In this essay we will discuss about the meaning of net radiation.
The difference between all incoming solar energy and ail outgoing terrestrial energy by both shortwave and long-wave radiation is called net radiation. It is apparent from the aforesaid discussion that the net radiation from the whole globe is at least theoretically zero but this is far from truth if we look at the regional distribution of insolation. There are some places where the receipt of solar energy is more than the energy lost because the solar energy comes at faster rate than the terrestrial energy goes out.
Similarly, in some areas the loss of energy through outgoing terrestrial radiation is faster than the gain of incoming solar radiation. This mechanism results in the development of areas of energy surplus (where incoming solar radiation exceeds outgoing terrestrial radiation and energy deficit (where outgoing terrestrial radiation exceeds incoming solar radiation).
The energy surplus and energy deficit areas may be identified and studied at two levels viz.:
(i) at the earth’s surface, and
(ii) in the atmosphere, latitudinal base being common in both the cases.
(i) The distribution of net radiation at the earth’s surface from equator towards the poles shows that:
(a) there is large energy surplus area between the zones of 20°N and 20°S where energy gain from the incoming solar radiation is more than the loss of energy through outgoing long-wave terrestrial radiation and the annual surplus energy amounts to 100 kilolangleys per year;
(b) net radiation rapidly decreases from the energy surplus areas of low latitudes towards mid-latitudes;
(c) net radiation becomes practically zero near 70° latitude in the northern and southern hemispheres, and
(d) the polar areas are the zones of perennial energy deficit.
(ii) The latitudinal distribution of net radiation in the atmosphere shows that:
‘The atmosphere is itself a net loser of radiation at all latitudes’. Thus, the atmosphere is the zone of perennial energy deficit because the deficit of energy always exceeds 60 kilolangleys per year. If the data of net radiation of both, the earth’s surface and the atmosphere, are combined together, the net radiation value for the combined ‘earth’s surface-atmosphere system’ may be calculated.
Based on the combined data the following energy zones are identified:
(a) Large region of surplus radiation extending between 40°N and 30°S latitudes,
(b) Northern high-latitudes of deficit radiation, and
(c) Southern high-latitudes of deficit radiation.
It means that “there must exist a two-way heat transfer; from the earth’s surface to the atmosphere and from the equator to the poles”. This can be achieved if heat is transported from the earth’s surface to the atmosphere and from the tropical and subtropical areas of surplus radiation to the high latitude zones of deficit radiation. The transport of heat from equatorial area towards the poles is called ‘meridional transport of heat’.
The meridional transport of heat energy in the form of sensible heat is accomplished by the atmospheric circulation and ocean currents which transport heat energy from the ‘low latitude surplus energy areas’ to the ‘high latitude deficit energy areas.’ The vertical transport of heat in the atmosphere is accomplished by ascending air in the form of sensible heat and latent heat. Table 33.5 portrays latitudinal transfer of sensible heat.