The ultimate source of atmospheric energy is in fact heat and light received through space from the Sun. This energy is known as solar insolation. The Earth receives only a tiny fraction of the total amount of Sun’s radiations. Only two billionths or two units of energy out of 1,00,00,00,000 units of energy radiated by the sun reaches the earth’s surface due to its small size and great distance from the Sun. The unit of measurements of this energy is Langley (Ly). On an average the earth receives 1.94 calories per sq. cm per minute (2 Langley) at the top of its atmosphere.
Incoming solar radiation through short waves is termed as insolation. The amount of insolation received on the earth’s surface is far less than that is radiated from the sun because of the small size of the earth and its distance from the sun. Moreover water vapour, dust particles, ozone and other gases present in the atmosphere absorb a small amount of insolation.
The amount of insolation received on the earth’s surface is not uniform everywhere. It varies from place to place and from time to time. The tropical zone receive the maximum annual insolation. It gradually decreases towards the poles. Insolation is more in summers and less in winters.
The following factors influence the amount of insolation received.
(i) The angle of incidence:-The angle formed by the sun’s ray with the tangent of the earth’s circle at a point is called
angle of incidence. It influences the insolation in two ways. First, when the sun is almost overhead, the rays of the sun are vertical. The angle of incidence is large hence, they are concentrated in a smaller area, giving more amount of insolation at that place. If the sun’s rays are oblique, angle of incidence is small and sun’s rays have to heat up a greater area, resulting in less amount of insolation received there. Secondly, the sun’s rays with small angle, traverse more of the atmosphere, than rays striking at a large angle. Longer the path of sun’s rays, greater is the amount of reflection and absorption of heat by atmosphere. As a result the intensity of insolation at a place is less.
(ii) Duration of the day. (daily sunlight period) :-The duration of day is controlled partly by latitude and partly by the season of the year. The amount of insolation has close relationship with the length of the day. It is because insolation is received only during the day. Other conditions remaining the same, the longer the days the greater is the amount of insolation. In summers, the days being longer the amount of insolation received is also more. As against this in winter the days are shorter the insolation received is also less. On account of the inclination of the earth on its axis at an angle of 23 ½ , rotation and revolution, the duration of the day is not same everywhere on the earth. At the equator there is 12 hours day and night each throughout the year. As one moves towards poles duration of the days keeps on increasing or decreasing. It is why the maximum insolation is received in equatorial areas.
(iii) Transparency of the atmosphere.Transparency of the atmosphere: Transparency of the atmosphere also determines the amount of insolation reaching the earth’s surface. The transparency depends upon cloud cover, its thickness, dust particles and water vapour, as they reflect, absorb or transmit insolation. Thick clouds hinder the insolation to reach the earth while clear sky helps it to reach the surface. Water vapour absorb insolation, resulting in less amount of insolation reaching the surface.
Heat Budget
Energy emitted by the Earth’s climate system tends to maintain a balance with solar energy coming into the system. This balance, known as the radiation budget, allows the Earth to maintain the moderate temperature range essential for life as we know it.
There is positive radiation balance between 35°S and 40°N, which drives the weather systems. Ocean currents even out the difference
When incoming short-wave solar radiation (Figure 3), known as insolation, enters the Earth’s climate system, a portion of it is absorbed at the Earth’s surface, causing the surface to heat up. Some of the absorbed energy is then radiated outward in the form of long-wave infrared radiation. Cloud layers trap some of the radiation from the Earth’s surface, and then emit long-wave radiation, both outward and back to the surface. The temperature of the Earth’s surface is about 33°C higher due to long-wave radiation contribution from the atmosphere .
The amount of radiation emitted by the Earth’s surface that makes it back to space is the result of many interrelated influences, such as the amount of cloud cover, cloud heights, characteristics of cloud droplets, amount and distribution of water vapor and other greenhouse gases, land features, surface temperature, and the transparency of the atmosphere. In the warm tropical areas, low values of outgoing longwave radiation (OLR) correspond to large amounts of high, cold clouds while high values of OLR correspond to relatively clear areas or cloudy areas with low, warm clouds. In the extra-tropics OLR values typically decrease with decreasing temperature.
Let us suppose that the total heat (incoming solar radiation) received at the top of the atmosphere is 100 units (see fig. 10.2) Roughly 35 units of it are reflected back into space even before reaching the surface of the earth. Out of these 35 units, 6 units are reflected back to space from the top of the atmosphere, 27 units reflected by clouds and 2 units from the snow and ice covered surfaces.
Out of the remaining 65 units (100-35), only 51 units reach the earth’s surface and 14 units are absorbed by the various gases, dust particles and water vapour of the atmosphere.
The earth in turn radiates back 51 units in the form of terrestrial radiation. Out of these 51 units of terrestrial radiation, 34 units are absorbed by the atmosphere and the remaining 17 units directly go to space. The atmosphere also radiates 48 units (14 units of incoming radiation and 34 units of outgoing radiation absorbed by it) back to space. Thus 65 units of solar radiation entering the atmosphere are reflected back into the space. This account of incoming and outgoing radiation always maintains the balance of heat on the surface of the earth.