The Sun's heat input to Earth

Everyone is interested in the weather. Deciphering its complexity and variability in both time and place presents scientists with one of their most challenging problems.

Imagine the Earth the size of a grapefruit: the atmosphere would be a layer only 1mm thick. You might think the shallowness of such a layer could not allow air movements to develop - yet in the real atmosphere the vertical movements of air control much of our weather.

Our atmosphere has been described as a giant heat engine driven by the sun's power. In outlining how the solar heat affects the atmosphere, we first note that the sun does not heat the atmosphere directly. Because air is largely transparent to sunshine, most of the sun's heat reaches the ground and heats the surface. The atmosphere then picks up this heat 'second hand.'

The earth, however, does not keep getting hotter and hotter from this sunshine, technically called solar radiation. The planet loses heat by radiation to the intense cold of outer space at a rate depending on the temperature.

While the earth's temperature varies from the equator to the poles, the earth is so much warmer than space that this relatively small temperature difference does not have much effect on radiation loss. The poles therefore lose nearly as much heat as the equator.

Consider the earth as a sphere in space: it is obvious that a given amount of sunshine in a beam falling on the equator, which points directly at the sun, has a much more intense effect than the glancing rays spread over a much larger area of the curving surface near the poles.

The greater sunshine intensity near the equator more than balances the heat loss there, so there is a net gain of heat. From latitude 37° north and south to the poles, the earth loses more heat than it gains from the sun. Yet the equatorial regions do not get steadily hotter, nor the polar regions colder: there is a redistribution of heat from the equator to the poles, carried by the atmosphere.