Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy.
That is true, but what most do not understand is the amazing amount of information the radiation contains. Information such as location, object type, and shape can all be obtained from measuring radiated energy intensity patterns.
As the earth is a geoid resembling a sphere, the sun’s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun’s energy. On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.
During its revolution around the sun, the earth is farthest from the sun (152 million km) on 4th July. This position of the earth is called aphelion.
On 3rd January, the earth is the nearest to the sun (147 million km). This position is called perihelion.
The amount and the intensity of insolation vary during a day, in a season and in a year. The factors that cause these variations in insolation are:
- the rotation of earth on its axis;
- the angle of inclination of the sun’s rays;
- the length of the day;
- the transparency of the atmosphere;
- the configuration of land in terms of its aspect.
The fact that the earth’s axis makes an angle of 66 with the plane of its orbit round the sun has a greater influence on the amount of insolation received at different latitudes. Note the variations in the duration of the day at different latitudes.
The second factor that determines the amount of insolation received is the angle of inclination of the rays. This depends on the latitude of a place. The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays.
Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds colour to the sky.
The red colour of the rising and the setting sun and the blue colour of the sky are the result of scattering of light within the atmosphere.
Spatial Distribution of Insolation at the Earth’s Surface. The insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles.
