Distance of the Earth from the Sun
Earth revolves around the sun on an elliptical path. As such the distance of the earth from the sun keeps on changing which causes slight variation in the solar output received at the top of the atmosphere. During its revolution around the sun, the earth is farthest from the (152 million km on 4th July). This position of the earth is called aphelion. The earth is nearest to the sun (147.3 million km) on January 3. In this position the earth is said to be in perihelion. Thus the insolation received on 3rd January is about 6.6 per cent more than the amount received on 4th July.
Heat Budget of the Earth
The earth maintains its temperature because the amount of heat received by the earth in the form of insolation is equal to the amount of heat lost by it through terrestrial radiation. The gains and losses in heat by incoming and outgoing radiation is known as heat budget.
It is estimated that 35% of the total insolation received at the outer limits of the atmosphere is returned to space in its original shortwave form by scattering and reection from cloud, small dust particles, molecules of air, and from the earth’s surface and so has no part in heating either the earth or its atmosphere (Fig. 3.4). This is called earth’s albedo. This 35% is comprised of 2% rected from the earth’s surface, 6% rected from the atmosphere and 27% from the clouds. Fourteen per cent of the incoming solar insolation is absorbed by the atmosphere most of it by water vapour, but small amounts by clouds, dust and permanent gases. e remaining 51% reaches the earth’s surface and is available for heating the atmosphere from below. Of this 51%, 34% is in the form of direct sunlight, while 17% is diused daylight or sky radiation. It is clear from the preceding analysis that only 65% of the total incoming solar radiation (14% absorbed by the atmosphere directly and 51% absorbed by the earth’s surface) can be available for heating the atmosphere. Further, a much larger percentage of 51% is absorbed by the earth’s surface as compared to only 14% absorbed by the atmosphere.
51% solar radiation absorbed by the earth is returned as terrestrial radiation in the form of long wave radiation. Of this, 51% as much as 17% is radiated to space from the earth and 34% is absorbed by the atmosphere. Of this 34% about 6% is absorbed by the atmosphere itself, 9% by turbulence and convection and 19% as latent heat of condensation. The 48% absorbed by the atmosphere (14 % from sun and 34% by terrestrial radiation) is radiated to space by atmosphere. Thus the total amount of heat energy received by the earth is equal to the amount of heat radiated by it. The balance of heat received and emitted by the earth is called its heat budget.
Temperature of a place is the degree of heat contained in the air at about one metre above the surface under standard condition. In simple words temperature is a measure of intensity or degree of hotness of a body. It is the condition that determines the heat energy from one substance to another and this w is always from substance at high temperature to substance of low temperature. Temperature is often erroneously treated as synonymous with insolation, but these two terms have dierent meanings and represent different concepts as is clear from the following description.
|1.||Insolation is heat energy||1.||Temperature measures|
|which makes things hotter.||the intensity of heat, i.e.|
|the degree of hotness.|
|2.||Insolation is measured||2.||Temperature is|
|in calories.||measured in degrees.|
|(Celsius or Fahrenheit).|
|3.||Insolation is the cause.||3.||Temperature is the eect.|
As already mentioned, temperature gradually decreases as we move from equator towards the poles. Based on this fact, the ancient Greeks divided the entire globe into three temperature zones. e zone within the tropics ( i.e.23½°N & 23½°S) is called the tropical or torrid zone. In this zone the sun’s rays are almost vertical and the temperature remains high throughout the year and there is practically no winter season.
Lies between Tropic of Cancer (23½°N) and Arctic Circle (66½°N) in the northern hemisphere and Tropic of Capricorn (23½°S) and Antarctic Circle (66½°S) in the southern hemisphere. Both summer and winter seasons are observed here. Frigid zone extends from the Arctic Circle (66½°N) to North Pole (90°N) in the northern hemisphere and from Antartic Circle (66 ½°S) to South Pole (90°S) in the Southern Hemisphere. This zone is characterized by extremely oblique rays of the sun and receives very little solar radiation. As a result of this, the temperatures are very low and winters are exceptionally cold. The ground remains frozen for sutly longer period during the year and this region is known as frigid region.