We turn next to variations in Mars' atmosphere. Mars' axial tilt is
similar to earth's, but its year is twice as long: as one might
expect, the long winters and summers are more extreme on Mars. The
radiative timescale upon which Mars's atmosphere equilibrates with its
radiative forcing is only 2 days, as opposed to 
25 days on
Earth. (Stone, 1996 11-27). With such a rapid radiative equilibration
time, Mars' atmosphere comes rapidly into equilibrium with seasonal
and even diurnal changes in shortwave flux. This short timescale is
essentially a result of the low heat capacity of Mars' atmosphere:
Earth's heavier atmosphere has much more thermal inertia. On Mars,
noon and afternoon temperatures differ by an enormous 80 K, and the
seasonal cycle is roughly 50 K in middle latitudes.
Mars' dramatic seasonal cycle has marked effect on atmospheric dynamics and composition. In the summer hemisphere, the atmosphere exhibits a Hadley cell ``direct circulation'', with little atmospheric wave activity, just as on the Earth. In contrast to Earth, the rapid atmospheric equilibration time means that the constantly-illuminated summer polar regions become warmer than the rest of the hemisphere, and the meridional temperature gradient reverses. In such conditions, the atmospheric potential vorticity (***) does not change sign anywhere, and baroclinic instability, the primary method of storm generation on Earth, cannot occur (Stone, 1996). However, the season corresponding to southern hemisphere summer is marked by regional and even global dust storms, obscuring the surface for weeks to months. The role of dust in Mars' atmosphere will be discussed shortly.
In the winter hemisphere, conditions favor baroclinic instability,
and atmospheric waves similar to Earth's winter storms occur, though
far more regular in space and time. In addition, a decidedly
unearthly effect also occurs: the winter pole temperature falls to the
freezing point of 
(150 K), and the begins to freeze out
as a polar cap. This can lead to variations in global atmospheric
mass, and therefore pressure, pressure of 25% through the year.
During the solstices, one polar cap is vaporizing while the other is
condensing, so there is a significant cross-equator mass flux.
The large eccentricity of Mars' orbit also affects the seasons. The
current configuration means aphelion occurs during northern-hemisphere
summer; as a result, northern summer is up to 30 degrees colder than
southern summer, and the amplitude of the seasonal cycle is 110 K in
southern midlatitudes but only 55 K in the north. However, Mars'
equinoxes precess just as Earth's do. A historically important model
developed to explain Mars' past warm climate (see section **PAST**)
was proposed by Sagan et al (Icarus 15, 1971; Science 181, 1973.).
This ``Long Winter'' model proposes that when the equinoxes and
aphelion/perihelion are in phase, as they are now, Mars' is
frozen out as a ice cap on the pole with cooler summers
(currently the north pole.) When aphelion/perihelion occur during
spring and fall, neither pole is cold enough to freeze , and it
is released into the atmosphere; the resulting greenhouse warming
creates warm conditions in which liquid water can exist. Spacecraft
observations later showed that the north pole is composed of water
ice, not ice, and that Mars' current cold state has persisted
for far longer than the precession cycle of 50,000 years. However,
the similarity between this model and the Milankovich model of Earth's
ice ages is striking, and Milankovich cycles are likely to be
important in Mars' climate, though to a less extreme degree than Sagan
et al proposed.