The Martian tropics...

The tropical circulation of Earth is forced by the so-called Hadley circulation. During the equinoxes, this consists of two cells of air rising at the equator, moving towards the poles, and descending in the subtropics. This circulation is what causes the so-called trade winds. Nearer the solstices, this pattern is replaced by one large cell composed of rising motion in the summer hemisphere, horizontal motion across the equator, sinking motion in the winter hemisphere, and horizontal motion back into the summer hemisphere.

On Earth, the seasonal change in the strength of this circulation is weakened a great deal by the oceans. On Mars, this doesn't happen, and this Hadley circulation can become very strong indeed. One indication of the strength of the Hadley circulation is the so-called overturning timescale. This term is related to the time taken for an imaginary particle to move completely around the cell (upwards, then horizontally from the summer to winter hemisphere, downwards, and back into the summer hemisphere). On Mars, this timescale is about 100 Martian days or sols, while on Earth it is well over a year.

Winds associated with the Martian Hadley circulation can reach very high speeds: mean winds in the Martian subtropics during southern summer solstice have been modeled using the Mars atmosphere general circulation model: about 200 metres above the surface winds exceed 100 mph. The southern subtropics are also a region where dust storms are thought to start.

Water vapour plays a relatively minor role in Martian meteorology, and hurricanes cannot form on Mars as they do on Earth. However, people have suggested that suspended dust in the atmosphere may play a similar role to water vapour in the development of cyclones by heating the atmosphere.