The Atmosphere

Abstract

The air is a mechanical mixture of gases and so highly compressible that its lower layers are much denser than overlying ones. It constitutes the atmosphere which is probably unique in the solar system because it contains water vapour, most of this being near its saturation point as a result of which cloudless conditions can change rapidly to obscure ones. Atmospheric water amounts to only 0–1% of the total quantity of water on the planetary surface but it is very significant in determining surficial conditions on the Earth. This is because of its effect upon impinging solar radiation and outgoing infrared radiation. About 84.4% of atmospheric moisture is derived from oceanic evaporation. The energy of the atmosphere appears to remain constant and only derives in part from the sun, another source being terrestrial heat escaping from the planetary interior. As regards the former (termed insolation), 28% is reflected and lost at once and while some of the remainder is intercepted by water vapour, ozone and dust in the atmosphere, 48% is absorbed by the Earth. It is apparent that the atmosphere is mainly heated from below because the Earth absorbs more of the insolation than the atmosphere. The component absorbed by the Earth is ultimately returned to space, but only after a complex transfer process. Convection currents carry sensible heat and evaporated water vapour upwards to the troposphere (defined below); the latter contains latent heat abstracted from the evaporating surface. Long wave radiant heat is also radiated by the planet but, unlike solar radiation, this is quickly absorbed by water and water vapour so that very little escapes through the atmosphere and only a minute fraction reaches space directly. The bulk is absorbed by water vapour or clouds and reradiated, nearly all of this returning to Earth where it is absorbed, converted back to heat and again radiated. Thus a heat exchange process is maintained between the planet and its atmosphere. That large quantity of heat retained in the lower atmosphere is a consequence of this greenhouse effect. Of course there is a considerable variation of the heat budget with latitude. Over the Earth as a whole the poles receive less heat from the sun than the equatorial belt, hence the difference in temperature between the two regions. It has been found that the polar caps (embracing the areas between 35°N and 35°S to the poles) have an annual heat deficit, about half of the planetary surface together with most densely settled areas suffer a net radiative loss of heat over the year but the equatorial belt (35°N to 35°S) shows a net gain (insolation exceeding loss by radiation). Atmospheric circulation prevents the poles from getting progressively cooler and the equator from getting progressively warmer. There is also a variation of temperature with altitude.