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Insolation is the energy received on the earth surface from the sun.
Although the entire amount of insolation reaching the earth has to pass through the atmosphere, very little of it is absorbed by the atmosphere before reaching the earth’s surface. This is because solar radiation is in the form of short waves for which the atmosphere almost acts as a transparent medium.
When the heated surface of the earth radiates this energy back it is in the form of long waves which is absorbed by the atmosphere.
Distribution of insolation:
Solar constant is the amount of solar energy received upon a unit area of surface held at right angles to the sun’s rays. Its value is 2 gm-calorie/sq.cm/minute. Solar constant, however, varies with sun spots cycle. Also, due to varying sun-earth distance, more energy reaches earth during perihelion than aphelion.
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The proportion of solar radiation reflected from earth. High for light colored surfaces and low for dark colored ones.
Latitudes affect insolation by affecting the angle of sun’s ray and determining the length of the day. Vertical rays provide more energy.
Slope of land also affects insolation.
So, tropical areas receive the maximum amount of insolation while the polar areas the minimum.
Earth’s temperature remains fairly constant despite insolation because it loses an amount of heat equal to that gained through insolation. This mechanism of maintaining the same temperature is called the heat budget or heat balance.
Long waves in terrestrial radiation.
There are latitudinal variations in heat budget. Tropical areas gain more heat than lost and polar areas lose more heat than gained. This imbalance is corrected by latitudinal transfer of energy. This takes place through air and water circulation.
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Temperature at a place, to a large extent, depends on the angle of incidence of sun’s radiation.
Earth receives only about 1/2000 millionth part of the total energy emitted by the sun.
Part of the incident energy is reflected back. Other is absorbed by the surface of earth which gets heated up and starts radiating energy. This makes the air near the earth surface hotter. This fact explains why it is cooler as one goes higher up.
Latitude, altitude, distances from sea, aspect of the land and nature of surface are some of the factors that affect the global distribution of temperature.
- Latitude: Highest temperature near the equator and lowest near poles
- Altitude: Temp decreases with height
- Nature of surface: Albedo. Land has higher albedo than water. However, if the angle of incidence is high and there is movement in water, its albedo might become higher than land.
- Distance from sea: Due to difference in specific heats of land and water bodies, land gets heated faster than oceans and cools faster as well. Hence, while oceans have moderate temperatures, continents experience extremes of temperatures. Effect of nearness to sea is called maritime influence while that of location in the interiors of a continent is called continental influence.
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|Various dynasties during 10th century.Delhi Sultanate India in 18th Century
Sikhs and Marathas after Mughals
Deccan, Sufism and Mughal art & architecture