1. 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.
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.
2. 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 below
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.
3. 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.
The sun-spot studies that have been carried so far establish that when the sun-spots appear in larger numbers, the intensity of the solar radiation received at the surface is increased. Naturally, therefore, as the number of sunspots decreases, the quantity of radiation received at the earth's surface declines.
The scientists are of the opinion that the number of sunspots increases or decreases on a regular basis, creating a cycle of 11 years. However, there is little doubt that the magnitude of the effect of the varying amount of the solar constant on the amount of solar radiation received here on earth seems to be too small.
4. Distance between the earth and sun:
Since the earth revolves around the sun in an elliptical orbit, the distance varies during the course of a year. The mean distance between the earth and sun is about 149,000,000 kilometers.
Each year, on about January 3, the earth comes closer to the sun (distance 147 million kilometers). This position is known as perihelion. On about July 4, the earth is a little farther from the sun when the distance becomes about 152 million kilometers. This position is called aphelion.
Although the amount of incoming solar radiation received at the outer boundary of the atmosphere is a little greater (7 percent) in January than in July, there are other major factors, such as the angle of incidence and the duration of sunshine that more than offset its effect on seasonal temperature variations.
It may be interesting to note that the earth is relatively closer to the sun during the northern hemisphere winter.
5. Transparency of the atmosphere:
Transparency of the atmosphere is an important control on the amount of insolation which reaches the earth's surface. Reflection from dust, salt, and smoke particles in the air is an important mechanism for returning shortwave solar radiation to space.
Similarly, reflection from cloud tops also depletes the amount of solar radiation that would otherwise be available to the earth. The effect of certain gases, water vapour, and dust particles on reflection, scattering, and absorption is well-known.
Obviously, areas with heavy cloudiness and turbid atmosphere will receive lesser amount of radiant energy at the surface. But the transparency of the atmosphere varies with time and place.
Transparency of the atmosphere is closely related to the latitude. In the higher latitudes the sun's rays are more oblique, so that they have to pass through relatively thicker layers of the atmosphere than at lower latitudes. In winter when the altitude of the sun is relatively lower, there is greater loss of incoming solar radiation than in summer.
Since atmospheric depletion plays a very significant role in the receipt of solar radiation at the earth's surface, a more detailed discussion of this factor follows.
francis1897 answered the question on
January 11, 2023 at 07:22