There are different reasons why a dog can lick air. It can be caused by a bloat or they are just trying to catch flying insects around them. They may also do it for fun to the sensory receptor on their tongues.
dogs have very keen sences sometimes they are simply tasting what's in the air. notice how dogs sniff like crazy when there head are out a window its like the dog gets a natural high by sticking its head out the car window and taking in all the smells