Insolation & Heat Budget – NDA Geography Notes

Introduction

The Sun is the ultimate source of energy for almost everything that happens on Earth — weather, ocean currents, winds, the growth of plants, and the warmth that makes life possible. But the way the Sun’s energy reaches the Earth, how it is absorbed, how it is reflected, and how the Earth maintains a balance between incoming and outgoing energy — this is what this chapter explains.

For a science student, this chapter connects directly to Physics — radiation, heat transfer, and energy balance are concepts you already know. The difference here is that we apply them at the scale of the entire Earth.

NDA tests this chapter through questions on albedo, the greenhouse effect, why the Earth’s maximum temperature occurs in the afternoon rather than at noon, why cloudy nights are warmer, the concept of terrestrial radiation, and the thermal equator. These are not pure memory questions — they test whether you understand why things happen the way they do.

Understand the Topic

What is Insolation?

Insolation stands for INcoming SOLar radiATION. It is the energy received by the Earth from the Sun.

The Sun produces energy through nuclear fusion and radiates it outward as electromagnetic radiation — including visible light, ultraviolet radiation, and infrared radiation.

Not all of this radiation reaches the Earth’s surface. As sunlight passes through the atmosphere, some is reflected back into space by clouds, dust, and the Earth’s surface. Some is absorbed by the atmosphere. Only a portion finally reaches and is absorbed by the Earth’s surface.

Short-wave vs Long-wave Radiation

The Sun is very hot — about 6,000°C at its surface. Hot objects radiate energy at short wavelengths — visible light and UV radiation. So the Sun’s energy that reaches Earth is short-wave radiation.

The Earth’s surface is much cooler — average about 15°C. Cool objects radiate energy at long wavelengths — infrared radiation. So the Earth radiates heat outward as long-wave radiation, also called terrestrial radiation.

This distinction — short-wave incoming, long-wave outgoing — is fundamental to understanding the greenhouse effect and why the atmosphere is heated the way it is.

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How the Atmosphere is Heated

The atmosphere is NOT heated directly by the Sun’s short-wave radiation to any significant extent. Instead, it is heated mainly from below — by the long-wave terrestrial radiation from the Earth’s surface.

The Earth’s atmosphere is mainly heated by long-wave terrestrial radiation. [NDA 2021-II]

This is why temperatures are highest at the surface and decrease with altitude in the troposphere — the atmosphere is heated from below, and heat decreases as you move upward away from the heat source.

Factors Affecting Insolation

Different parts of the Earth receive different amounts of insolation. The factors that determine how much solar energy a place receives include:

Latitude — the most important factor. At the equator, the Sun is nearly overhead and rays strike the surface at a nearly vertical angle. A given amount of solar energy is concentrated on a small area — so energy intensity per unit area is high. At higher latitudes toward the poles, the Sun is at a lower angle in the sky — the same amount of energy is spread over a larger area, so intensity is lower. This is why tropical regions are hot and polar regions are cold. [NDA 2011-II]

Length of Day — longer days mean more hours of sunlight and more total insolation.

Transparency of the Atmosphere — clouds, dust, and water vapour reduce the solar radiation reaching the surface. Clear skies allow more insolation.

Altitude — at higher altitudes, the atmosphere is thinner and clearer. Less absorption and scattering. Mountain areas receive more intense solar radiation.

Factors causing variation in insolation:
Rotation of the Earth on its axis, length of day, and distribution of land and water on the Earth all cause variation in the amount of insolation received. [NDA 2025-II]

Why Maximum Temperature Occurs at 2–4 PM, Not at Noon

This is one of the most interesting and frequently tested concepts in this chapter.

The Earth receives maximum solar radiation at noon — when the Sun is highest in the sky and rays are most direct. So logically you might expect the maximum temperature to also occur at noon. But the hottest part of the day is usually around 2:00–4:00 PM.

Why?

The Earth’s surface takes time to heat up. At noon, it is receiving maximum solar energy, but that energy has to be absorbed and converted to heat first. The surface continues to gain more energy than it loses until about 2–4 PM. After that, the loss of energy through outgoing radiation exceeds the incoming solar energy, and temperature starts to fall.

Two key reasons: [NDA 2012-I]

1. Transformation of solar energy into heat requires some time — TRUE
2. Energy received from solar radiation continues to exceed energy lost through outgoing radiation up to 4:00 PM — TRUE

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Albedo

Albedo is the proportion of solar radiation that is reflected by a surface back into space without being absorbed.

If a surface reflects 30% of the light that hits it, its albedo is 30%.

High albedo = reflects a lot = absorbs less heat = stays cooler.
Low albedo = reflects little = absorbs more heat = gets warmer.

The term albedo implies the proportion of shortwave solar radiation reflected by a surface. [NDA 2013-II]

The Earth’s reflectivity (albedo) is highest in snow-covered areas. Fresh snow reflects about 80–90% of incoming sunlight. [NDA 2007-II]

A snow-covered mountain has the highest albedo among commonly compared surfaces. [NDA tested via CDS]

Albedo values for different surfaces:

Surface	Approximate Albedo
Fresh snow	80–90%
Ice	50–70%
Sand desert	30–40%
Cropland	15–25%
Forest	10–15%
Ocean	5–10%
Dark soil	5–10%

When is albedo highest?

Albedo is relatively higher in early morning and late evening. When the Sun is at a low angle, sunlight travels through more atmosphere and hits the surface at a glancing angle. Reflectivity is higher at low angles of incidence. [NDA tested via CDS]

Earth’s overall albedo:

About 30% of incoming solar radiation is reflected back to space — this is Earth’s albedo. The remaining 70% is absorbed by the atmosphere and surface. This 30% reflected fraction is called Earth’s albedo. [NDA tested via CDS]

The Heat Budget of the Earth

The Earth maintains a relatively stable average temperature over long periods. This means it must be releasing as much energy as it receives — otherwise it would keep getting hotter or cooler.

The balance between incoming solar radiation and outgoing terrestrial radiation is called the heat budget or radiation balance.

If we take total incoming solar radiation as 100 units:

• About 30 units are reflected back to space (Earth’s albedo = 30%)
• About 20 units are absorbed by the atmosphere
• About 50 units are absorbed by the Earth’s surface

The surface and atmosphere together radiate back 70 units (20 + 50) as long-wave terrestrial radiation — maintaining the balance.

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Why Cloudy Nights are Warmer Than Clear Nights

On a clear night, the Earth’s surface radiates heat freely into space as long-wave radiation. The heat escapes easily and the surface cools rapidly.

On a cloudy night, the clouds act like a blanket — they absorb the long-wave radiation from the surface and radiate some of it back downward. This keeps the surface warmer.

Cloudy nights are warmer than clear nights because of terrestrial radiation — clouds trap the outgoing long-wave terrestrial radiation and radiate some back toward the surface. [NDA 2010-I]

The Greenhouse Effect

The natural greenhouse effect is essential for life. Without it, the Earth’s average temperature would be about –18°C instead of the current +15°C.

How it works:

The Sun’s short-wave radiation passes through the atmosphere and is absorbed by the Earth’s surface. The surface warms and radiates this energy back as long-wave (infrared) radiation. Greenhouse gases in the atmosphere — particularly CO₂, water vapour, methane, and ozone — absorb this outgoing long-wave radiation and radiate some of it back toward the surface. This keeps the surface warmer than it would otherwise be.

Carbon dioxide is called a greenhouse gas because it absorbs infrared radiation. [NDA tested via CDS]

The major role of a greenhouse gas is that it lets incoming sunlight pass through but stops outgoing infrared radiation. [NDA tested via CDS]

Average surface temperature of Earth = 15°C. [NDA 2012-II]

Without the atmosphere:
If the Earth did not have an atmosphere, temperature extremes between day and night would increase dramatically. [NDA tested via CDS]

Greenhouse effect in a glass building:

A greenhouse (glass structure) works because:

• Shorter wavelength infrared radiation from the Sun can enter through the glass
• Longer wavelength infrared radiation from the ground and plants inside CANNOT pass back out through the glass
• Heat is trapped inside

This is the exact analogy for how greenhouse gases work in the atmosphere. [NDA tested via CDS]

In absorption of insolation, the most significant part is played by carbon dioxide. [NDA tested via CDS]

Temperature Distribution and Isotherms

An isotherm is a line on a map connecting all points that have the same temperature at the same time.

Annual range of temperature:

The equator receives fairly consistent insolation throughout the year — the Sun is always relatively high in the sky. So the temperature is high and stable throughout the year — minimum annual range. [NDA 2006-II]

As you move toward the poles, the difference between summer and winter temperatures increases — the annual range of temperature increases with latitude. [NDA 2010-I]

Isotherm patterns:

In the Southern Hemisphere (mostly ocean), isotherms are more parallel to latitude because the ocean moderates temperature and creates a more uniform pattern. [NDA 2016-II]

In the Northern Hemisphere, isotherms are deflected toward lower latitudes over continents in winter because land cools faster than ocean.

In winter, the minimum temperature is recorded in interior continental areas like Northern Canada and Siberia — not on the coasts. [NDA 2016-II]

Environmental Temperature Lapse Rate:
Air temperature is measured at a standard height of 1.2 m (4 feet) above the ground surface. The average rate of temperature decrease with height is called the environmental temperature lapse rate. [NDA 2024-I]

Thermal Equator

The thermal equator (or heat equator) is the line connecting all points of highest mean annual temperature around the Earth. It is NOT the same as the geographical equator (0° latitude).

The thermal equator lies north of the geographical equator — at approximately 5°N to 10°N. This is because the Northern Hemisphere has more landmass than the Southern Hemisphere. Land heats up faster and to higher temperatures than ocean. So the zone of maximum temperature is shifted northward.

The thermal equator is found north of the geographical equator. [NDA tested via CDS]

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Adiabatic Temperature Changes

When a parcel of air rises, it moves into lower pressure regions. Lower pressure means the air expands. Expanding air cools — without exchanging heat with its surroundings. This cooling is called adiabatic cooling.

When air sinks, the opposite happens — it is compressed and warms. This is adiabatic warming.

The cause of adiabatic temperature changes is expansion and compression of air. [NDA tested via CDS]

Adiabatic processes involve NO heat exchange with surrounding air. The cooling or warming is purely due to pressure changes.

Adiabatic cooling explains why high mountains are cold even in the tropics — rising air expands and cools, causing precipitation on the windward side and a rain shadow on the leeward side.

Zones of Insolation

The Earth’s surface is divided into zones based on how much solar radiation they receive:

NDA 2025-II asked to arrange these zones in ascending order of latitudinal extent:

1. Equatorial zone (approximately 0°–5°N/S) — narrowest
2. Midlatitude zone (35°–65°N/S)
3. Subarctic zone (65°–80°N/S)
4. Tropical zone (5°–35°N/S)

The NDA answer is (d) 3 – 1 – 2 – 4 — meaning from smallest to largest latitudinal extent: Subarctic (3) → Equatorial (1) → Midlatitude (2) → Tropical (4). This seems counterintuitive since the Tropical zone covers a wider belt than the Midlatitude zone in terms of degrees. Follow the NDA PYQ answer as given.