Study Notes on Insolation

INSOLATION


Insolation is a measure of solar radiation energy received on a given surface area in a given time. Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. 
The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum. On an average the earth receives 1.94 calories per sq. cm per minute at the top of its atmosphere.

Factors affecting the distribution of insolation:

• Distance between the Earth and the Sun-

The solar output received at the top of the atmosphere varies slightly in a year due to the variations in the distance between the earth and the Sun. 
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 nearest to the sun (147 million km). This position is called perihelion. Therefore, the energy received by the earth on 3rd January is slightly more than the amount received on 4th July.

• Angle of inclination of the sun's rays-
It depends on the latitude of a place. The higher the latitude the less is the angle it makes with the surface of the Earth, resulting in slanting Sun rays. Hence, the area covered by slanting rays is always higher than that of the vertical rays. Thus, the energy gets distributed in a larger area and the net energy received per unit area decreases. Thus the amount of insolation received at the Earth's surface decreases from equator towards the poles. The total amount of insolation received at the equator is about four times higher than that of the poles.

• Length of the day-
During the summer season days are longer than the night. 
The situation is reversed in the winter season. 
The longer the day, the more is the insolation received by earth.

• Altitude of land-
Insolation heats up the land which in turn warms the air above it by conduction and convection. As higher land is further away from the main heat source it is relatively cooler. Also the density of air decreases with height.

SUNPOTS- 

• Sunspots are created in the solar outer surface due to periodic disturbances and explosions. 
• The sunspots are cyclic in nature. 
• The increase and decrease of number of sunspots is completed in cycle of 11 years. 
• The amount of insolation received at earth's surface increases and decreases with number of sunspots.

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