Re: What conditions lead to dust storms on Mars, how often do they occur?

Well, this is a great question, so I decided to defer to my friend, Dr. Mike Wolff, who is actually an expert on Mars dust. In fact, he's part of the Mars MER team that will actually send a rover to Mars. He says:

i wish that there was a good answer to this...it is a topic of considerable study... large dust storms are typically linked with perihelion [when Mars is closest to the sun] and are driven by the added thermal input (flux varies by 40% from aphelion to perihelion). once dust is raised, it absorbs more energy and adds more heat to the system. however, dust also has a "nuclear winter" effect and will cool the surface. so, dust provides feedbacks of both signs in the atmosphere. the exact nature and evolution is not well known (to say the least ;-). however, Mars Global Surveyor has shown us that dust storms on the small (regional) scale can occur all year long...

You may also want to check out the Mars' climate FAQ, which says that the frequency of dust storms is quite variable.