What is insolation?


Insolation is a type of atmospheric phenomenon where the sunray plays an essential role. The main source of heat in the atmosphere is the sun. Sun’s rays cannot heat the atmosphere directly. The earth’s surface is first heated by the sun’s rays and then the atmosphere is heated indirectly due to the heat radiation of the warm surface. 

The radiation helps in heating the air or warming the air by transport, convection, and absorption processes. The temperature of the air is measured with the help of maximum and minimum thermometers. But the temperature of the atmosphere is not equal everywhere on the earth’s surface. Insolation is the primary reason for this variation in atmospheric temperature.

The main source of all thermal energy on earth is the sun. Energy from the sun heats the atmosphere. It has been observed that only 200 million of the total rays of the sun reach the earth, which heats the earth’s atmosphere. This method is insolation. The insolation process is described in detail below (1) & (4). 

What is insolation?

Insolation means incoming solar radiation. The rays coming from the sun or solar energy are the source of thermal energy of the earth’s surface and the atmosphere. This energy from the sun enters space in the form of light waves. A small part of the amount of radiation from the sun, which is only one part in 200 million, reaches the earth.

All the animals and plants of the earth depend on this energy. This energy from the sun is transmitted to the earth in the form of small waves at a speed of 186,000 miles/second, which is the source of the earth’s thermal energy. This incoming solar energy which is effective to heat the earth is called insolation (2).

Insolation definition

The solar radiation constantly emitted by the sun reaches the earth’s surface in the form of small waves (one part in 2oo million), due to which the atmosphere is heated. This incoming solar energy from the sun to the earth’s surface is called insolation (1).

Interesting facts about Insolation

  • Insolation is an atmospheric process that warms the earth’s surface.
  • It is the solar energy from the sun that is received and obstructed by the earth’s surface.
  • In this process, about 51% of the solar energy reaches the surface of the earth and 35% of the remaining 49% is reflected back to space by the atmosphere.
  • Only about 52% of this insolation reaches the earth’s surface (3).

Controlling factors of insolation

1. The angle of incidence of the sun rays

The angle of incidence of the sun’s rays depends on where the sun is located in the sky. Oblique sunlight spreads more than perpendicular sunlight. A larger area receives solar energy from oblique rays, but the thermal output per unit area is less. Moreover, since the oblique rays travel longer distances, their thermal effect decreases. 

On the other hand, because the perpendicular rays spread over a shorter area, the thermal effect of these rays per unit is more. Just as the thermal effect of the sun’s rays is controlled by the position of the sun in the sky, it is also controlled by the latitudinal position of a place, the length of the day, the season, etc.

2. Duration of the sun rays

If the length of the day is longer, the amount of thermal effect in a certain area is also higher because the incoming sunrays get a longer duration. The latitudinal position of a region, the rotation, and the orbital speed of the earth are responsible for the length of the day being shorter and longer. 

In the equatorial region, the length of the day and night is almost the same throughout the year i.e. the variation is less. That is why the annual variation of the thermal effect of the sun’s rays or insolation is less in this region.

In the northern hemisphere, during the northward movement in the tropics, the amount of insolation increases as the days are longer. The number of insolation increases during southward movement in the tropic region of the southern hemisphere. So it can be seen that the shorter the night, the longer the day will be and as a result, the amount of insolation will also increase. 

3. Solar constant

Due to variations in solar radiation at different times, the insolation also varies. The reason for the variation in the solar constant is the periodic eruptions and similar events on the solar surface. If the number of solar flares decreases, the solar radiation decreases and the increase in the number of solar flares increases the amount of solar radiation. Scientists believed that the increase and decrease of solar flares occur regularly in a seasonal cycle of the year. 

It is notable that the decrease in the number of solar flares has a negligible effect on insolation. Only 1 billion of the energy that is continuously radiated from the sun reaches the earth. 

The fraction of vertical solar radiation that falls above the earth’s atmosphere from the mean distance of the sun is called the solar constant.

4. Distance between the earth and the sun

Due to the revolution of the earth, the distance from the sun decreases during the orbit. As the sun is farthest from the earth during aphelion, the total amount of insolation is low. In perihelion, the earth comes to the closest position to the sun, that is, as the distance between the earth and the sun decreases, the number of insolation increases.

5. Transparency of the weather 

The less dust, smoke, or cloudiness in the atmosphere, the greater the amount of insolation on the surface. Because the sunlight that will be reflected from dust particles, clouds, etc. will not reach the earth’s surface. Therefore, the amount of insolation is naturally high in a cloudless clear sky.

6. Nature of water and land

Different types of local surface heat radiation amount is different. For example, it varies by 25% in grasslands, 30-40% in deserts, 9-18% in forested areas, and 2-3% in water bodies. Since the specific heat of water is higher than that of land, if the heat of the sun is less than 15°C, more than 50% is changed by water. 

7. Influence of landforms

Insolation varies due to differences in topography. Earth’s mountain regions or mountain shield regions receive a few hours of sunlight. So all those places are cooler due to location at one latitude (2) & (4).

Impact of insolation

  • The insolation process warms 66% of the atmosphere, called effective solar radiation. The remaining 34% is returned to space, which is called albedo. During the day the surface heats up by absorbing heat from the sun and cools down again at night by radiating heat.
  • Ocean water circulation occurs due to the unequal heating of ocean water by insolation.
  • This atmospheric process received by the earth is responsible for all forms of activity on the earth.
  • The insolation process also plays a significant role in atmospheric circulation (1) & (4).

Solar insolation

Solar insolation is the amount of solar radiation or electromagnetic energy from the sun on the surface of the earth. The energy emitted from the sun’s ring fire reaches the earth in the form of electromagnetic waves at high speed, which is called solar insolation. 

Sun’s surface temperature is 6000°C. The energy emitted by the sun reaches the earth in the form of electromagnetic waves at high speed. This electromagnetic wave’s speed is 297000 km per second. Here the light energy is converted into heat energy. So it is called solar insolation (3).


Solar insolation has calculated the formula of I= S cosZ

Angle of insolation

Due to the spherical shape of the earth, the sun’s rays fall on the earth’s surface either perpendicularly or diagonally. Perpendicular rays have more heating capacity than diagonal rays. The causes for this are-

  1. Dust particles, water particles, smoke, etc. are floating in the air. When the sun’s rays reach the earth’s surface through the outer layer, some part of that sun’s rays is absorbed by dust particles and water vapor. Perpendicular rays enter less part of the atmosphere and fall to the earth’s surface. Heat is absorbed less by perpendicular rays. So the surface is heated more by the perpendicular rays than by diagonal rays.
  2. The same number of rays are spread over a larger area if they fall diagonally. But if it falls vertically, it spreads to a relatively small area. So the place where the sun rays fall perpendicularly gets hotter. But the earth’s surface is not heated as much by the diagonally incident sun rays (4).


1. What is insolation?

The sun is the source of all thermal energy on earth. Energy from the sun heats the atmosphere. It has been observed that only one part in 200 millionths of the total rays of the sun reaches the earth, which heats the earth’s atmosphere. This atmospheric phenomenon is known as insolation.

2. Which climate zone receives the highest insolation?

Tropical deserts climate zone receives the highest insolation

3. What is solar insolation?

The amount of solar radiation incident on a unit horizontal surface at a given time for a given area is called solar insolation. It depends on the angle of the sun’s rays, and on the ratio of the actual distance to the mean distance from the earth to the sun.

4. Which climate zone receives the lowest insolation?

The polar climate zone receives the lowest insolation.

5. What is the angle of insolation?

The angle at which the sun’s rays strike a particular place on earth is known as the angle of insolation. The angle of the insolation depends on the position of the sun in the sky. 


1. Savindra Singh. Climatology. Pravalika Publications, Allahabad. Chapter 3: Insolation and energy (heat) balance. Page No: 34.

Written By: Manisha Bharati.