It clearly indicates that at different latitudes different amounts of insolation are received at different times of the year.
Diurnal total amount of solar and sky radiation on a horizontal surface on a clear day (gram cal/cm2)
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On these dates the sun, because of refraction, is visible on the horizon and adequate amount of solar radiation is received from the sky light. So the actual value of these zeros is more.
The mean values of the zonal distribution of the incoming solar radiation received at the earth’s surface of the northern hemisphere were calculated by Baur and Phillips. While making the calculations, they also took into account the amount of cloudiness and the turbidity factor.
In the mean values of incoming solar radiation received at the outer boundary of the atmosphere and the earth’s surface in different latitude zones can be seen.
A comparison of the mean values of insolation as shown in part A and B Latitudinal and seasonal variation in the amount of the table throws sufficient light on the of insolation received at the outer margin of the atmosphere, evidence that a certain percentage of the incoming solar radiation is lost by various factors such as the amount of cloudiness, reflection and atmospheric turbidity.
The comparative values of both the parts of also indicate that at the time of summer solstice the highest values of insolation received at the outer boundary of the atmosphere occur at the poles.
However, at the surface the situation is otherwise. It is between latitudes 30° and 40° north, that on June 21 the maximum amount of insolation is received on the surface of the earth. Because of the presence of sub-tropical high-pressure belt there is the minimum amount of cloudiness in these latitudes.
It is mainly due to the difference in cloudiness that the maximum amount of insolation at the time of vernal equinox is received in the latitudinal belt extending from latitudes 10° to 20° north.
But at the time of autumnal equinox the maximum insolation belt shifts to 20°-30° North latitudinal belt. The conditions are almost the same in the southern hemisphere also.
In fact, the total annual insolation is greatest at the equator, and there is a general decrease pole-ward. The total amount of insolation received at the equator is roughly about four times that received at either of the poles.
The zone of maximum possible insolation, as it were, is tied with the sun. In tropical regions the amount of insolation is not only constantly large, but also there is little seasonal variation.
Since all places between the Tropic of Cancer and the Tropic of Capricorn experience overhead sun twice during the course of a year, there are naturally two maxima of insolation. In the belt lying between latitudes 23V2° and 66V20 the periods of maximum and minimum insolation coincide with the summer and winter solstices respectively.
However, so far as the actual insolation received at the surface is concerned, there is a slight departure from a simple latitudinal pattern. Because of aridity and clear skies the maximum amount of incoming solar radiation is received at about latitude 20° north and south.
It may be noted that it is simply on account of relatively clearer air at higher elevations that high mountain ranges and plateaus are always in receipt of a larger amount of insolation.
Beyond the Arctic Circle, the sun is always seen in the sky on mid-summer days, but the altitude of the mid-day sun decreases steadily till it is 231/2° at the poles.
However, the longer duration of the period of sunshine more than compensates the low noon-time elevation of the sun. That is why on its midsummer day the insolation received at the poles is greatest among all the latitudes.