Maarg Manthan · Topic 6.3

Heat Transfer and Heat Budget of the Earth

Geography › Atmosphere and Temperature · Topic 6.3

The Earth receives energy from the Sun as short-wave insolation and loses an equal amount as long-wave terrestrial radiation, so its average temperature stays steady. Heat moves by radiation, conduction, convection and advection. Of 100 units of insolation, 35 are reflected as albedo, 14 are absorbed by the atmosphere and 51 by the surface, which then returns all of it to space, giving a balanced heat budget.

6 min read · UPSC Prelims · Geography · Atmosphere and Temperature

At a Glance

Incoming energy Short-wave solar radiation (insolation)
Reflected (albedo) 35 of 100 units in the standard budget
Absorbed by atmosphere 14 units
Absorbed by surface 51 units
Modes of heat transfer Conduction, convection, advection, radiation
Exam link UPSC Prelims, Geography: Atmosphere and Temperature

Where Does It Fit?

This is the third topic of the category on the atmosphere and temperature. The previous topic described the layers of the atmosphere, and this one explains how it is heated. The next topic covers the factors that decide temperature at a place. Previous topic: Structure of the Atmosphere. Next topic: Factors Affecting Temperature Distribution.

How Does the Earth Receive Heat?

In short: the Sun’s short-wave radiation warms the Earth, which sends the heat back as long-wave radiation.

The Sun is the source of almost all the Earth’s energy. The energy that reaches the Earth in the form of short waves is called insolation, an abbreviation of incoming solar radiation. The Earth intercepts only a tiny part, about two billionths, of the Sun’s output. The amount at the top of the atmosphere on a surface facing the Sun is called the solar constant, about 1,361 watts per sq m, or around 1.94 calories per sq cm per minute. Insolation reaches the Earth in short waves, mostly visible light, and the Earth, being much cooler than the Sun, sends heat back in long waves, called terrestrial radiation, mostly infrared. The Earth’s position in its orbit affects the amount: it is slightly higher at perihelion in January and lower at aphelion in July, as in Revolution, Inclination of the Axis and Seasons.

How Does Heat Move: Conduction, Convection, Advection and Radiation?

Mode	Meaning	Example
Radiation	Transfer of energy by waves, needing no medium	Sunlight reaching the Earth; the Earth sending heat to space
Conduction	Transfer through direct contact from hotter to colder part	The ground heating the air layer touching it; works mostly near the surface
Convection	Vertical movement of heated air that rises while cooler air sinks	Rising warm air that makes clouds and thunderstorms
Advection	Horizontal movement of air or water carrying heat	The summer loo of north India; sea breeze; ocean currents

Convection and advection are the main ways of transferring heat in the atmosphere. In the tropics, advection carries heat poleward, and in meteorology, the term advection is used for horizontal movement, which carries the warm and cold air masses described in Air Masses and Fronts.

What Is the Heat Budget of the Earth?

The Earth neither warms up nor cools down year after year, which means that the amount of heat received is balanced by the amount lost. The standard heat budget takes 100 units of insolation at the top of the atmosphere.

Fate of insolation	Units
Reflected by clouds and the atmosphere	27
Reflected by snow and ice-covered surfaces	2
Scattered back to space	6
Total reflected, the albedo of the Earth	35
Absorbed by the atmosphere	14
Absorbed by the Earth’s surface	51

The 51 units absorbed by the surface return to the atmosphere and space: 17 are radiated directly to space, and 34 are absorbed by the atmosphere, of which 6 by direct absorption of terrestrial radiation, 9 by convection and turbulence and 19 as latent heat of condensation. The atmosphere radiates 48 units to space, from its own 14 plus the 34 it received. The total returned to space is 35 + 17 + 48 = 100 units, and so the budget balances. Albedo is the share of insolation reflected by a surface. Fresh snow reflects about 80 to 90 per cent, clouds up to about 80 per cent, and dark forests and oceans only a small part. Modern satellite measurements put the Earth’s albedo at about 29 to 30 per cent, a little lower than the textbook figure of 35 per cent, but the structure of the budget is the same.

What Is the Greenhouse Effect and the Energy Imbalance?

The atmosphere is largely transparent to short-wave insolation but absorbs much of the long-wave radiation from the Earth, particularly through water vapour, carbon dioxide, methane and other greenhouse gases. These gases re-radiate part of the heat downward, which keeps the average surface temperature at about 15 degrees Celsius instead of about minus 18 degrees without them. This natural greenhouse effect is essential for life. Human emissions have added greenhouse gases, as in Evolution and Composition of the Atmosphere, so that the Earth now absorbs more energy than it emits, an energy imbalance of roughly one watt per sq m measured by satellites, which has raised the global temperature by about 1.1 to 1.3 degrees Celsius above the pre-industrial level. The year 2024 was the warmest on record according to the World Meteorological Organization, at about 1.55 degrees above the 1850 to 1900 average. As an analytical point, the budget is a balance of large flows, so even a small shift of less than one per cent can drive climate change.

Why Is the Heat Budget Not Balanced Everywhere?

Although the Earth as a whole is in balance, there are surpluses and deficits by latitude. The tropics gain more energy than they lose, while the polar regions lose more than they gain. The atmosphere and the oceans move heat from the surplus areas to the deficit areas through winds and currents, which prevents the tropics from getting hotter and the poles from getting colder. This is the subject of Horizontal Distribution of Temperature and Latitudinal Heat Balance.

Why Is This Topic Important?

Numbers 35, 14, 51 asked
Climate Greenhouse effect and warming
Concepts Albedo and advection

What Came Next?

The next topic describes the factors that affect temperature distribution. Next topic: Factors Affecting Temperature Distribution.

Key People and Terms

Insolation Incoming solar radiation
Albedo Share of light reflected
Terrestrial radiation Long-wave heat from the Earth
Latent heat Heat released on condensation
Advection Horizontal heat transport
Solar constant Energy at top of atmosphere

Exam Corner

Points to Remember

Albedo is 35 units; atmosphere absorbs 14; surface absorbs 51.
Insolation is short-wave; terrestrial radiation is long-wave.
Convection is vertical; advection is horizontal.
Fresh snow has high albedo.
2024 was the warmest year on record.

Do Not Confuse With

Convection and advection Vertical versus horizontal
Insolation and terrestrial radiation Short-wave in versus long-wave out
Albedo and absorption Reflected versus taken in

Memory Hook

Thirty-five bounce, fourteen caught, fifty-one warm the ground; what comes in must go out.

Mains Angle

Questions ask for heat budget and the greenhouse effect. These points add depth.

Balance Incoming equals outgoing
Greenhouse Natural and enhanced
Transport Heat moved poleward

A Question You May Face

An original practice question, not a past paper question.

“Explain the heat budget of the Earth and show how greenhouse gases affect it.”

How to Answer

Inputs Insolation.
Budget 35, 14, 51.
Balance Return to space.
Greenhouse Imbalance today.

GS Relevance

Prelims: heat budget numbers, albedo, greenhouse effect, modes of heat transfer. GS Paper 1: atmosphere and climate; GS Paper 3: global warming.

Frequently Asked Questions

What is insolation?

Insolation is the incoming solar radiation received by the Earth in the form of short waves. The Earth intercepts only a very small part of the Sun's output, and the amount received varies with latitude, season, the length of day and the angle of the Sun's rays.

What is the heat budget of the Earth?

The heat budget is the balance between incoming solar radiation and outgoing terrestrial radiation. Of 100 units of insolation, 35 are reflected, 14 are absorbed by the atmosphere and 51 by the surface, and all of it is returned to space, so the Earth's temperature remains steady.

What is albedo?

Albedo is the fraction of incoming solar radiation reflected by a surface or by the Earth as a whole. Fresh snow and clouds have high albedo, while dark forests and oceans have low albedo. The Earth's albedo is about 30 to 35 per cent.

What is the difference between convection and advection?

Convection is the vertical movement of heated air, in which warm air rises and cooler air sinks. Advection is the horizontal movement of air or water that carries heat from one place to another, such as sea breezes, warm winds and ocean currents.

What is the greenhouse effect?

The greenhouse effect is the warming caused by gases such as water vapour, carbon dioxide and methane, which let short-wave sunlight pass but absorb long-wave heat from the Earth and send part of it back. It keeps the Earth's average temperature near 15 degrees Celsius.

PYQ Practice — Statement Analysis

1 The Earth's albedo in the standard heat budget is 35 per cent.

True

That is the share reflected.

2 Terrestrial radiation is in the form of short waves.

False

It is long-wave radiation.

3 Advection is the vertical movement of heat in the atmosphere.