Crucial to atmospheric circulation, is the fact that the atmosphere is mostly heated from the bottom. That is, atmosphere receives most of it's thermal energy from the sun, indirectly by transfer from the earth's surface.
On average, about a two-thirds of the sun's incoming energy is is absorbed; the remainder is immediately reflected. Of this two-thirds, about 20% appears directly as thermal energy in the atmosphere; the remainder appears first as thermal energy in the earth's surface which subsequently warms the air near the surface.
Systematic local departures these averages convert thermal energy in the atmosphere into mechanical energy in the form of well-defined currents. These are:
1) the amount of solar energy reaching the earth's surface depends on:
A) the angle of incidence of the sun's rays with the earth's surface:
This angle varies with latitude -- resulting in more solar energy transfer at the equator than at the poles -- and seasonally at each latitude -- more in summer, less in winter. Two factors -- geometry and length of the sun's path through the atmosphere -- contribute to this dependence.B) the amount of water vapor (clouds) and aerosols present in the atmosphere;
This also depends on latitude indirectly in that greater quantities of water evaporate from the earth' surface near the equator where more solar energy reaches the earth's surface. Debris from volcanic eruptions appears to be one of the most significant aerosols here ( Bryson '89).2) of the solar energy reaching the earth's surface the amount absorbed as thermal energy depends on characteristics of the surface.
For example, snow and ice surfaces absorb less than half to as little as 10% of solar energy arriving at their surfaces while bare ground absorbs as much as 90%. The albedo of a surface refers to the percentage of solar energy reflected (not absorbed) by the surface.
