文档介绍：CLBE001-ESS2E November 11, 2006 5:15
176 Encyclopedia of the Solar System
On the other hand, this condensation and rainout of can often dominate midlatitude winter precipitation, form
water dehydrates the air, so the descending branch of the when large regions of warm air are forced upward over
Hadley cell, which occurs in the subtropics at ∼20–30◦ lat- colder air masses during growing baroclinic instabilities. In
itude, is relatively dry. Because of the descending motion many cases, the forced ascent associated with these instabil-
and dry conditions, little precipitation falls in these regions, ities produces predominantly sheet-like stratus clouds and
which explains the abundance of arid biomes at 20–30◦ lat- steady rainfall lasting for several days, although sometimes
itude, including the deserts of the African Sahara, southern the forced ascent can trigger local convection events (.,
Africa, Australia, central Asia, and the southwestern United thunderstorms).
States. However, the simple Hadley cell is to some degree a What causes the jet stream? This is a subtle question. At
theoretical idealization, and many regional 3-dimensional the crudest level, poleward-moving equatorial air deflects
time-variable phenomena—including monsoons, equa- eastward due to the Coriolis acceleration (or, equiva-
torial waves, El Nino, ˜ and longitudinal overturning lently, due to the air’s desire to conserve angular momentum
circulations associated with continent–ocean and sea- about the ary rotation axis), so the formation of east-
surface-temperature contrasts—affect the locations of trop- ward winds in the midlatitudes is a natural response to the
ical thunderstorm formation and hence the climatic rainfall Hadley circulation. These strong eastward winds in mid-
patterns. latitudes are also consistent with the large latitudinal ther-
Satellite images (Fig. 3) dramatically illustrate the sig- mal gradients in midlatitudes via the thermal-wind