|MadSci Network: Astronomy|
That is a very excellent question! Interestingly enough, dust storms are very much affected by the minimal atmosphere and air pressure -- as well as seasonal variations due to Mars' more elliptical orbit. Since you're learning about Mars, you've probably already discovered that the Martian spin axis is tilted from its orbital plane roughly the same (23 degrees) as Earth is. However, its orbit is much more elliptical (the Earth's orbit is very close to being a circle). This means that Mars has more extreme seasons than Earth does.
What does this have to do with dust storms? Well, let's look first at the circulation of the Martian atmosphere. On Earth, the jet streams stay in one hemisphere. On Mars however, the jet streams actually cross the equator at certain times of year! This allows for transport of material (such as dust) from one hemisphere to the other. But does this matter for the dust storms? For smaller ones, it doesn't come into play. However, for the global storms, it really can!
Smaller dust storms - otherwise known as dust devils - form on Mars similar to the way they form on Earth. During the day, the ground heats up, which then heats the air above the ground. The warm air then rises and can begin to spin - this then picks up dust from the Martian surface, forming the spinning column we call a dust devil. These typically last for only a few days, but sometimes they can grow to regional scales.
Once at regional scale size, a dust storm has the chance of growing to global scale, though most often even the regional storms die down before becoming that large. Global storms almost always occur during summer in the southern hemisphere on Mars, since Mars is nearest to the sun at that time and there is the greatest solar heating at that time (summer in the northern hemisphere does not get as hot since then Mars is further away from the Sun). Once the regional dust storm is large enough, it can be transported across the equator via the jet streams -- and since dust itself is a good absorber of heat, it is possible that as more dust that goes into the atmosphere this can cause further heating of the atmosphere itself, leading to more warm air rising and a larger dust storm. This is one way that Mars is different from the Earth -- Earth's atmosphere is dominated by water vapor, but since Mars has very little water in its atmosphere, the dust dominates instead.
However, I should say that we really do not have a complete idea of how dust storms form and grow into global dust storms on Mars. There have been a lot of computer simulations on the subject, and the MGS/TES instruments have provided a lot of clues but we are still working toward a solid answer.
Here are some links which help to detail dust storms on Mars a little more:
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