Go with either
Weathershield or
Noah material car covers. Weathershield is a thin and very light car cover which is great for dust/dirt/moisture protection. Noah is a heftier and thicker car cover (which I currently use) which offers all the protection of the Weathershield. I've had both materials before and I personally prefer Noah because it's thicker, offers some ding protection and won't flap around as much in the wind (= less scratches should you have to put the cover on with some dust/dirt on the car). Both are suitable for outdoor use, though the more robust material of the Noah makes it stand up to the elements a little better. There's also a new material out called
Weathershield HD which might be of interest to you; it's recommended for use in "intense sun climates."
Here's a few links on these materials:
http://www.covercraft.com/weathershield.htm
http://www.covercraft.com/weathershield_hd.htm
http://www.covercraft.com/NOAH%AE.htm