Radiation can be defined as a process whereby energy (or heat) is transferred by some sort of wave motion. Wave motion varies according to the length of the waves concerned. Thus, related to the title, solar radiation is short wave radiation, whereas terrestrial radiation, otherwise the heat which is lost from the ground to the atmosphere, particularly during a clear night, is long wave radiation.
Incoming solar radiation, or insolation, arrives at the earth as magnetic energy waves and high speed particles. It provides virtually all the energy available to the earth and is affected by four things.
1. The solar output, a very intense, mainly short wave radiation of between 0.2 and 4 microns. The figure will vary with the behaviour of the sun, and during sun spot activity up to twenty times more radiation may be emitted in certain wave lengths.
2. The earth’s distance from the sun varies because of the eccentricity of its orbit, and about 7% more solar radiation arrives at the earth on 4 January than on 4 July. This effect on world temperatures is nullified, mainly by the circulation of the atmosphere.
3. The altitude of the sun has a definite effect on insolation amounts received, as when the sun is high in the sky the energy received at the earth’s surface is more concentrated. Thus more heat is felt from the sun during the middle of a day compared with the beginning and end of a day.
4. The length of a day will affect the amount of insolation received, as a longer period of daylight will provide a longer period of incoming radiation.
Other factors which affect the amount of insolation received at a specific point on the earth’s surface include variations in the atmosphere’s thickness, cloud cover, altitude, areas of land and sea, and aspect, i.e. the angle of the land in relation to the sun.
It is thought that of all the incoming solar radiation, only 47% reaches the earth’s surface. The remainder is absorbed in the stratosphere, mainly by ozone, or in the troposphere by ozone, water vapour and clouds, or by radiation back into the atmosphere from clouds.
What is little realised by most people is that the atmosphere in which we live receives the greater part of its heat energy by radiated long wave energy from the earth below, and not by incoming short wave radiation directly from the sun. This explains why ambient temperatures are lower at higher altitudes.
Energy can be transmitted by the processes of radiation, advection, conduction and convection, and it is the first of these, in the form of long wave radiation, which transmits most of the heat from the earth’s surface to the atmosphere.
The differences then, between solar radiation and terrestrial radiation, are their different wave lengths and the differing amounts of effect they have on the heating of the lower levels of the atmosphere in which people live.