Solar Radiation, Heat Balance and Temperature

1.Solar Radiation

• Insolation and Short-Wave Solar EnergyImportance 3/5
  What is insolation, and in which wavelengths does the Earth mostly receive it? Understand why the Earth intercepts only a small part of the Sun's energy, and why the Earth's shape makes the Sun's rays fall obliquely at the top of the atmosphere. Remember the solar constant, in calories per sq. cm per minute.
• Aphelion and PerihelionImportance 4/5
  Remember the names and distances of the Earth's farthest and nearest positions from the Sun, and which one falls in early January and which in early July. On which of the two dates is insolation slightly higher? Be able to explain why this difference has little effect on daily weather. Does the Earth's nearest position to the Sun fall in the northern hemisphere's summer?
• Factors Causing Variation in InsolationImportance 3/5
  Insolation changes during a day, a season and a year. Remember the five factors behind this. Which two have less influence than the others? Also understand how the length of the day changes insolation at different latitudes.
• Tilt of the Earth's Axis and InsolationImportance 4/5UPSC 4 Years
  What angle does the Earth's axis make with the plane of its orbit? Understand how this tilt changes the insolation received at different latitudes through the year. At which latitude is the Sun overhead at noon on 21 June?
• Angle of Inclination of the Sun's RaysImportance 4/5
  How does latitude decide the angle of the Sun's rays? Be able to explain the two reasons why slant rays give less energy per unit area than vertical rays: the area they cover, and the depth of atmosphere they pass through.
• Absorption of Incoming Radiation in the TroposphereImportance 2/5
  Is the atmosphere largely transparent or opaque to short-wave solar radiation? Which gas is the main absorber of near infrared radiation in the troposphere? Where is most ozone found, and which kind of radiation does it absorb?
• Scattering and the Colour of the SkyImportance 3/5
  Which wavelengths of visible light does the atmosphere scatter most, and do gas molecules or larger particles scatter them? In which directions is the light scattered? Be able to explain why the sky looks blue and why the rising and setting Sun looks red.
• Insolation from the Tropics to the PolesImportance 4/5
  Remember the surface insolation at the tropics and at the poles, in watts per sq. metre. Over which regions is it highest, and why? Does the equator get more or less insolation than the tropics, and why?
• Insolation Over Continents and Oceans and by SeasonImportance 2/5
  At the same latitude, does insolation reach a higher level over land or over the oceans? How does winter insolation differ from summer insolation in the middle and higher latitudes?

2.Heating and Cooling of Atmosphere

• ConductionImportance 2/5
  Understand how heat passes between two bodies in contact, and when this transfer stops. Which part of the atmosphere does conduction mainly heat?
• ConvectionImportance 3/5UPSC 2024
  How does rising air heat the atmosphere vertically? Understand the mechanism, and remember the one layer of the atmosphere to which convective transfer is confined.
• AdvectionImportance 3/5
  How does advection differ from convection, and which of the two matters more for heat transfer? Remember its role in the day and night weather changes of the middle latitudes. Which hot local wind of northern India in summer results from advection?
• Short-Wave and Long-Wave RadiationImportance 3/5UPSC 2023
  Why does the Sun give out short waves while the Earth gives out long waves? Understand how a body's temperature relates to the energy it radiates and the wavelength of that radiation. Be clear about incoming short-wave and outgoing long-wave radiation.
• Terrestrial Radiation and Heating of the Atmosphere from BelowImportance 4/5UPSC 2024
  Is the atmosphere heated mainly directly by the Sun or indirectly by the Earth? Be able to explain how long-wave radiation from the heated Earth warms the air, and which gases absorb it. Understand how this explains why temperature falls with height.
• Clouds and Day and Night TemperaturesImportance 4/5UPSC 2001, 2019, 2022
  Why are cloudy days usually cooler, and cloudy nights warmer, than clear ones? Understand what clouds do to incoming insolation by day and to terrestrial radiation by night. How does a large amount of aerosols change the insolation reaching the surface by day?
• Heat Balance of the EarthImportance 4/5
  Why does the Earth neither warm up nor cool down over time, despite the huge transfer of heat? Understand the condition that must hold between insolation received and terrestrial radiation lost. Follow the model in which the insolation at the top of the atmosphere is taken as 100 units.
• Reflected Insolation and the Albedo of the EarthImportance 4/5UPSC 2008
  How many of the 100 units are reflected back to space before reaching the surface? Remember the three shares: cloud tops, scattering by the atmosphere, and snow and ice-covered areas. What is albedo, and what is the Earth's measured average albedo, in per cent?
• Albedo of Different SurfacesImportance 3/5
  Which of these surfaces have a high albedo and which a low albedo: fresh snow, sea ice, desert sand, forests, oceans and cities? Understand whether each kind of surface leads to cooling or to warming.
• Absorption of Insolation by the Atmosphere and the SurfaceImportance 2/5
  After the reflected share is removed, how many units remain? Remember how these are divided between absorption within the atmosphere and absorption by the Earth's surface.
• Terrestrial Radiation and the Balance of 65 UnitsImportance 2/5
  Of the units the surface radiates back, how many go straight to space, and how many does the atmosphere take up? Remember the three ways the atmosphere takes up its share (direct absorption, convection with turbulence, and latent heat of condensation). Understand how the atmosphere's outgoing radiation and the Earth's outgoing radiation add up to balance the 65 units received.
• Surplus and Deficit LatitudesImportance 3/5
  Between which latitudes does the Earth show a surplus of net radiation, and which regions show a deficit? Remember the latitude at which the surplus ends.
• Poleward Transfer of Surplus HeatImportance 3/5
  Why do the tropics not keep getting hotter, and why do the high latitudes not freeze permanently? Know what carries surplus heat towards the poles. Understand which latitudes see most of this transfer and what that means for storms there.
• Heat and TemperatureImportance 2/5
  What is the difference between heat and temperature? Be clear about which one relates to the movement of molecules and which one measures how hot or cold something is.
• Factors Controlling Temperature DistributionImportance 3/5
  Remember the five main factors that control the temperature of the air at a place. Understand how latitude acts through insolation. Also be aware of local factors such as vegetation cover, the direction a slope faces, local winds and built-up areas.
• Altitude and the Normal Lapse RateImportance 4/5UPSC 2012
  Why do places at higher elevation usually record lower temperatures than places near sea level? Be able to explain this through heating from below. Remember the normal lapse rate in degrees Celsius per 1,000 m.
• Distance from the Sea and ContinentalityImportance 4/5UPSC 2013, 2023
  Why do land and sea heat up and cool down at different speeds, and what does this do to temperature range near the coast and far inland? Remember the meaning of specific heat and how it helps explain the difference. Understand how land and sea breezes moderate coastal temperatures.
• Air Masses and Ocean Currents as Controls of TemperatureImportance 2/5
  How do warm and cold air masses change the temperature of the places they cover? What do warm and cold ocean currents do to the temperature of coastal places?
• Isotherms and Their General TrendImportance 3/5
  What are isotherms? Understand why they generally run parallel to the latitudes. In which month and hemisphere do they deviate most from this pattern, and what does the size of the northern hemisphere's land area have to do with it?
• January Isotherms in the Northern HemisphereImportance 4/5UPSC 2024, 2025
  In January, in which direction do isotherms bend over the oceans, and in which over the continents? Understand the case of the North Atlantic, with the Gulf Stream and North Atlantic Drift, and the case of the Siberian plain. Remember the broad January temperatures for the equatorial oceans, the tropics, the middle latitudes and the Eurasian interior.
• January Isotherms in the Southern HemisphereImportance 2/5
  Why are the isotherms in the southern hemisphere more parallel to the latitudes, with more gradual change than in the north? Remember the latitudes along which the 20°C, 10°C and 0°C isotherms run.
• July Distribution of TemperatureImportance 2/5
  How do the isotherms run in July? In the northern hemisphere, in which direction do they bend over the oceans and over the continents, and why? Remember the temperatures over the equatorial oceans and over the subtropical continental region of Asia along 30° N.
• Annual Range of TemperatureImportance 4/5UPSC 2007, 2013
  What is the annual range of temperature? Where is it highest and why? Where is it least, and at roughly what value? Be able to explain why the range is so high in the Siberian plains.

3.Inversion of Temperature

• Meaning and Conditions of Temperature InversionImportance 4/5
  What happens to the normal lapse rate during an inversion? Understand why a long winter night with clear sky and still air is ideal for it. Where is inversion almost permanent, and in which season is it strongest? What are the main types of inversion, and where does each form?
• Effects of Surface InversionImportance 3/5
  How does surface inversion affect the stability of the lower atmosphere? Understand what happens to smoke and dust, and why dense morning fogs are common in winter. For how long does it usually last, and what ends it? Also know its effect on visibility.
• Air Drainage in Hills and ValleysImportance 3/5
  How does cold night air on hills and mountains behave, and where does it collect? Understand why warm air ends up above the cold air in valley bottoms. Where is the risk of frost greatest, and which part of a hill is better protected? Why are frost-sensitive crops often planted on slopes?
• Pollution Dome EffectImportance 3/5
  Why do pollutants stay trapped near the ground during a temperature inversion? Understand why the air cannot mix vertically. Remember the season in which this usually happens and its effects on health.
• Heat DomeImportance 2/5
  What is a heat dome? Understand how a stagnant high-pressure system makes air sink and warm up by compression. Remember the kind of weather and impacts this brings.

1. 1.

   Consider the following statements:
   1. The earth is farthest from the sun, at about 152 million km, on 4th July; this position is called aphelion.
   2. The annual insolation received by the earth on 3rd January is slightly less than that received on 4th July.
   3. The variation in solar output due to the earth–sun distance does not have a great effect on daily weather.
   Which of the statements given above is/are correct?

   1. A1 only
   2. B1 and 3 only
   3. C2 and 3 only
   4. D1, 2 and 3
2. 2.

   Among the factors that cause variations in insolation, which two have the least influence?

   1. AThe rotation of the earth and the transparency of the atmosphere
   2. BThe angle of the sun's rays and the configuration of land (aspect)
   3. CThe length of the day and the transparency of the atmosphere
   4. DThe transparency of the atmosphere and the configuration of land (aspect)
3. 3.

   Consider the following statements:
   Statement-I: Places at higher latitudes receive less net solar energy per unit area.
   Statement-II: At higher latitudes, slant rays spread over a larger area and pass through a greater depth of atmosphere.
   Which one of the following is correct in respect of the above statements?

   1. ABoth Statement-I and Statement-II are correct and Statement-II explains Statement-I
   2. BBoth Statement-I and Statement-II are correct and Statement-II does not explain Statement-I
   3. CStatement-I is correct but Statement-II is incorrect
   4. DStatement-I is incorrect but Statement-II is correct
4. 4.

   With reference to the spatial distribution of insolation at the earth's surface, consider the following statements:
   1. Maximum insolation is received over the equator.
   2. At the same latitude, insolation is generally more over the continents than over the oceans.
   Which of the statements given above is/are correct?

   1. A1 only
   2. B2 only
   3. CBoth 1 and 2
   4. DNeither 1 nor 2
5. 5.

   Consider the following pairs:

   Process	Feature
   1. Convection	Confined only to the troposphere
   2. Advection	Produces the local wind 'loo' in northern India in summer
   3. Conduction	Transfer of heat through horizontal movement of air

   How many of the pairs given above are correctly matched?

   1. AOnly one
   2. BOnly two
   3. CAll three
   4. DNone
6. 6.

   Consider the heat budget of the earth, taking insolation at the top of the atmosphere as 100 units. Consider the following statements:
   1. Roughly 35 units are reflected back to space before reaching the earth's surface.
   2. 51 units are absorbed by the earth's surface.
   3. Of the 51 units radiated back by the earth, 17 units are radiated directly to space.
   Which of the statements given above is/are correct?

   1. A1 only
   2. B1 and 2 only
   3. C2 and 3 only
   4. D1, 2 and 3
7. 7.

   Of the 34 units of terrestrial radiation absorbed by the atmosphere in the heat budget, consider the following pairs:

   Means of absorption	Units
   1. Absorbed directly by the atmosphere	6
   2. Through convection and turbulence	9
   3. Through latent heat of condensation	19

   How many of the pairs given above are correctly matched?

   1. AOnly one
   2. BOnly two
   3. CAll three
   4. DNone
8. 8.

   With reference to the latitudinal variation in the net radiation balance of the earth–atmosphere system, consider the following statements:
   1. There is a surplus of net radiation balance between 40° north and south.
   2. The regions near the poles have a surplus.
   Which of the statements given above is/are correct?

   1. A1 only
   2. B2 only
   3. CBoth 1 and 2
   4. DNeither 1 nor 2
9. 9.

   The normal lapse rate, the rate at which temperature decreases with height, is

   1. A3.5°C per 1,000 m
   2. B6.5°C per 1,000 m
   3. C10°C per 1,000 m
   4. D1°C per 1,000 m
10. 10.

    Consider the following statements:
    Statement-I: The variation in temperature over the sea is less than over land.
    Statement-II: Compared to land, the sea gets heated slowly and loses heat slowly.
    Which one of the following is correct in respect of the above statements?

    1. ABoth Statement-I and Statement-II are correct and Statement-II explains Statement-I
    2. BBoth Statement-I and Statement-II are correct and Statement-II does not explain Statement-I
    3. CStatement-I is correct but Statement-II is incorrect
    4. DStatement-I is incorrect but Statement-II is correct
11. 11.

    With reference to the distribution of temperature, consider the following statements:
    1. Deviation of isotherms from the latitudes is more pronounced in January than in July, especially in the northern hemisphere.
    2. In January, isotherms bend towards the south over the North Atlantic Ocean because of warm ocean currents.
    Which of the statements given above is/are correct?

    1. A1 only
    2. B2 only
    3. CBoth 1 and 2
    4. DNeither 1 nor 2
12. 12.

    Consider the following statements about mean January temperatures:
    1. Equatorial oceans record over 27°C.
    2. The Eurasian continental interior records –18°C to –48°C.
    3. Along 60°E longitude, it is –20°C at both 80°N and 50°N latitudes.
    4. The middle latitudes record 10°C to 12°C.
    How many of the above statements are correct?

    1. AOnly one
    2. BOnly two
    3. COnly three
    4. DAll four
13. 13.

    With reference to the distribution of temperature in the southern hemisphere, consider the following statements:
    1. Isotherms deviate from the latitudes more sharply than in the northern hemisphere.
    2. The 0°C isotherm runs parallel to 45°S latitude.
    Which of the statements given above is/are correct?

    1. A1 only
    2. B2 only
    3. CBoth 1 and 2
    4. DNeither 1 nor 2
14. 14.

    Consider the following statements about inversion of temperature:
    1. A long winter night with clear skies and still air is ideal for inversion.
    2. Over polar areas, temperature inversion is normal throughout the year.
    3. Surface inversion promotes instability in the lower layers of the atmosphere.
    4. Smoke and dust particles collect beneath the inversion layer.
    How many of the above statements are correct?

    1. AOnly one
    2. BOnly two
    3. COnly three
    4. DAll four
15. 15.

    The highest annual range of temperature, more than 60°C, is found over

    1. AThe equatorial oceans
    2. BThe southern hemisphere oceans along 45°S
    3. CThe subtropical continental region of Asia along 30°N
    4. DThe north-eastern part of the Eurasian continent

1. 1.

   UPSC 2024

   Consider the following statements :
   Statement-I : Thickness of the troposphere at the equator is much greater as compared to poles.
   Statement-II : At the equator, heat is transported to great heights by strong convectional currents.
   Which one of the following is correct in respect of the above statements ?

   1. ABoth Statement-I and Statement-II are correct and Statement-II explains Statement-I
   2. BBoth Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
   3. CStatement-I is correct, but Statement-II is incorrect
   4. DStatement-I is incorrect, but Statement-II is correct
2. 2.

   UPSC 2023

   With reference to the Earth's atmosphere, which one of the following statements is correct?

   1. AThe total amount of insolation received at the equator is roughly about 10 times of that received at the poles.
   2. BInfrared rays constitute roughly two-thirds of insolation.
   3. CInfrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.
   4. DInfrared waves are a part of visible spectrum of electromagnetic waves of solar radiation.
3. 3.

   UPSC 2025

   Consider the following statements :
   Statement I : In January, in the Northern Hemisphere, the isotherms bend equatorward while crossing the landmasses, and poleward while crossing the oceans.
   Statement II : In January, the air over the oceans is warmer than that over the landmasses in the Northern Hemisphere.
   Which one of the following is correct in respect of the above statements?

   1. ABoth Statement I and Statement II are correct and Statement II explains Statement I
   2. BBoth Statement I and Statement II are correct but Statement II does not explain Statement I
   3. CStatement I is correct but Statement II is not correct
   4. DStatement I is not correct but Statement II is correct
4. 4.

   UPSC 2024

   Consider the following statements :
   Statement-I : The atmosphere is heated more by incoming solar radiation than by terrestrial radiation.
   Statement-II : Carbon dioxide and other greenhouse gases in the atmosphere are good absorbers of long wave radiation.
   Which one of the following is correct in respect of the above statements ?

   1. ABoth Statement-I and Statement-II are correct and Statement-II explains Statement-I
   2. BBoth Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
   3. CStatement-I is correct, but Statement-II is incorrect
   4. DStatement-I is incorrect, but Statement-II is correct
5. 5.

   UPSC 2024

   Which of the following is/are correct inference/inferences from isothermal maps in the month of January ?
   1. The isotherms deviate to the north over the ocean and to the south over the continent.
   2. The presence of cold ocean currents, Gulf Stream and North Atlantic Drift make the North Atlantic Ocean colder and the isotherms bend towards the north.
   Select the answer using the code given below :

   1. A1 only
   2. B2 only
   3. CBoth 1 and 2
   4. DNeither 1 nor 2
6. 6.

   UPSC 2024

   On June 21 every year, which of the following latitude(s) experience(s) a sunlight of more than 12 hours ?
   1. Equator
   2. Tropic of Cancer
   3. Tropic of Capricorn
   4. Arctic Circle
   Select the correct answer using the code given below :

   1. A1 only
   2. B2 only
   3. C3 and 4
   4. D2 and 4
7. 7.

   UPSC 2023

   Consider the following statements :
   Statement–I : The temperature contrast between continents and oceans is greater during summer than in winter.
   Statement–II : The specific heat of water is more than that of land surface.
   Which one of the following is correct in respect of the above statements?

   1. ABoth Statement–I and Statement–II are correct and Statement–II is the correct explanation for Statement–I
   2. BBoth Statement–I and Statement–II are correct and Statement–II is not the correct explanation for Statement–I
   3. CStatement–I is correct but Statement–II is incorrect
   4. DStatement–I is incorrect but Statement–II is correct
8. 8.

   UPSC 2022

   In the northern hemisphere, the longest day of the year normally occurs in the :

   1. AFirst half of the month of June
   2. BSecond half of the month of June
   3. CFirst half of the month of July
   4. DSecond half of the month of July
9. 9.

   UPSC 2019

   On 21st June, the Sun

   1. Adoes not set below the horizon at the Arctic Circle
   2. Bdoes not set below the horizon at Antarctic Circle
   3. Cshines vertically overhead at noon on the Equator
   4. Dshines vertically overhead at the Tropic of Capricorn
10. 10.

    UPSC 2013

    Variations in the length of daytime and night-time from season to season are due to

    1. Athe earth's rotation on its axis
    2. Bthe earth's revolution round the sun in an elliptical manner
    3. Clatitudinal position of the place
    4. Drevolution of the earth on a tilted axis
11. 11.

    UPSC 2013

    The annual range of temperature in the interior of the continents is high as compared to coastal areas. What is/are the reason/reasons?
    1. Thermal difference between land and water
    2. Variation in altitude between continents and oceans
    3. Presence of strong winds in the interior
    4. Heavy rains in the interior as compared to coasts
    Select the correct answer using the codes given below.

    1. A1 only
    2. B1 and 2 only
    3. C2 and 3 only
    4. D1, 2, 3 and 4
12. 12.

    UPSC 2012

    Normally, the temperature decreases with the increase in height from the Earth's surface, because
    1. the atmosphere can be heated upwards only from the Earth's surface
    2. there is more moisture in the upper atmosphere
    3. the air is less dense in the upper atmosphere
    Select the correct answer using the codes given below :

    1. A1 only
    2. B2 and 3 only
    3. C1 and 3 only
    4. D1, 2 and 3
13. 13.

    UPSC 2008

    Consider the following statements:
    1. The albedo of an object determines its visual brightness when viewed with reflected light.
    2. The albedo of Mercury is much greater than the albedo of the Earth.
    Which of the statements given above is/are correct?

    1. A1 only
    2. B2 only
    3. CBoth 1 and 2
    4. DNeither 1 nor 2
14. 14.

    UPSC 2007

    Consider the following statements:
    1. The annual range of temperature is greater in the Pacific Ocean than in the Atlantic Ocean.
    2. The annual range of temperature is greater in the Northern Hemisphere than in the Southern Hemisphere.
    Which of the statements given above is/are correct?

    1. A1 only
    2. B2 only
    3. CBoth 1 and 2
    4. DNeither 1 nor 2
15. 15.

    UPSC 2001

    Cloudy nights are warmer compared to clear cloudless nights, because clouds

    1. Aprevent cold waves from the sky from descending on Earth
    2. Breflect back heat given off by Earth
    3. Cproduce heat and radiate it towards Earth
    4. Dabsorb heat from the atmosphere and send it towards Earth
16. 16.

    UPSC 2022

    Consider the following statements :
    1. High clouds primarily reflect solar radiation and cool the surface of the Earth.
    2. Low clouds have a high absorption of infrared radiation emanating from the Earth's surface and thus cause warming effect.
    Which of the statements given above is/are correct ?

    1. A1 only
    2. B2 only
    3. CBoth 1 and 2
    4. DNeither 1 nor 2
17. 17.

    UPSC 2019

    Why are dewdrops not formed on a cloudy night?

    1. AClouds absorb the radiation released from the Earth’s surface.
    2. BClouds reflect back the Earth’s radiation.
    3. CThe Earth’s surface would have low temperature on cloudy nights.
    4. DClouds deflect the blowing wind to ground level.