Maarg Manthan · Topic 5.3

Insolation: Aphelion, Perihelion, Angle of Inclination of Sun’s Rays, Length of Day

Geography › Climatology · Topic 5.3

Insolation, short for incoming solar radiation, is the solar energy that reaches the Earth's surface in the form of short waves. The Earth receives only about one two-billionth of the Sun's output. The amount at a place depends on the angle of the Sun's rays, the length of the day, the transparency of the atmosphere, altitude and the distance from the Sun, which varies between perihelion (about 3 January, nearest) and aphelion (about 4 July, farthest). Insolation is greatest in the tropics and least at the poles.

5 min read · UPSC Prelims · Geography · Climatology

At a Glance

Insolation Incoming short-wave solar radiation
Perihelion About 3 January; about 147.1 million km
Aphelion About 4 July; about 152.1 million km
Main controls Angle of rays and day length
Highest insolation Clear-sky subtropical deserts
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows the structure of the atmosphere and begins the study of heat. It describes the energy that enters the system, while the next topic follows its fate through the heat budget. Previous topic: Structure of the Atmosphere: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere. Next topic: Heat Balance of the Atmosphere: Conduction, Convection, Advection, Terrestrial Radiation, Albedo.

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 Inclination of the Earth’s Axis and Its Effects.

How Do Aphelion and Perihelion Affect Insolation?

The Earth’s orbit is an ellipse, so its distance from the Sun changes during the year. The Earth is nearest to the Sun at perihelion, about 3 January, when the distance is about 147.1 million km. It is farthest at aphelion, about 4 July, at about 152.1 million km, and the mean distance is 149.6 million km. The difference in insolation between the two positions is about 7 per cent. It is small compared with the effect of the tilt of the axis, which is the main cause of seasons, as in Inclination of the Earth’s Axis and Its Effects. The slight difference does mean that the Earth receives a little more total insolation in the northern winter than in the northern summer.

What Controls the Amount of Insolation at a Place?

Factor	Effect
Angle of inclination of the Sun’s rays	Vertical rays pack energy into a small area and pass through less air, so they heat more. Slant rays spread over a larger area and lose more energy in the air. Insolation falls from the equator to the poles
Length of day	The longer the day, the more energy received. At the equator the day is about 12 hours throughout the year, and in the polar regions in summer it is 24 hours
Transparency of the atmosphere	Clouds, dust and haze reflect and scatter radiation. Clear skies over deserts give the highest insolation
Altitude	Thin air at height absorbs less, so mountain sites receive more radiation
Land and sea	Surface type controls how much is reflected and how much is absorbed
Distance from the Sun	Aphelion and perihelion, a small effect

How Is Insolation Distributed Over the Earth?

Solar constant: the energy reaching the top of the atmosphere on a surface at right angles to the Sun’s rays is about 1,361 watts per sq m. The Earth intercepts only about one two-billionth of the energy the Sun gives out.
Latitude: annual insolation is highest in the tropics and falls towards the poles. The equator receives somewhat less than the subtropics because of its cloud cover.
Hot deserts: the Sahara, Arabia and the Thar receive the highest insolation, since the skies are clear.
Seasons: the zone of maximum insolation shifts between the Tropic of Cancer and Tropic of Capricorn through the year.
Continents and oceans: insolation is nearly the same over land and sea at the same latitude, but the surface heats differently, as in Factors Controlling the Distribution of Temperature.

As an analytical point, insolation is the source of almost all energy at the surface, and solar power projects are sited where it is highest, such as Rajasthan and Gujarat. How the energy is then moved and balanced is in Heat Balance of the Atmosphere: Conduction, Convection, Advection, Terrestrial Radiation, Albedo.

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 explains how the heat is transferred and balanced in the atmosphere. Next topic: Heat Balance of the Atmosphere: Conduction, Convection, Advection, Terrestrial Radiation, Albedo.

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: insolation, perihelion and aphelion dates, factors controlling insolation. GS Paper 1: heat budget and temperature.

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 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 are perihelion and aphelion?

Perihelion is the position of the Earth nearest to the Sun, about 3 January, at a distance of roughly 147.1 million km. Aphelion is its farthest position, about 4 July, at about 152.1 million km. The Earth receives about 7 per cent more insolation at perihelion than at aphelion.

Why does insolation decrease from the equator to the poles?

Insolation decreases poleward because the Sun's rays strike the surface at a smaller angle, so the same energy is spread over a larger area and travels through a thicker layer of air. Polar regions also have long dark winters, and ice reflects much of the radiation.

Which factors control insolation?

The main factors are the angle of the Sun's rays, the length of the day, the transparency of the atmosphere, altitude and the distance between the Earth and the Sun. Clouds, dust and haze reflect radiation, so clear skies give higher insolation than cloudy ones at the same latitude.

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 The Earth is nearest to the Sun at perihelion, in early January.

True

About 147.1 million km.

3 Insolation is a form of long-wave radiation.

False

It is short-wave radiation.

4 The equator receives more insolation than the subtropical deserts annually.