Insolation & Heat Budget – NDA Geography Notes

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 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.

NDA tests this chapter through questions on albedo, the greenhouse effect, why maximum temperature occurs in the afternoon, 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.

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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. Some is reflected back into space by clouds and dust. Some is absorbed by the atmosphere. Only a portion reaches and is absorbed by the Earth’s surface.

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Short-Wave and 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 arriving at 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.

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Factors Affecting Insolation

Different parts of the Earth receive different amounts of insolation.

Latitude: the most important factor. At the equator, the Sun is nearly overhead and rays strike the surface at a nearly vertical angle. Energy is concentrated on a small area. At higher latitudes, 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 solar radiation reaching the surface. Clear skies allow more insolation.

Altitude: at higher altitudes, the atmosphere is thinner and clearer, so less absorption and scattering occurs.

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]

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Why Maximum Temperature Occurs at 2–4 PM

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. So logically, maximum temperature should also occur at noon. But the hottest part of the day is usually around 2:00–4:00 PM.

Two key reasons: [NDA 2012-I]

Reason 1: Transformation of solar energy into heat requires some time. The surface does not instantly convert solar energy to heat.

Reason 2: Energy received from solar radiation continues to exceed energy lost through outgoing radiation up to approximately 4:00 PM. The surface keeps gaining more energy than it loses until this point. Then the loss exceeds the gain and temperature falls.

★ IMPORTANT NDA 2012-I tested this with three statements and the answer was: Statement 1 (transformation takes time) = TRUE, Statement 3 (incoming exceeds outgoing until 4 PM) = TRUE, but Statement 2 (loss exceeds gain at 4 PM) is worded incorrectly. Loss exceeds gain after 4 PM. Answer = 1 and 3 only. [NDA 2012-I]

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Albedo

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

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]

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

Albedo is relatively higher in early morning and late evening, when the Sun is at a low angle, sunlight hits the surface at a glancing angle and reflectivity is higher.

Earth’s overall albedo: About 30% of incoming solar radiation is reflected back to space. The remaining 70% is absorbed by the atmosphere and surface.

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The Heat Budget of the Earth

The Earth maintains a relatively stable average temperature. This means it must be releasing as much energy as it receives.

If total incoming solar radiation = 100 units: About 30 units reflected back to space (Earth’s albedo). About 20 units absorbed by the atmosphere. About 50 units absorbed by the Earth’s surface. The surface and atmosphere together radiate back 70 units as long-wave terrestrial radiation, maintaining the balance.

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Why Cloudy Nights are Warmer

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, 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]

Note: there is no solar radiation at night, so clouds blocking sunlight is irrelevant here. It is entirely about outgoing terrestrial radiation.

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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. [NDA 2012-II]

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 energy back as long-wave (infrared) radiation. Greenhouse gases, particularly CO₂, water vapour, methane, and ozone, absorb this outgoing long-wave radiation and radiate some back toward the surface, keeping it warmer than it would otherwise be.

Carbon dioxide is called a greenhouse gas because it absorbs infrared radiation.

The major role of a greenhouse gas: It lets incoming sunlight pass through but stops outgoing infrared radiation.

Without the atmosphere: If Earth had no atmosphere, temperature extremes between day and night would increase dramatically: daytime much hotter, nighttime much colder.

How a glass greenhouse building works: Short-wave radiation from the Sun enters through the glass. Long-wave radiation from the warm interior CANNOT pass back out. Heat is trapped. This is the exact analogy for how greenhouse gases work in the atmosphere.

In absorption of insolation, the most significant part is played by carbon dioxide.

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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 consistent insolation throughout the year. The Sun is always relatively high in the sky. Temperature is high and stable, 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 uniformly. [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]

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The 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.

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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.

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.

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