Earth as a Planet: Atmosphere and Oceans 177
Triton. The atmospheres of Venus and Mars have been sam-
pled by entry probes, landers, orbiting spacecraft, and tele-
scopic studies. Basic questions like why Venus’ atmosphere
rotates up to 60 times faster than does the planet, or why
Jupiter and Saturn have superrotating equatorial jets, do
not have completely satisfactory explanations. However, by
comparing and contrasting each planet’s weather, a general
picture has begun to emerge.
Theoretical studies and comparative planetology show
that planetary rotation rate and size exert a major control
over the type of global atmospheric circulations that occur.
When the rotation rate is small, Hadley cells are unconfined
and stretch from the equator to pole. Venus, with a rotation
period of 243 days, seems to reside in such a state. Titan
rotates in 16 days and, according to circulation models, its
Hadley cell extends to at least∼ 60 ◦latitude, a transitional
regime between Venus and Earth. On the other hand, fast
rotation confines the Hadley cell to a narrow range of lati-
tudes (0–30◦on Earth) and forces baroclinic instabilities to
take over much of the heat transport between low latitudes
and the poles. Increasing the rotation rate still further—or
making the planet larger—causes the midlatitudes to break
into series of narrow latitudinal bands, each with their own
east–west jet streams and baroclinic instabilities. The faster
the rotation rate, the straighter and narrower are the bands
and jets. This process helps explain the fact that Jupiter and
Saturn, which are large and rapidly rotating, have∼30 and
20 jet streams, respectively (as compared to only a few jet
streams for Earth). Fast rotation also contributes to smaller
structures because it inhibits free movement of air toward
or away from pressure lows and highs, instead causing the
organization of vortices around such structures. Thus, a
planet identical to Earth but with a faster or slower ro-
tation rate would exhibit different circulations, equatorial
and polar temperatures, rainfall patterns, and cloud pat-
terns, and hence would exhibit a different distribution of
deserts, rainforests, and other biomes.
The giant planets Jupiter and Saturn exhibit numerous
oval-shaped windstorms that superficially resemble terres-
trial hurricanes. However, hurricanes can generate abun-
dant rainfall because friction allows near-surface air to spiral
inward toward the low-pressure center, providing a source
of moist air that then ascends inside thunderstorms; in turn,
these thunderstorms release energy that maintains the hur-
ricane’s strength against the frictional energy losses. In con-
trast, windstorms like Jupiter’s Great Red Spot and the
hundreds of smaller ovals seen on Jupiter, as well as the
dozens seen on Saturn and the couple seen on Neptune,
do not directly require moist convection to drive them and
hence are not hurricanes. Instead, they are simpler sys-
tems that are closely related to three types of long-lasting,
high-pressure “storms,” or coherent vortices, seen on Earth:
blocking highs in the atmosphere and Gulf Stream rings
and Mediterranean salt lenses (“meddies”) in the ocean.
Blocking highs are high-pressure centers that stubbornly
settle over continents, particularly in the United States and
Russia, thereby diverting rain from its usual path for months
at a time. For example, the serious 1988 drought in the U.S.
Midwest was exacerbated by a blocking high. Gulf Stream
rings are compact circulations in the Atlantic that break off
from the meandering Gulf Stream, which is a river inside the
Atlantic Ocean that runs northward along the eastern coast
of the United States and separates from the coast at North
Carolina, where it then jets into the Atlantic in an unsteady
manner. Seen in three dimensions, the Gulf Stream has the
appearance of a writhing snake. Similar western bound-
ary currents occur in other ocean basins, for example, the
Kuroshio Current off the coast of Japan and the Agulhas
Current off the coast of South Africa. Jet streams in the
atmosphere are a related phenomenon. When Gulf Stream
rings form, they trap phytoplankton and zooplankton inside
them, which are carried large distances. Over the course of
a few months, the rings dissipate at sea, are reabsorbed into
the Gulf Stream, or run into the coast, depending on which
side of the Gulf Stream they formed. The ocean plays host
to another class of long-lived vortices, Mediterranean salt
lenses, which are organized high-pressure circulations that
float under the surface of the Atlantic. They form when
the extra-salty water that slips into the Atlantic from the
shallow Mediterranean Sea breaks off into vortices. After
a few years, these meddies eventually wear down as they
slowly mix with the surrounding water. The mathematical
description of these long-lasting vortices on Earth is the
same as that used to describe the ovals seen on Jupiter,
Saturn, and Neptune. [SeeAtmospheres of the Giant
Planets.]
Given that we know that atmospheric motions are funda-
mentally driven by sunlight, and we know that the problem
is governed by Newton’s laws of motion, why then are atmo-
spheric circulations difficult to understand? Several factors
contribute to the complexity of observed weather patterns.
In the first place, fluids move in an intrinsically nonlinear
fashion that makes paper-and-pencil analysis formidable
and often intractable. Laboratory experiments and numeri-
cal experiments performed on high-speed computers are
often the only means for making progress on problems
in geophysical fluid dynamics. In the second place, me-
teorology involves the intricacies of moist thermodynamics
and precipitation, and we are only beginning to understand
and accurately model the microphysics of these processes.
And for the terrestrial planets, a third complexity arises
from the complicated boundary conditions that the solid
surface presents to the problem, especially when moun-
tain ranges block the natural tendency for winds to orga-
nize into steady east–west jet streams. For oceanographers,
even more restrictive boundary conditions apply, namely,
the ocean basins, which strongly affect how currents be-
have. The giant planets are free of this boundary problem
because they are completely fluid down to their small rocky
cores. However, the scarcity of data for the giant planets,