Definition and Explanation of Insolation

Most of the energy obtained by the earth’s surface is in short wavelengths. The energy that the earth gets from the sun is known as insolation or incoming solar radiation. The sun’s rays fall at the top of the atmosphere obliquely because the planet is a geoid, and the earth captures only a little part of the sun’s energy. At the top of its atmosphere, the planet receives an average of 1.94 calories per sq. cm each minute. 

What is insolation?

In simple terms, insolation can be defined as the incoming solar radiation from the sun, which is received on earth in short waves. As the size of the earth and its distance from the sun are both relatively small, it receives significantly less quantity of insolation.

Only two billionths of a percent of total solar radiation reach the earth’s surface, which is two units of energy out of 1,000,00,000 units of energy radiated by the sun! This small part of solar radiation is critical, since it is the primary source of energy on the planet for most physical and biological processes. Furthermore, a small amount of insolation is also absorbed by dust particles, ozone, water vapour, and other gases in the atmosphere.

Major characteristics of insolation

Having understood what is insolation, let’s know more about its fundamental characteristics: 

1) Only 51% of the sun’s insolation reaches the earth’s surface.

2) The atmosphere reflects 35% of the remaining 49% into space.

Factors influencing insolation

Earth’s surface does not receive the amount of insolation uniformly. It varies across locations on the planet. Annual insolation is highest in the tropical zone. It steadily decreases from the tropics to the poles. Moreover, insolation is lesser in winters than in summers.

The amount of insolation received is influenced by the following factors:

• The incidence angle 

Because the Earth is spherical, the rays of the sun fall on the surface at various angles in different locations. The angle of incidence is the angle formed at a place by the rays of the sun and the tangent of the earth’s circle. It has two effects on insolation- 

1. Sun Rays are vertical when the sun is almost overhead. As the angle of incidence is large, they are concentrated in a smaller area, resulting in more insolation. When sun rays are oblique, the angle of incidence is small, and they heat a larger area, resulting in less insolation.
2. Sun Rays falling at a small angle travel the atmosphere more than sun rays striking at a large angle. The greater the quantity of heat reflection and absorption by the atmosphere, the longer the journey of sun rays. As a result, a location’s intensity of insolation is reduced.

• Duration of the day

The day varies depending on where you are and the ongoing season. It determines how much sunlight reaches the earth’s surface. The amount of insolation received is proportional to the length of the day. 

• The atmosphere’s transparency

The amount of insolation reaching the earth’s surface is also determined by the atmosphere’s transparency. The atmosphere’s transparency is affected by the cloud cover, dust particles, water vapour, and cloud thickness. For example, thick clouds in the sky prevent sunlight from reaching the earth, while a clear sky allows the sunlight to reach the surface easily.

Heating and cooling of the atmosphere

Four different heating processes cause the heating of the atmosphere, viz—Radiation, Conduction, Convection and Advection.

• Radiation

The mechanism by which solar energy is received and reflected by the earth is radiation. Radiation occurs when a source of heat sends heat directly to an object via heat waves. Radiation transports a large portion of the heat energy entering and exiting the earth.

• Conduction

Conduction is the process of transfer of heat energy from a warmer object to a colder object. The zone of contact between the atmosphere and the earth’s surface is where conduction occurs in the atmosphere.

• Convection

Heat is transferred by moving a mass or substance from one place to another. Convection is the movement of air from one location to another, usually in a vertical direction. 

• Advection

Advection is the horizontal movement of heat by winds. If a location is in the path of warm winds, the temperature of the location will rise. If the location is the path of cold winds, the temperature will drop. 

Conclusion

We’ve learned that the sun is the Earth’s primary energy source. Insolation is the term for the energy from the sun that reaches the earth in short waves. The amount of insolation is determined by three factors: angle of incidence, length of the day, and the atmosphere’s transparency. Radiation, conduction, convection, and advection are the processes that contribute to the heating and cooling of the atmosphere.