There's no basis for the idea that there is never fog in the desert, as long as there's sufficient water vapor. But a very dry desert climate cannot fog up for lack of water vapor.

Remember that fog forms when temperature and dew point collide. But without water vapor, there's no meaningful dew point.

http://en.wikipedia.org/wiki/Dew_point

Quote: "The dew point is the temperature below which the water vapor in a volume of humid air at a given constant barometric pressure will condense into liquid water at the same rate at which it evaporates."