Terrestrial radiation

Also called: Earth radiation · Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"

Meaning

Terrestrial radiation (also called Earth radiation) is the heat that the Earth's surface gives off as long-wave (infrared) radiation after it has been warmed by insolation (incoming solar radiation, the Sun's energy that reaches the Earth). It matters because it is the main way the atmosphere is heated. The ground heats the air from below, not the Sun directly. This is why air near the ground is warmest, and it is the base of the greenhouse effect.

Explanation

How it forms: cause → process → result

• Cause: the Sun heats the ground, not the air.
• Insolation arrives mostly as short waves.
• Air lets most short waves pass straight through, so the Sun heats the air only a little.

• The short waves reach the ground and heat it.

• Process: the warm ground sends out long waves.

• The heated ground gives off energy as long waves. This is terrestrial radiation.
• Air does not let long waves pass easily. Greenhouse gases (gases that trap heat), mainly carbon dioxide (CO₂) and water vapour, absorb them.

• This absorbed energy heats the air.

• Result: the atmosphere is heated from below.

• 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).
• The warmed atmosphere then sends heat out to space. This balances the energy that comes in.

Short waves in, long waves out

Feature	Insolation	Terrestrial radiation
Source	Sun	Earth's heated surface
Wave type	Short waves	Long waves (infrared)
Effect on air	Mostly passes through	Mostly absorbed (CO₂, water vapour)
Role	Heats the ground	Heats the atmosphere

• Sunlight that the ground or clouds simply reflect back is not terrestrial radiation. It is still short-wave sunlight. Terrestrial radiation is only the energy the ground gives off after it has absorbed sunlight and warmed up.

Its place in the heat balance

• Averaged over the whole globe, the surface gains more radiant energy than it loses, and the atmosphere loses more than it gains [2].
• Two flows from the surface to the air close this gap [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).

• The greenhouse effect is built on terrestrial radiation.

• CO₂, water vapour and clouds absorb the outgoing long waves and keep some of the heat near the surface.
• Without greenhouse gases and clouds, the Earth's surface would be about 33°C cooler [2].

Conditions that change it

• By day: the ground absorbs more insolation than it loses, so it warms up and sends out more long waves.
• At night: there is no insolation, but the ground keeps giving off long waves. So the ground cools.

• Through conduction (heat passing between two bodies that touch), the cold ground then takes heat from the air touching it. This helps form ground frost and dew.

• Clear skies vs cloudy skies:

• Clear sky: long waves escape easily to space, so nights get colder.

• Cloudy sky: clouds absorb the long waves and send some back to the ground, so nights stay warmer.

• Land vs water:

• Land heats and cools fast, so its terrestrial radiation changes quickly between day and night.
• Water's specific heat (energy needed to raise 1 gram of a substance by 1°C) is about 4–5 times that of rock and soil. So the sea warms and cools slowly.

Where it occurs

• Everywhere on the Earth's surface. All heated ground and water give off terrestrial radiation. It is the main way the whole atmosphere is heated.
• Troposphere (worldwide): air is warmest near the ground and cools with height. This is why peaks stay snow-capped even in the tropics and why hill stations are cool.
• Equatorial belt (Amazon basin, Congo basin, Indonesia):
• Strongly heated ground warms the air above it.

• The warm air rises by convection, cumulus clouds form, and convectional rain falls almost every afternoon.

• West Bengal and Odisha: summer heating of the ground sets off the pre-monsoon Kalbaisakhi (nor'wester) thunderstorms.

• Plains of north-west India:
• The heated, dry lands send out strong terrestrial radiation and warm the air above them.
• Wind carries this hot air east over the plains as the loo.
• Heat waves mainly occur from March to June, with May as the peak month [3].

• Heat-wave-prone states include Punjab, Haryana, Delhi, Uttar Pradesh, Rajasthan, Bihar, Madhya Pradesh, Odisha, Andhra Pradesh and Telangana [3].

• Indo-Gangetic Plains (IGP) in winter:

• As a Western Disturbance (a low-pressure storm system that comes from the Mediterranean region) approaches, clouds form and night temperatures rise [4]. The cloud cover traps outgoing long waves.
• After it passes, skies clear and cold north-westerlies set in, so minimum temperatures fall [4]. Under clear skies, long waves escape easily at night.

• December and January are the coldest months over north India [4].

• Delhi vs Mumbai: inland Delhi heats and cools quickly and has a large yearly temperature range. Coastal Mumbai, next to the slow-changing sea, has a small range.

Don't confuse with

• Insolation: this is short-wave energy coming in from the Sun. Terrestrial radiation is long-wave energy going out from the Earth. The air is heated mainly by the second.
• Reflected sunlight: reflected sunlight is still short-wave and is not absorbed by the ground. Terrestrial radiation is long-wave energy given off after the ground has absorbed sunlight and warmed up.
• Conduction / convection / advection: these move heat by contact, by rising air and by wind. Terrestrial radiation moves heat as long waves, with no contact needed.
• Greenhouse effect: terrestrial radiation is the outgoing long-wave energy itself. The greenhouse effect is the trapping of that energy by CO₂, water vapour and clouds, which keeps the Earth about 33°C warmer [2].

Prelims Hooks

• The atmosphere is heated mainly by long-wave terrestrial radiation, not directly by short-wave insolation. The common trap answer is "direct solar radiation".
• The two main gases that absorb terrestrial radiation are CO₂ and water vapour.
• Short waves in (insolation), long waves out (terrestrial radiation). The air is largely transparent to short waves but absorbs long waves.
• Because the air is heated from below, temperature falls with height in the troposphere.
• Averaged over the globe, the surface gains more radiant energy than it loses, and the atmosphere loses more than it gains. Sensible heat and latent heat close the gap [2].
• Cloudy nights are warmer than clear nights. For example, an approaching Western Disturbance raises minimum temperatures, and clear skies after it passes make them fall [4].

Mains Points

• Why the troposphere is warmest at the bottom (GS-I):
• The ground absorbs short-wave insolation and gives off long-wave terrestrial radiation, which CO₂ and water vapour absorb.
• Conduction and convection then carry this heat upward, so temperature falls with height.

• This explains snow-capped tropical peaks, cool hill stations and the afternoon convectional rain of the equatorial belt.

• Terrestrial radiation, greenhouse gases and global warming (GS-III, environment):

• Greenhouse gases keep the Earth about 33°C warmer than it would otherwise be [2].

• More CO₂ traps more outgoing long waves. This upsets the heat balance and causes global warming.

• Heat waves and cold waves in India (GS-III, disasters):

• In summer, the heated dry lands of north-west India warm the air above them, and the loo carries it east. Heat waves occur from March to June and peak in May [3].
• In winter, clear skies after a Western Disturbance let long waves escape at night, and cold north-westerlies bring cold waves to the IGP [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 the early-warning responses [3].

Related concepts

• Conduction
• Convection
• Advection
• Specific heat

Read more

• Heating and cooling of the atmosphere