the occurrence of urban smogs.
  • When smog is mixed with air pollutants such as sulphur dioxide it becomes poisonous and deadly health hazard to human beings.
  • Fogs reduce atmospheric visibility and thus they are responsible for several cases of accidents of air crafts while taking off and landing and ships in the oceans.
  • Though generally fogs are unfavourable for many agricultural crops such as grams, peas, mustard plants, wheat, etc. but sometimes they are also favourable for some crops such as coffee plants in Yemen hills of Arabia where fogs protect coffee plants from direct strong sun’s rays.
  • Inversion of temperature causes frost when the condensation of warm air due to its cooling by cold air below occurs at temperature below freezing point.

Frost is definitely economically unfavourable weather phenomenon mainly for crops because fruit orchards and several agricultural crops such as potatoes, tomatoes, peas etc. are totally damaged overnight.

  • The valley floors in the hills of Brazil are avoided for coffee cultivation because of frequent frosts. Alternatively, coffee is planted on the upper slopes of the valleys.
  • The upper parts of the valleys are inhabited in Switzerland while lower parts are avoided.
  • Inversion of temperature causes atmospheric stability which stops upward (ascent) and downward (descent) movements of air. The atmospheric stability discourages rainfall and favours dry condition.

3.10.3. Areas Affecting Inversion of Temperature

  1. The heat of the day is radiated off during the night, and by early morning hours, the earth is cooler than the air above.
  2. Over polar areas, temperature inversion is normal throughout the year.
  3. Snow covered ground surface, so that there is maximum reflection of incoming solar radiation.

3.10.4. EFFECTS

  • In the mountain valleys, the trees are frost-bitten along the lower

slopes, whereas those at higher levels are free from it.

  • Air pollutants such as dust particles and smoke do not disperse in the valley bottoms.
  • Because of these reasons, houses and farms in intermontane valleys are generally situated along the upper slopes, avoiding the cold and foggy valley bottoms.
  • For example, mulberry planters in the Suwa Basin of Japan and apple growers in the mountain states of the Himalayas avoid lower slopes.
  • Similarly, the hotels at holiday resorts in the Himalayas are built on the upper slopes.

3.11. DIURNAL OR DAILY RANGE OF TEMPERATURE

  • The difference between the maximum and minimum temperature of one day is the diurnal range of temperature.
  • The diurnal range is much larger on land than at sea.
  • A cloudy day has smaller daily range of temperature than a clear day.
  • STABILITY: Atmospheric stability is directly related to the fluctuations of daily range of temperature. The inversion of temperature lowers the daily range of temperature.
  • NATURE OF THE SURFACE: The place with marine influence, have smaller diurnal range of temperature. Therefore the place situated far away from the ocean has the moderate influence of the oceans have higher diurnal range of temperature.
  • WIND SPEED: Maximum temperature on a windy day is certainly on a day with gusty winds, the diurnal range of temperature is relatively smaller.
  • WATER VAPOUR CONTENT: Larger amount of water vapour in the air absorbs a large percentage of radiant heat from the earth’s surface. Therefore if the humid air is more, lesser is the diurnal range of temperature, drier the air and larger is the diurnal range.
  • EFFECT OF LATITUDE: The diurnal range is the highest near the ground and decreases upward.

Greatest in desert regions which record high daytime temp followed by a rapid heat loss through radiation at night, owing to clear skies.

3.12. ANNUAL RANGE OF TEMPERATURE

  • The Difference between the mean temperature of the hottest month and the mean temperature of the coldest month is the annual range of temperature.

Controlling factors:

  • The following are the factors that affect and control the annual range of temperature in the same way as they as they do the horizontal distribution of temperature:

latitude, height above the mean sea level; ocean currents; prevailing winds; precipitation and cloudiness; local relief; and distance from the sea.

1 LATITUDE

  • It increases from the equator to the poles.
  • The mid-latitude regions, where the seasonal variation in temperature is greatest, record the highest annual range of temperature.
  • In Equator, sun’s rays are always direct and so it is always hot. So less Annual Range of temperature is observed here.
  • Largest Range occurs in the subpolar locations, in Siberia, where range 640C have been recorded.

2 HEIGHTS ABOVE MEAN SEA LEVEL

  • At high elevations, the rarity of the air, larger amount of precipitation and cloudiness combine together to lower down the average temperature even during the warmer months of the year.
  • But the mean values of temperature for the colder part of the year are not affected by these factors.
  • Thus, places situated at higher elevations have lower annual ranges of temperature.

3 PREVALING WINDS

  • Off-shore winds bring about an increase in the annual range of temperature of the adjacent land, while the on-shore winds carry the moderating influence of the oceans far inland and impose a restriction on the annual range.

4 PRECIPITATIONS AND CLOUDINESS

  • In those regions where the rains are falling or where the skies are covered with clouds, the summer temperatures are relatively lower.
  • But during the winter, the clouds check the loss of heat by terrestrial radiation. Thus, in cloudy regions the winter time temperatures are not allowed to fall much. Therefore in such regions the annual range of temperature is relatively smaller than those regions where the weather is clear and dry.

5 LOCAL RELIEF

  • The slopes facing the sun have higher temperatures during summer months, and the slopes protected from the sun have much lower temperatures during winter. Thus, this local factor also affects the annual range of temperature.

6 DISTANCES FROM THE SEA

  • Water is heated or cooled in a longer period of time than land.
  • The coastal areas enjoy a moderate climate, and the difference in temperature of the warmest and the coldest months is not very large.
  • On the contrary, the interior locations have extremely hot summers and cold winters. Thus, with increasing distance from the sea-coast, there is a corresponding increase in the seasonal variation of temperatures.
  • Its effect is more marked in the temperate regions.

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