Solar Radiation, Heat Balance and Temperature

1. Solar radiation and insolation

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• The Earth gets almost all its energy from the Sun. Over time it sends the same amount back to space, so it neither warms nor cools. Section 5 shows that this balance is now slightly upset.
• Insolation (incoming solar radiation) reaches the Earth mostly as short waves. The Earth is nearly a sphere, so rays strike the top of the atmosphere at a slant. The Earth catches only about one two-billionth of the Sun's output.
• Solar constant (the energy that falls at the top of the atmosphere on a surface facing the Sun, at the average Earth-Sun distance): NCERT gives 1.94 cal/sq cm/min, which is about 1,353 W/sq m. The modern satellite value is about 1,361 W/sq m (Kopp and Lean, 2011).
• Spread over the whole globe, day and night, this comes to about 340 W/sq m, one quarter of the solar constant.

• It changes about 0.1% over the 11-year sunspot cycle and about 7% between perihelion and aphelion.

• Planck's law (Class 11 box): the hotter a body, the more energy it radiates and the shorter the wavelength. Beyond NCERT, this idea comes from two laws:

• Stefan-Boltzmann law: energy rises with the fourth power of the absolute temperature.

• Wien's law: the peak wavelength gets shorter as temperature rises.

• Shortwave and longwave radiation:

	Sun (about 5,800 K)	Earth (about 288 K)
Peak wavelength	about 0.5 µm (visible light)	about 10 µm (infrared)
What the atmosphere does	lets most of it pass	absorbs much of it (water vapour, CO₂)

• Aphelion (the point of the orbit farthest from the Sun): about 152 million km, around 4 July. Perihelion (the nearest point): about 147 million km, around 3 January. Insolation is slightly higher in January.
• Land-sea spread and air circulation hide this effect, so it hardly changes daily weather.
• The Northern Hemisphere has winter at perihelion. This proves that distance does not cause seasons.
• Perihelion together with a new or full moon gives very large tides.

2. Why insolation varies: tilt, angle of rays, day length and aspect

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• Factors affecting insolation (Class 11 lists five): (i) the Earth's rotation; (ii) the angle of the Sun's rays; (iii) the length of the day; (iv) how clear the atmosphere is; (v) the aspect of the land. The last two matter least.
• Inclination of the Earth's axis: the axis makes 66.5° with the plane of the orbit (23.5° from the vertical). It always points the same way, so the overhead noon Sun moves between the two Tropics during the year.
• Summer solstice (about 21 June): the noon Sun is overhead at the Tropic of Cancer (23.5°N).
• The Northern Hemisphere has its longest day.
• North of the Arctic Circle there is 24-hour daylight (Class 11, Fig. 8.1).

• Winter solstice, about 22 December: Sun over the Tropic of Capricorn. Equinoxes, about 21 March and 23 September: Sun over the equator, with equal day and night everywhere.

• In summer, days get longer as you go further from the equator. So Chandigarh has longer June days than Thiruvananthapuram.

• Angle of incidence of the Sun's rays (the angle at which sunlight hits the ground) depends mainly on latitude.
• Vertical rays heat a small area strongly.

• Slanting rays spread the same energy over a bigger area. They also pass through more air, so more energy is lost to absorption and scattering.

• Aspect (the direction a slope faces): in the Northern Hemisphere, south-facing slopes (adret) are sunnier, warmer and drier than north-facing slopes (ubac).

• In Himalayan valleys, villages, terraced fields and orchards are on south-facing slopes.
• North-facing slopes keep thicker forest and a lower snowline.

3. Sunlight in the atmosphere and its spatial distribution

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• Most short waves pass through the air. In the troposphere, water vapour, ozone and other gases absorb much of the near-infrared. Ozone higher up absorbs ultraviolet.
• Scattering of sunlight (tiny gas molecules and particles bounce light in all directions):
• Rayleigh scattering is much stronger for short blue waves, so the daytime sky is blue.
• At sunrise and sunset, light travels a much longer path through the air. Most of the blue is scattered away, and red and orange light is left.

• Larger haze, dust and cloud particles scatter all colours equally (Mie scattering). This is why polluted or cloudy skies look white or grey.

• Spatial distribution of insolation (how surface sunlight varies from place to place): about 320 W/sq m in the tropics and about 70 W/sq m at the poles.

• The maximum is over subtropical deserts, which have the fewest clouds. The equator is cloudier, so it gets less than the tropics.
• At the same latitude, continents get more than oceans.

• Middle and high latitudes get much less in winter.

• India's hottest month is May, not June, because monsoon clouds cut insolation from June onwards.

• Global dimming (beyond NCERT): sunlight reaching the surface fell by a few per cent between the 1950s and the 1980s.
• The main cause was aerosol pollution, such as sulphates, and the brighter clouds it creates. This partly hid greenhouse warming.
• After about 1990, cleaner air brought "brightening" to Europe and North America. Dimming went on longer over South and East Asia, including India (Wild, 2009; IPCC AR6).

4. Heating and cooling of the atmosphere

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• The atmosphere is heated mainly from below, not directly by the Sun.
• Short-wave insolation heats the ground.
• The ground gives off long waves, called terrestrial radiation.

• CO₂, water vapour and other greenhouse gases absorb these long waves. So the air is heated indirectly, and it then radiates heat out to space.

• Conduction: heat flows between bodies that touch, from the warmer to the cooler. It continues until their temperatures are equal or the contact breaks. It heats only the thin layer of air touching the ground.

• Convection: warmed air rises in currents and carries heat upward. It happens only in the troposphere. It builds cumulus clouds and gives afternoon convectional rain.
• Advection: heat is carried sideways by moving air. This sideways movement matters more than vertical movement.
• In middle latitudes, it causes most day-to-day weather change.

• The hot, dry loo of north India in May-June is caused by advection.

• Specific heat (Class 11 box): the energy needed to raise the temperature of 1 g of a substance by 1°C.

• Water's specific heat is about 4-5 times that of rock and soil.
• Water also mixes heat downward, lets sunlight pass into it and loses heat through evaporation.
• So land heats and cools fast, while the sea heats and cools slowly. This is the root of continentality, land and sea breezes, and the monsoon.

5. The heat budget of the Earth and albedo

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• Heat budget of the Earth (Class 11, Fig. 8.2). Take the insolation at the top of the atmosphere as 100 units:

Flow	Units
Reflected to space (albedo)	35 = 27 from cloud tops + 2 from snow and ice + 6 scattered back by the air (NCERT leaves out the 6)
Absorbed by the atmosphere	14
Absorbed by the surface	51
Surface → straight to space	17
Surface → into the atmosphere	34 = 6 absorbed directly + 9 by convection and turbulence + 19 as latent heat of condensation
Atmosphere → space	48 (14 + 34)
Total outgoing	17 + 48 = 65, which equals the 65 units absorbed

• Albedo (the share of incoming light reflected without heating anything): NCERT gives 35%. The modern figure is about 29-30% (NASA/CERES).
• Fresh snow reflects up to 80-90%, sea ice 50-70%, desert sand about 40%, grassland about 25%, forest 10-20% and open ocean under 10%.

• Ice-albedo feedback: melting ice uncovers darker surfaces that absorb more heat. This is why the Arctic is warming about 3-4 times faster than the global average.

• Latitudinal heat balance (Fig. 8.3): between about 40°N and 40°S, the Earth gains more radiation than it loses (a surplus). Closer to the poles there is a deficit.

• Winds (Hadley, Ferrel and polar cells; cyclones) and ocean currents (such as the Gulf Stream) carry the extra heat poleward.

• So the tropics do not keep heating up, and the poles do not freeze for good.

• Earth's energy imbalance (beyond NCERT): greenhouse gases now trap more outgoing heat, so the Earth absorbs more than it gives off. The gap is roughly 0.5-1 W/sq m, and it about doubled from 2005 to 2019 (Loeb and others, 2021; verify current).

• About 90% of the extra heat goes into the oceans.

6. Temperature and the factors controlling it

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• Heat is the energy of moving molecules. Temperature measures, in degrees, how hot or cold something is.
• Factors controlling temperature distribution: latitude, altitude, distance from the sea, air masses, warm and cold ocean currents, and local aspects (slope, relief, land cover).
• Normal lapse rate (how fast temperature falls with height): 6.5°C per 1,000 m, or about 1°C per 165 m. Darjiling (about 2,000 m) has about 4°C in January, while Agra, at a similar latitude, has about 16°C.
• Adiabatic lapse rate (beyond NCERT): rising air cools as it expands, without gaining or losing heat.
• Dry air cools about 9.8°C per km.
• Saturated air cools only about 4-7°C per km, because condensation releases latent heat.

• Air is stable if the surrounding air cools more slowly than the rising parcel, and unstable if it cools faster.

• Continentality (the effect of being far from the sea): inland places have large daily and yearly temperature ranges. Examples are Delhi, Kanpur, Amritsar, Siberia, and Leh and Jodhpur in Class 9 Table 3.3.

• Equable climate (small gaps between summer and winter): Mumbai, the Konkan, Kerala and the Andamans. Thiruvananthapuram's monthly means stay near 26-29°C (Class 9, Table 3.3).
• Ocean currents and air masses:
• Warm currents raise coastal temperatures. The Gulf Stream and North Atlantic Drift keep Norway's coast ice-free.

• Cold currents cool coasts: the Humboldt off Peru, the Benguela and the California current.

• Microclimate: the climate of a small area, such as a valley floor, forest, lake shore or city block, that differs from its region. It matters for crops, frost and health.

• Urban heat island: a city is several degrees warmer than the countryside around it, especially at night. Causes include concrete and asphalt, little greenery, trapped heat and waste heat from vehicles and air-conditioners.
• Responses: the Ahmedabad Heat Action Plan (2013, the first in South Asia), NDMA guidelines and the Telangana Cool Roof Policy (2023).

7. Global distribution of temperature: isotherms and ranges

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• Isotherm: a line joining places with equal temperature (reduced to sea level on world maps).
• Isotherms broadly follow latitudes.

• They bend more in January than in July, and more in the land-heavy Northern Hemisphere.

• January (Fig. 8.4a):

• Isotherms bend poleward over the warm North Atlantic (Gulf Stream, North Atlantic Drift) and equatorward over cold continents, most sharply over Siberia.
• Along 60°E, it is −20°C at both 80°N and 50°N.
• Temperatures: over 27°C on equatorial oceans, over 24°C in the tropics, 2 to 0°C in middle latitudes, and −18 to −48°C in inner Eurasia.

• In the Southern Hemisphere, isotherms run nearly parallel to latitudes: 20°C, 10°C and 0°C along about 35°S, 45°S and 60°S.

• July (Fig. 8.4b):

• Isotherms are mostly parallel to latitudes, with over 27°C on equatorial oceans.
• Over subtropical Asia it is above 30°C along about 30°N.

• NCERT's printed line "10°C along 40°N" should be checked against the map.

• Thermal equator (beyond NCERT): the belt of highest temperature. It lies about 5°N on average, because the Northern Hemisphere has more land.

• It moves with the overhead Sun, much further over land (into South Asia and the Sahel in July).

• It tracks the ITCZ.

• Annual range of temperature (warmest-month mean minus coldest-month mean; Fig. 8.5):

• Over 60°C in north-east Eurasia (Verkhoyansk-Oymyakon).

• About 3°C between 20°S and 15°N.

• Diurnal range (the gap between a day's highest and lowest temperature): large in deserts with clear skies, such as the Thar. It is small on coasts, such as Kerala (about 7-8°C).

• Safdarjung (1951-80): January mean 14.2°C, May mean 32.75°C, range 18.55°C.

• Records:

• The 58°C reading at Al Aziziyah (El Azizia), Libya, on 13 September 1922 was rejected by the WMO in 2012. The record is now 56.7°C at Death Valley, USA (10 July 1913).
• Lowest: −89.2°C at Vostok, Antarctica (21 July 1983).
• India's highest: 51.0°C at Phalodi, Rajasthan (19 May 2016; verify current).

8. Inversion of temperature and air drainage

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• Temperature inversion: temperature rises with height instead of falling. This is the reverse of the normal lapse rate.
• How it forms: on long winter nights with clear skies and still air, the ground loses its daytime heat. By early morning the ground is colder than the air above it.

• It usually lasts only until the Sun warms the ground. Over polar areas it is normal all year.

• Other types (beyond NCERT):

• Subsidence inversion: sinking air under subtropical highs, which stops rain over deserts.
• Frontal inversion: warm air rides over cold air.

• Advection inversion.

• Effects:

• The lower air becomes stable, so smoke and dust collect under the inversion and spread sideways.
• Dense winter morning fog forms.

• In the Indo-Gangetic plain and Delhi-NCR, stubble smoke and other emissions get trapped under a shallow inversion on calm days. This causes smog and disrupts flights, trains and roads.

• Air drainage:

• At night, cold, heavy air flows down hill slopes like water.
• It collects in valley bottoms, under warmer air. This creates a valley inversion.

• Valley floors become frost pockets. So tea, coffee and apple orchards are planted on the slopes (the "thermal belt"), and Himalayan villages prefer mid-slopes.

• NCERT says air drainage "protects plants from frost". This is true for the slopes, not for the valley floor.

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Exam angles

Prelims — high-yield facts and traps

• Aphelion about 4 July (152 million km); perihelion about 3 January (147 million km). The Northern Hemisphere's winter falls at perihelion, so distance does not cause seasons.
• Solar constant: NCERT 1.94 cal/sq cm/min; now about 1,361 W/sq m. It is not truly constant.
• Axis at 66.5° to the orbital plane. On 21 June the Sun is overhead at 23.5°N. Chandigarh has longer June days than Thiruvananthapuram.
• The atmosphere is heated mainly by long-wave terrestrial radiation, from below. Convection happens only in the troposphere. Advection causes the loo.
• Heat budget: 35 reflected (27 by clouds + 2 by snow and ice + 6 by the air); 14 absorbed by the atmosphere; 51 by the surface; 17 + 48 = 65 returned; latent heat 19 units.
• Albedo: NCERT 35%; modern value about 30%. Fresh snow is highest; the ocean is lowest.
• Maximum surface insolation is over subtropical deserts, not the equator. Continents get more than oceans at the same latitude.
• Surplus between 40°N and 40°S; deficit closer to the poles.
• Lapse rates: normal 6.5°C/km; dry adiabatic about 9.8°C/km; saturated about 4-7°C/km.
• January isotherms bend poleward over warm oceans and equatorward over continents. Southern Hemisphere isotherms are nearly parallel to latitudes.
• Annual range: greatest (over 60°C) in north-east Eurasia; least (about 3°C) between 20°S and 15°N.
• Blue sky comes from Rayleigh scattering; red sunsets from the long light path; white haze from Mie scattering.
• FALSE: "El Azizia's 58°C is the world record." It was rejected in 2012; the record is now Death Valley, 56.7°C.
• FALSE: "Inversion never occurs by day or lasts long." It is normal all year over polar areas.
• FALSE: "India's hottest month is June." It is May.
• FALSE: "Air drainage protects valley floors from frost." Valley floors become frost pockets; it is the slopes that are protected.

Mains — GS-I themes

1. How do latitude and the tilt of the Earth's axis control insolation and seasons?
2. Explain the Earth's heat budget. Why don't the tropics keep heating up? How is the balance changing now (energy imbalance, ocean heat)?
3. Compare January temperature distribution in the Northern and Southern Hemispheres (the Class 11 exercise).
4. Temperature inversion: causes, types and effects on air quality, fog and farming in north India.
5. Urban heat islands in Indian cities: causes, health risks, and responses (heat action plans, cool roofs).
6. Why are the hottest places on Earth in the subtropics and not at the equator? Why is India hottest in May?
7. Continentality versus equable climate: compare Indian examples (Delhi and Leh versus Mumbai and Thiruvananthapuram).

Current-affairs hooks

• Record heat waves in India and the world, IMD heat-wave criteria and new temperature records (verify current).
• Reports on Earth's energy imbalance and ocean heat content (verify current).
• Arctic amplification, shrinking sea ice and the albedo feedback.
• Delhi-NCR winter fog and smog episodes, and inversion forecasting.
• City heat action plans, cool-roof programmes (Telangana 2023) and urban greening.
• The solar cycle 25 maximum and its small effect on insolation.