The sun, like all the other stars in the universe, is a self-luminous mass of gases that emit radiant energy. The sun is the major source of energy, either directly or indirectly, for the entire earth system.

The energy emitted by the sun comes from nuclear reactions that takes place in its interior. There, under high pressure, hydrogen is changed into helium through nuclear fusion in a process similar to that in a hydrogen bomb. This nuclear reaction releases tremendous amounts of energy that radiate out from the sun in all directions at the speed of light.

Energy emitted by the sun is in the form of Electromagnetic Radiation. About 41% of the of this spectrum of waves is in the form of visible light rays, but much of the sun's radiation cannot be seen by the human eye.

As the sun's energy passes through the earth's atmosphere, it loses over half of its intensity through various processes.

Insolation

Incoming solar radiation through short waves is termed as insolation. The amount of insolation received on the earth’s surface is far less than that is radiated from the sun because of the small size of the earth and its distance from the sun. Moreover water vapour, dust particles, ozone and other gases present in the atmosphere absorb a small amount of insolation.

The amount of insolation received on the earth’s surface is not uniform everywhere. It varies from place to place and from time to time. The tropical zone receive the maximum annual insolation. It gradually decreases towards the poles. Insolation is more in summers and less in winters. 

The following factors influence the amount of insolation received:

  1. The angle of incidence
  2. Duration of the day (daily sunlight period)
  3. Transparency of the atmosphere

Heating of Atmosphere

Sun is the ultimate source of atmospheric heat and energy, but its effect is not direct. There are four heating processes directly responsible for heating the atmosphere. They are:

  1. Radiation
  2. Conduction
  3. Convection
  4. Advection

Radiation

Radiation is the process by which solar energy reaches the earth and the earth loses energy to outer space. When the source of heat transmits heat directly to an object through heat waves, it is known as radiation process. Insolation reaches the earth’s surface in short waves and heat is radiated from the earth in long waves.

Conduction

When two objects of unequal temperature come in contact with each other, heat energy flow from the warmer object to the cooler object and this process of heat transfer is known as conduction. The conduction in the atmosphere occurs at zone of contact between the atmosphere and the earth’s surface. However, this is a minor method of heat transfer in terms of warming the atmosphere since it only affects the air close to the earth’s surface.

Convection

Transfer of heat by movement of a mass or substance from one place to another is called convection. The air of the lower layers of the atmosphere get heated either by the earth’s radiation or by conduction. The heating of the air leads to its expansion. Its density decreases and it moves upwards. Continuous ascent of heated air creates vacuum in the lower layers of the atmosphere. As a consequence, cooler air comes down to fill the vacuum, leading to convection.

Advection

Winds carry the temperature of one place to another. The temperature of a place will rise if it lies on the path of winds coming from warmer regions. The temperature will fall if the place lies on the path of the winds blowing from cold regions. This process of horizontal transport of heat by winds is known as advection.

Heat Budget

Suppose that the total heat (incoming solar radiation) received at the top of the atmosphere is 100 units. Roughly 35 units of it are reflected back into space even before reaching the surface of the earth. Out of the remaining 65 units, only 51 units reach the earth’s surface and 14 units are absorbed by the various gases, dust particles and water vapour of the atmosphere.

The earth in turn radiates back 51 units in the form of terrestrial radiation. Out of these 51 units of terrestrial radiation, 34 units are absorbed by the atmosphere and the remaining 17 units directly go to space.  The atmosphere also radiates 48 units (14 units of incoming radiation and 34 units of outgoing radiation absorbed by it) back to space. Thus 65 units of solar radiation entering the atmosphere are reflected back into the space.

This account of incoming and outgoing radiation always maintains the balance of heat on the surface of the earth. Heat budget is the balance between insolation (incoming solar radiation) and terrestrial radiation.