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To date the main identified and indicated components of the Martian general circulation — based on both observations and modelling (Zurek et al, 1992).

• one or two Hadley cells (depending on the season),
• baroclinic eddies in the winter hemisphere,
• stationary eddies induced by topographical and other surface variations,
• condensation/sublimation flow between the CO2 polar caps,
• thermal tides, and
• normal mode oscillations.

The atmosphere is dry in absolute terms (the H2O column thickness is less than 100 µm, typically 10 µm), but relative humidity can often reach 100%, hence condensation clouds and near-surface fogs composed of H2O ice do occur (e.g., Savijärvi, 1995, 1999). H2O has been measured by Mariner 9, Viking, Mars Global Surveyor and Mars Express orbiters; condensation clouds have been monitored with imaging from various orbiters. No H2O measurements have been made on the surface, however. The major source of H2O is the northern permanent polar cap and according to recent observations the southern cap is a H2O source as well (Titus et al, 2003; Bibring et al, 2004). The H2O content varies by latitude and season with hemispherical asymmetry, the peak occurring in the northern polar region in the spring. Mars Odyssey GRS has also observed H (most likely H2O) in the regolith (Boynton et al, 2004). The role of surface-atmosphere H2O exchange is not well understood, as no surface humidity observations have been made.

Dust is the dominant aerosol species and its source is the surface, which has a temporally and spatially varying dust cover. Dust is lifted into the atmosphere by different processes resulting in dust storm activity in widely varying spatial and temporal scales (ranging from short-lived dust devils through regional dust storms of days to weeks duration to hemispherical, even global dust storms lasting up to several months). Consequently suspended dust is a ubiquitous component of the Martian atmosphere. Observations and models indicate favoured seasons and (smaller-scale) areas of dust storm activity, which points to regional or mesoscale characteristics as well. Hellas impact basin and the polar cap edges are among the areas that exhibit frequent dust storm activity. The dust optical thickness has an approximate lower limit of 0.2, but during dust storms optical thickness is 5 or even more (Kahn et al, 1992). The dust effects on solar and thermal radiation (in turn highly dependent on the dust radiative characteristics) alter drastically the atmospheric thermal structure and consequently the circulation patterns.

Due to the low temperatures and the composition of the atmosphere, CO2 is exchanged between the seasonal polar ice caps and the atmosphere: CO2 condenses to the winter hemisphere polar cap and is sublimated from the summer hemisphere cap. This results in atmospheric mass being cycled between the caps and in variation of the mass of the atmosphere. Both of these processes are visible in, e.g., surface pressure observations, as demonstrated already by the Viking landers. The condensation/sublimation flow is hence also a manifestation of the CO2 cycle.