in the manual there is written that mud has the effect of halved action points and rain should make aircraft attacks not possible.
However that does not seem to be the case in my game as the attached pictures show. Or am I Missing sth.?

I also tested the mud thing by putting a unit just on muddy ground without any kind of roads or streets and it still seem to have no effect at all.
Same goes for Sandstorms. I can fly aircrafts without any penalties in those conditions.