176 Encyclopedia of the Solar System
On the other hand, this condensation and rainout of
water dehydrates the air, so the descending branch of the
Hadley cell, which occurs in the subtropics at∼20–30◦lat-
itude, is relatively dry. Because of the descending motion
and dry conditions, little precipitation falls in these regions,
which explains the abundance of arid biomes at 20–30◦lat-
itude, including the deserts of the African Sahara, southern
Africa, Australia, central Asia, and the southwestern United
States. However, the simple Hadley cell is to some degree a
theoretical idealization, and many regional 3-dimensional
time-variable phenomena—including monsoons, equa-
torial waves, El Ni ̃no, and longitudinal overturning
circulations associated with continent–ocean and sea-
surface-temperature contrasts—affect the locations of trop-
ical thunderstorm formation and hence the climatic rainfall
patterns.
Satellite images (Fig. 3) dramatically illustrate the sig-
nature of the Hadley cell and midlatitude baroclinic insta-
bilities as manifested in clouds. In Fig. 3, the east–west
band of clouds stretching across the disk of Earth just
north of the equator corresponds to the rising branch of
the Hadley cell (this cloud band is often called the in-
tertropical convergence zone). These clouds are primarily
the tops of thunderstorm anvils. In the midlatitude regions
of both hemispheres (30–70◦latitude), several arc-shaped
clouds up to 3000–5000 km long can be seen. These are
associated with baroclinic instabilities. These clouds, which
FIGURE 3 Visible-wavelength image of Earth from theGOES
geostationary weather satellite, illustrating the clouds associated
with the Hadley cell, baroclinic instabilities, and other weather
systems. North America can be seen at the upper right and
South America (mostly obscured by clouds) is at the lower right.
can often dominate midlatitude winter precipitation, form
when large regions of warm air are forced upward over
colder air masses during growing baroclinic instabilities. In
many cases, the forced ascent associated with these instabil-
ities produces predominantly sheet-like stratus clouds and
steady rainfall lasting for several days, although sometimes
the forced ascent can trigger local convection events (e.g.,
thunderstorms).
What causes the jet stream? This is a subtle question. At
the crudest level, poleward-moving equatorial air deflects
eastward due to theCoriolis acceleration(or, equiva-
lently, due to the air’s desire to conserve angular momentum
about the planetary rotation axis), so the formation of east-
ward winds in the midlatitudes is a natural response to the
Hadley circulation. These strong eastward winds in mid-
latitudes are also consistent with the large latitudinal ther-
mal gradients in midlatitudes via the thermal-wind equation
mentioned in Section 3.2. However, these processes alone
would tend to produce a relatively broad zone of eastward
flow rather than a narrow jet. Nonlinear turbulent motions,
in part associated with baroclinic instabilities, pump mo-
mentum up-gradient into this eastward-flowing zone and
help to produce the narrow jet streams.
Although the Earth’s equator is hotter than the poles
at the surface, it is noteworthy that, in the upper tropo-
sphere and lower stratosphere (∼18 km altitude), the re-
verse is true. This seems odd because sunlight heats the
equator much more strongly than the poles. In reality, the
cold equatorial upper troposphere results from a dynamical
effect: Large-scale ascent in the tropics causes air to expand
and cool (a result of decreased pressure as the air rises),
leading to the low temperatures despite the abundant sun-
light. Descent at higher latitudes causes compression and
heating, leading to warmer temperatures. Interestingly, this
means that, in the lower stratosphere, the ascending air is
actually denser than the descending air. Such a circulation,
called a thermally indirect circulation, is driven by the ab-
sorption of atmospheric waves that are generated in the
troposphere and propagate upward into the stratosphere.
There is a strong planetary connection because all four gi-
ant planets—Jupiter, Saturn, Uranus, and Neptune—are
also thought to have thermally indirect circulations in their
stratospheres driven by analogous processes.3.4 Insights from other Atmospheres
Planetary exploration has revealed that atmospheric circu-
lations come in many varieties. Perhaps ironically, Earth
is observed to have the most unpredictable weather of
all. The goal of planetary meteorology is to understand
what shapes and maintains these diverse circulations. The
Voyagerspacecraft provided close-up images of the at-
mospheres of Jupiter, Saturn, Uranus, and Neptune and
detailed information on the three satellites that have at-
mospheres thick enough to sport weather—Io, Titan, and