Heat does have a factor also. Im sure many of you guys that played football in middle school, high school, or even college know how hard it is to keep your fatigue up when your running in the dead heat of August. In Ohio when i played football, i always hated practicing during the afternoon. Mornings where not so bad, but the 2-a-days where horrible when it was about 95 degrees and most of it was pure humidity.
But i agree, wind, snow, rain, its all bad weather to play in. Wind really only effects the kickers, and if its strong enough could cause the QB some trouble also. Rain, i think is the toughest one to deal with. It effects nearly every player on the field if the rain is coming down hard enough.
Its pretty hard to run with all the mud, the RBs and QBs are always at a risk of fumbling the ball when its wet and of course kickers planting their foot to kick it are at a greater risk of slipping, even a slight slip, can cause the ball to miss.