174 Encyclopedia of the Solar System
3. Atmospheric Circulation
3.1 Processes Driving the Circulation
The atmospheric circulation on Earth, as on any planet, in-
volves a wealth of phenomena ranging from global weather
patterns to turbulent eddies only centimeters across and
varies over periods of seconds to millions of years. All this
activity is driven by absorbed sunlight and loss of infrared
(heat) energy to space. Of the sunlight absorbed by Earth,
most (∼70%) penetrates through the atmosphere and is
absorbed at the surface; in contrast, the radiative cooling
to space occurs not primarily from the surface but from
the upper troposphere at an altitude of 5–10 km. This
mismatch in the altitudes of heating and cooling means
that, in the absence of air motions, the surface tempera-
ture would increase while the upper tropospheric temper-
ature would decrease. However, such a trend produces an
unstable density stratification, forcing the troposphere to
overturn. The hot air rises, the cold air sinks, and thermal
energy is thus transferred from the surface to the upper
troposphere. This energy transfer by air motions closes the
“energy loop,” allowing the development of a quasi-steady
state where surface and atmospheric temperatures remain
roughly steady in time. This vertical mixing process is fun-
damentally responsible for near-surface convection, tur-
bulence, cumulus clouds, thunderstorms, hurricanes, dust
devils, and a range of other small-scale weather pheno-
mena.
At global scales, much of Earth’s weather results not sim-
ply from vertical mixing but from the atmosphere’s response
to horizontal temperature differences. Earth absorbs most
of the sunlight at the equator, yet it loses heat to space
everywhere over the surface. This mismatch makes the
near-surface equatorial air hot and the polar air cold. This
configuration is gravitationally unstable—the hot equatorial
air has low density and the cold polar air has high density.
Just as the cold air from an open refrigerator slides across
your feet, the cold polar air slides under the hot equato-
rial air, lifting the hotter air upward and poleward while
pushing the colder air downward and equatorward. This
overturning process transfers energy between the equator
and the poles and leads to a much milder equator-to-pole
temperature difference (about 30 K at the surface) than
would exist in the absence of such motions. On average, the
equatorial regions gain more energy from sunlight than they
lose as radiated heat, while the reverse holds for the poles;
the difference is transported between equator and pole by
the air and ocean. The resulting atmospheric overturning
causes many of Earth’s global-scale weather patterns, such
as the 1000 km long fronts that cause much midlatitude
weather and the organization of thunderstorms into clus-
ters and bands. Horizontal temperature and density con-
trasts can drive weather at regional scales too; examples
include air-sea breezes and monsoons.
3.2 Influence of Rotation
The horizontal pressure differences associated with
horizontal temperature differences cause a force (the
“pressure-gradient force”) that drives most air motion at
large scales. However, how an atmosphere responds to this
force depends strongly on whether the planet is rotating. On
a nonrotating planet, the air tends to directly flow from high
to low pressure, following the “nature abhors a vacuum” dic-
tum. If the primary temperature difference occurs between
equator and pole, this would lead to a simple overturning
circulation between the equator and pole. On the other
hand, planetary rotation (when described in a noninertial
reference frame rotating with the solid planet) introduces
new forces into the equations of motion: the centrifugal
force and the Coriolis force. The centrifugal force naturally
combines with the gravitational force and the resultant
force is usually referred to as simply the gravity. For rapidly
rotating planets, the Coriolis force is the dominant term that
balances the horizontal pressure-gradient force in large-
scale circulations (a balance called geostrophy). Because
the Coriolis force acts perpendicular to the air motion,
this leads to a fascinating effect—the horizontal airflow is
perpendicular to the horizontal pressure gradient.A north–
south pressure gradient (resulting from a hot equator and a
cold pole, for example) leads primarily not to north–south
air motions but to east–west air motions! This is one reason
why east–west winds dominate the circulation on most
planets, including Earth. For an Earth-sized planet with
Earth-like wind speeds, rotation dominates the large-scale
dynamics as long as the planet rotates at least once every
10 days.
Physically, the Coriolis force acts in the following way. Air
moving eastward (i.e., in the same direction as the planet’s
rotation, but faster) experiences a force that moves it away
from the rotation axis—namely, equatorward—just as a
child experiences an outward force on a spinning merry-
go-round. Conversely, air moving westward (in the same
direction as the planet’s rotation, but slower) would experi-
ence a poleward force. And, just as an ice skater spins faster
as she pulls in her arms, air that moves toward the plan-
etary rotation axis—namely, poleward—spins faster, which
is equivalent to saying that it deflects eastward. Conversely,
air that moves away from the planetary rotation axis (equa-
torward) deflects westward. If one pays attention to the
directions of the force in each of these cases, one sees that,
in the northern hemisphere, this rotationally induced force
is always to the right of the air motion, while in the southern
hemisphere, it is always to the left of the air motion.
Two other important effects of rapid rotation are the sup-
pression of motions in the direction parallel to the rotation
axis, called the Taylor–Proudman effect, and the coupling of
horizontal temperature gradients with vertical wind shear, a
3-dimensional relationship described by the thermal-wind
equation.