One thing I picked up from following the MER site weather reports that Mars Global Surveyor puts out every week or so is that the dust storms are less frequent in "winter". I thought it odd that they'd refer to winter across the whole planet, instead of by hemisphere, but it seems they do mean the same 'season' within the martian year, when they refer to that. I'm thinking the concept of winter for mars, when they refer to it without specifically naming the hemisphere, is the southern hemisphere's fall/winter which happens to coincide with apehelion, when the solar radiation is at a significally lower power. That also seems to be the 'season' for water/ice clouds.

here's the site for MSSS's weather reports:

http://www.msss.com/mars_images/moc/mer_weather/index.html