Atmospheric Weight and Heat

The atmosphere weighs an estimated 5,000 million million tons, and about half of this total mass is in the lower layers, within 5km of the Earth's surface. At sea level the average atmospheric pressure is 1.05kg/cm2 (or 1,013 millibars) - that is, the weight of air above each square centimeter is 1.05kg.

The pressure (and density) of the atmosphere decreases with increasing altitude; at a height of 5| km the average pressure is 500 millibars - about half that at sea level - and at 16km above the ground it is only 100 millibars.

Variations in pressure are also caused by temperature changes. The chief source of heat is solar radiation, although little heat comes directly from the Sun's short wavelength radiation. Of the radiation that reaches the outer atmosphere, only about 46 per cent reaches the Earth's surface, most of the rest having been scattered or reflected back into space.

At the surface, however, solar radiation is absorbed (thereby heating the surface) then re-radiated in the form of longer wavelength radiation. It is this long-wavelength radiation that is absorbed by the carbon dioxide, water vapor and clouds in the lower atmosphere, creating the greenhouse effect. Hence the atmosphere is heated principally from below and, as a result, temperatures decrease with increasing altitude in the lower part of the atmosphere.

Heating by long-wavelength radiation near ground level makes the air expand so that it becomes less dense than the overlying cold air. As a result, the warm air tends to rise, leaving behind an area of comparatively low pressure. This contrasts with cold, dense air, which tends to sink, creating relatively high air pressure.