Enumerate the factors influencing distribution of insolation with the help of a mind map.
Answers
The actual amount of insolation received at a place on the earth varies according to the conditions of the atmosphere as well as the seasons. The following astronomical and geographical factors govern the amount of insolation received at any point on the earth's surface:
(1) Angle of incidence (2) Duration of sunshine
(3) Solar constant (4) Distance between the earth and the sun
(5) Transparency of the atmosphere.
Angle of incidence:
The altitude of the sun, i.e. the angle between its rays and a tangent to the earth's surface at the point of observation, controls the amount of insolation received at the earth's surface (Figure 22.3).
As the elevation angle decreases, the area over which the radiation is distributed increases. The vertical rays of the sun heat the minimum possible area, but on the contrary, the oblique rays are spread over a relatively larger area, so that the amount of area over which the available solar energy has to be distributed in increased and the energy per unit area on the earth's surface is decreased.
In addition, the oblique rays have to traverse a larger distance through the atmosphere before they strike the surface of the earth. The longer their path, the larger the amount of energy lost by various processes of reflection, absorption, and scattering, etc.
Thus, it is clear that the larger amount of radiant energy is destroyed in case of slanting rays than in vertical rays. Similar effect of the varying angle of sun's rays can be seen in the daily march of the sun across the sky.
At mid-day the intensity of insolation is maximum, but in the morning and evening hours it is reduced because of the slanting rays of the sun. So is the case in winter and at high latitudes, when the-amount of insolation received at the surface of the earth is small.
This is simply the effect of the low angle of incidence. The major factors that determine the sun's altitude or the angle of incidence are the latitude of the place, the time of the day and the season.
Duration of sunshine:
The duration of sunlight hours determines the length of the day, which also affects the amount of solar radiation received at the surface. Undoubtedly, the longer period of sunshine ensures larger supply of radiation which a particular area of the earth will receive.
Obviously, the latitudes exercise the most dominant control over the duration of sunshine and thereby the length of the day. The latitudinal and monthly variations in the length of days have been shown in Table 22.4.
Table 22.4: Maximum length of day in different latitudes.
Latitude
Longest day
Latitude
Longest day
or night
or night
0
12 hours
63.4
20 hours
17
13 hours
66.5
24 hours
31
14 hours
67.4
1 month
41
15 hours
69.8
2 months
49
16 hours
78.2
4 months
58.5
18 hours
90.0
6 months
The inclination of the earth's axis, its parallelism, the earth's rotation and revolution, all these factors combine together to bring about seasonal changes. It is to be remembered that these astronomical factors not only cause differences in the altitude of the sun, but also differences in the length of day from the equator pole-ward.
At the equator the length of days and nights is 12 hours. On the autumnal and vernal equinoxes that occur on September 21 and March 21 respectively, the mid-day sun is overhead at the equator.
On these days all over the earth the days and nights are equal. On these two days, the maximum amount of insolation is received at the equator, and the amount goes on decreasing towards the poles.
But from the winter solstice (December 22) onward the length of day increases in the northern hemisphere till the summer solstice (June 21). On the contrary, during this period the length of day in the southern hemisphere decreases and the nights are longer.
From June 21 to December 22 the length of day in the northern hemisphere decreases, and in the southern hemisphere it increases. In other words, at the summer solstice the northern hemisphere has the longest day and the shortest night.
The condition is reversed in the southern hemisphere. On the contrary, at the winter solstice the southern hemisphere has the longest day, and the northern hemisphere has the longest night.
At the respective summer solstice, under cloudless skies, a polar area may receive more radiation per 24 hour-day than other latitudes. It may be pointed out that because of the albedo of ice and snow surfaces the net radiation used for heating is largely reduced.
Thus, the longer the period of sunshine and shorter the night, the greater the amount of solar radiation received, all other conditions being equal.
Solar constant:
As the energy emitted by the sun varies, the amount of insolation received at the surface also changes. But the percentage of change in the solar constant is rather negligible. The variations in the solar constant are caused by periodic disturbances and explosions in the solar surface.
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