Heating and cooling of the atmosphere

Detail

1. The atmosphere is heated from below

• Insolation (incoming solar radiation: the Sun's energy that reaches the Earth) comes in mostly as short waves.
• Air lets most of these short waves pass straight through, so the Sun does not heat the air directly very much.
• How the air is heated (step by step):
• Short-wave insolation reaches the ground and heats it.
• The warm ground gives off energy as long waves. This is terrestrial radiation (heat given off by the Earth's surface as long-wave, infrared radiation).
• Greenhouse gases (gases that trap heat), mainly carbon dioxide (CO₂) and water vapour, absorb these long waves. This heats the air.

• The warmed atmosphere then sends heat out to space. This balances what comes in.

• So the air is heated indirectly, by the ground. The correct answer to "the atmosphere is mainly heated by…" is long-wave terrestrial radiation.

• Result: air close to the ground is warmest. Temperature falls as you go higher in the troposphere (the lowest layer of the atmosphere, where all weather happens).
• Global picture:
• Averaged over the whole globe, the surface gains more radiant energy than it loses, and the atmosphere loses more than it gains [2].
• The gap is closed by two flows from the surface to the air [2]:
  • sensible heat (heat you can feel as a temperature change), moved by conduction and convection;
  • latent heat (heat stored in water vapour during evaporation and released when the vapour condenses into cloud).
• Without greenhouse gases and clouds, the Earth's surface would be about 33°C cooler [2].

2. Conduction

• Conduction: heat passes between two bodies that touch, always from the warmer to the cooler body.
• It goes on until both are at the same temperature, or until they stop touching.
• Process: the ground is heated by day → the thin layer of air touching it is warmed by contact.
• Limit: air conducts heat poorly, so conduction heats only the lowest few layers of air.
• At night the process runs in reverse: the ground cools fast → it takes heat from the air touching it → that air cools (this helps cause ground frost and dew).

3. Convection

• Convection: heat carried upward by rising currents of warm air.
• Cause → process → result:
• Air warmed by the ground expands and becomes lighter.
• It rises in vertical currents, carrying heat upward. Cooler air sinks to take its place.
• The rising air cools, its water vapour condenses, and cumulus clouds (tall, puffy, cauliflower-shaped clouds) form.

• Often there is afternoon convectional rain with thunder.

• Convection happens only in the troposphere.

• Where it is seen:
• Equatorial belt (Amazon basin, Congo basin, Indonesia): convectional rain falls almost every afternoon.
• India: pre-monsoon thunderstorms in summer, e.g. the Kalbaisakhi (nor'westers) of West Bengal and Odisha.

4. Advection

• Advection: heat carried sideways (horizontally) by moving air, i.e. by wind.
• Sideways movement of air matters more than vertical movement.
• Middle latitudes: most day-to-day weather change comes from advection. For example, warm air moving north or cold air moving south brings sudden warm or cold spells.
• Loo (north India, May–June):
• The loo is a hot, dry local wind of north and north-west India in summer.
• Hot air from the heated dry lands of the north-west is carried east over the plains. This is an advection effect.
• May is the peak month for heat waves in India. Heat waves mainly occur from March to June, and rarely in July [3].

• Heat-wave-prone areas are the plains of north-west India, central, east and north Peninsular India. These include Punjab, Haryana, Delhi, Uttar Pradesh, Rajasthan, Bihar, Madhya Pradesh, Odisha, Andhra Pradesh and Telangana [3].

• Cold-air advection (winter cold waves):

• In winter, north-westerly winds blow over the Indo-Gangetic Plains (IGP). They come from the colder lands of Central Asia and the Hindu Kush, so temperatures fall [4].
• Western Disturbance (WD): a low-pressure storm system that comes from the Mediterranean region and brings winter rain and snow to north-west India.
  • As a WD approaches: clouds form and moist winds blow in from the Arabian Sea and/or the Bay of Bengal. Night temperatures rise and the cold wave weakens [4].
  • After a WD moves away (its "rear"): skies clear and cold north-westerlies set in. Minimum temperatures fall [4].
• December and January are the coldest months over north India [4].

Process	Direction of heat flow	Medium	Where it matters most
Radiation (terrestrial)	Ground → air → space	Long waves; no contact needed	Main way the whole atmosphere is heated
Conduction	Warmer → cooler body in contact	Direct contact	Thin layer of air touching the ground
Convection	Vertical (upward)	Rising air currents	Troposphere; tropics; summer afternoons
Advection	Horizontal (sideways)	Wind	Middle latitudes; loo; cold waves

5. How IMD defines heat waves and cold waves (temperature events made worse by advection)

Event	Basic condition	Normal event	Severe event
Heat wave (IMD) [3]	Maximum temperature ≥ 40°C in plains, ≥ 30°C in hills	Departure from normal +4.5 to +6.4°C, or actual maximum ≥ 45°C	Departure > +6.4°C, or actual maximum ≥ 47°C
Heat wave, coastal [3]	Actual maximum ≥ 37°C	Departure ≥ +4.5°C	—
Cold wave (IMD) [4]	Minimum temperature ≤ 10°C in plains, ≤ 0°C in hills	Departure −4.5 to −6.4°C, or actual minimum ≤ 4°C (plains)	Departure −6.4°C or lower, or actual minimum ≤ 2°C
Cold wave, coastal [4]	Actual minimum ≤ 15°C	Departure −4.5°C or lower	—

• An event is declared only when the criteria are met at at least 2 stations in a meteorological sub-division for at least 2 days in a row. It is declared on the second day [3][4].
• IMD's "normal" temperatures are based on the 1991–2020 climate record [3].
• Cold wave days are highest over the plains of north-west and nearby central India, at an average of 6–8 days a year (data 1971–2019) [4].

6. Specific heat: why land and sea behave differently

• Specific heat: the energy needed to raise the temperature of 1 gram of a substance by 1°C.
• Water's specific heat is about 4–5 times that of rock and soil. Water needs much more energy to warm up by the same amount.
• Three more reasons why water warms slowly:
• Mixing: waves and currents carry heat down into deeper water, so the surface does not get very hot.
• Transparency: sunlight passes several metres into water, so the heat is spread through a thick layer. On land, heat stays in a thin top layer.

• Evaporation: water uses up heat as it evaporates, which keeps the surface cool.

• Result: land heats and cools quickly, while the sea heats and cools slowly.

• What this leads to:
• Continentality (inland places have hot summers, cold winters and a large yearly temperature range). Compare Delhi (inland) with Mumbai (coastal), which has a small range.
• Land and sea breezes:
  • By day, land heats faster → low pressure over land → a cool breeze blows from the sea to the land.
  • At night, land cools faster → the breeze blows from the land to the sea.
• Monsoon:
  • In summer, land heats faster → low pressure over north-west India.
  • Moist south-west winds are drawn in from the Indian Ocean.
  • This brings monsoon rain.

Feature	Land	Water (sea)
Specific heat	Low	About 4–5 times higher
Depth heated	Thin top layer	Deep (sunlight passes in; mixing)
Heat lost by evaporation	Little	A lot
Heating and cooling	Fast	Slow
Temperature range	Large (continental)	Small (maritime)

Prelims Hooks

• The atmosphere is heated mainly by long-wave terrestrial radiation from the ground, not directly by short-wave insolation.
• The two main gases that absorb terrestrial radiation are CO₂ and water vapour.
• Conduction heats only the air touching the ground. Convection is vertical and happens only in the troposphere. Advection is horizontal.
• In middle latitudes, most day-to-day weather change is caused by advection, not convection.
• The loo (north India, May–June) is an advection effect. May is the peak heat-wave month in India [3].
• Convection → cumulus clouds → afternoon rain (equatorial belt). Do not mix this up with the loo, which is advection.
• IMD heat wave: max ≥ 40°C (plains), ≥ 30°C (hills), ≥ 37°C (coast). Severe heat wave if the actual maximum is ≥ 47°C [3].
• IMD cold wave: min ≤ 10°C (plains), ≤ 0°C (hills). Severe cold wave if the actual minimum is ≤ 2°C in the plains [4].
• An approaching Western Disturbance raises minimum temperatures. After it passes, minimum temperatures fall [4].
• Water's specific heat is about 4–5 times that of land. This is the root cause of continentality, land and sea breezes and the monsoon.

Mains Points

• Why the troposphere is warmest at the bottom:
• The air is heated from below by terrestrial radiation, conduction and convection.

• So temperature falls with height. This explains snow-capped peaks even in the tropics and why hill stations are cool.

• Advection and climate risk in India (GS-III, disasters):

• Hot north-westerly advection (the loo) causes heat waves from March to June over north-west, central and east India [3].
• Cold north-westerlies after Western Disturbances cause cold waves over the Indo-Gangetic Plains [4].

• Both put health, farm labour, crops (e.g. wheat at the grain-filling stage) and power demand at risk. IMD's colour-coded warnings and heat action plans are early-warning responses [3].

• Land–sea heating contrast as the driver of Indian climate:

• Differences in specific heat create low pressure over hot land in summer. This sets off the south-west monsoon.

• It also explains the gap between the mild coasts (Mumbai, Chennai) and the extreme interior (Delhi, Rajasthan).

• Energy balance and the greenhouse effect:

• Greenhouse gases keep the Earth about 33°C warmer than it would otherwise be [2].
• More CO₂ traps more terrestrial radiation, which upsets the heat balance and causes global warming.