RADIATION ENERGY BALANCE FOR SOLAR RADIATION

As far as earth is concerned, radiation means both incoming solar radiation and outgoing radiation emitted by earth’s surface. Before radiation reaching the earth’s surface, it travels through the atmosphere. When it passes the atmosphere, some of the radiation energy is reflected by clouds, and some are scattered and absorbed by gases and particles in the atmosphere. The remaining radiation finally reaches the earth; even some of the radiation reaching the earth’s surface or water surface is reflected back. The amount of radiation reflected depends upon the surface characteristics. The reflected solar radiation is called, albedo. Albedo of a water body is different from that of a land surface; albedo of a forest is different from that of a desert and so on. That is, albedo values vary with the type of the surfaces on which the radiation strikes. The scattered radiation by atmospheric constituents, may reach the earth and this is called diffuse radiation. On a cloudy day, the light we receive is not direct radiation but diffused radiation passing through the clouds.

Now let us turn our attention towards the amount of radiation received and the amount of radiation emitted by the earth. Let us assume that total energy received by outer atmosphere at a given location is 100 units (say 100%). Of these, some are reflected, absorbed, and/or scattered. Some units finally reach the earth’s surface directly or through diffuse radiation. The earth re-emits either all or part of the received radiation. All the above details constitute radiation balance.

On average, out of all the energy received at the edge of the outer atmosphere, only 31% directly reaches the earth’s surface (or water surface; hereafter, the term earth’s surface includes water surface too). This energy is called direct radiation. Almost 30% is reflected by the atmosphere to the outer space. Clouds reflect approximately 25% and the dust particles, about 5%. 17% of the energy is absorbed by clouds and other atmospheric constituents (dust particles and gaseous molecules). Some of the particles and gases scatter the radiation in the atmosphere. Some of the scattered radiation eventually reaches the earth’ surface. It is called diffuse radiation. Diffuse radiation reaching the earth’s surface accounts for about 22%. Thus, earth receives on average, 53% of the solar energy both directly and through diffuse radiation.

The heating of the troposphere is mainly caused by the direct radiation of the solar energy and re-radiation of the solar energy from the earth’s surface. In general, radiation from the Sun is responsible for heating the earth’s surface. Yet, direct solar radiation accounts for only a small part of heat the atmosphere absorbs. Most radiative heating of the atmosphere is result of re-radiated energy from the earth’s surface. In troposphere, the temperature or heating is caused by the solar radiation absorbed by the constituents of the troposphere. The troposphere is the lowest portion of earth’s atmosphere having average depth 17 km in the middle latitudes. CO2 and Water molecules absorb the direct solar radiation and emit much of it back towards the earth again. This delicate exchange of energy between the earth’s surface and atmosphere keeps the average global temperature from changing drastically from year to year.