The predominant flow within the atmosphere is the general circulation, described in § 26.1. An associated flow, the jet stream, whose existence relies on the thermal wind (considered in § 26.2) is investigated in § 26.3. These flows are powered by the “workhorse” of atmospheric flow: the cumulonimbus rain cloud, which has been investigated in § 24.3.

General Circulation

If the Earth were not rotating, the general circulation would consist of a Hadley cell in each hemisphere, with rising motion near the equator, descending motion near the poles, poleward motion at high elevation and equatorward motion near the surface. However, due to the action of the Coriolis force, the general circulation consists of three cells in each hemisphere, a Hadley cell near the equator, a mid-latitude Ferrel cell and the aptly named polar cell at high latitude, as illustrated in Figure 26.1 for the northern hemisphere. The flow in the southern hemisphere is the mirror image, reflected about the equator.

Circulation of the atmosphere is driven primarily by convective motions in the tropics. Thesemotions drive a circulation of roughly 5 to 8×1010 kg·s−1 in each of the Hadley cells (Liu et al., 2012) and these motions convey poleward a heat flow of roughly 4 PW. The strength of circulation in these cells has been increasing in the past 150 years, possibly due to global warming. The trade winds within the Hadley cells on either side of the equator blow toward the equator and tend to concentrate convective motions in a band close to the equator called the inter-tropical convergence zone (ITCZ) by meteorologists and the doldrums by sailors. Descending motions at the boundary between the Hadley and Ferrel cells produce a band of weak surface winds called the horse latitudes by sailors. These descending motions inhibit convection, so that rainfall is low near the horse latitudes.Most of the world's deserts occur near this latitude, both north and south of the equator.

The moist adiabatic lapse rate is smaller in magnitude than the dry adiabatic lapse rate.