The sun is the primary source of energy on the earth.
This energy is radiated in all directions into space through short waves.
This is known as solar radiation.
Only two billionths or (two units of energy out of 1,00,00,00,000 units of energy radiated by the sun) of the total solar radiation reaches the earth’s surface.
This small proportion of solar radiation is of great importance, as it is the only major source of energy on the earth for most of the physical and biological phenomena.
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.
Factors influencing 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 receives 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.
 The angle of incidence.
 Duration of the day. (daily sunlight period)
 Transparency of the atmosphere.
 The Angle of Incidence
Since the earth is round, the sun’s rays strike the surface at different angles at different places.
The angle formed by the sun’s ray with the tangent of the earth’s circle at a point is called angle of incidence.
It influences the insolation in two ways.
First, when the sun is almost overhead, the rays of the sun are vertical.
The angle of incidence is large hence; they are concentrated in a smaller area, giving more amount of insolation at that place.
If the sun’s rays are oblique, angle of incidence is small and sun’s rays have to heat up a greater area, resulting in less amount of insolation received there.
Secondly, the sun’s rays with small angle, traverse more of the atmosphere, than rays striking at a large angle.
Longer the path of sun’s rays, greater is the amount of reflection and absorption of heat by atmosphere.
As a result the intensity of insolation at a place is less.
 Duration of the day
Duration of the day varies from place to place and season to season.
It decides the amount of insolation received on earth’s surface.
The longer the duration of the day, the greater is the amount of insolation received.
 Transparency of the atmosphere
Transparency of the atmosphere also determines the amount of insolation reaching the earth’s surface.
The transparency depends upon cloud cover, its thickness, dust particles and water vapour, as they reflect, absorb or transmit insolation.
Thick clouds hinder the insolation to reach the earth while clear sky helps it to reach the surface.
Water vapours absorb insolation, resulting in less amount of insolation reaching the surface.