What causes heat lightning, rather than the lightning that comes with a rain storm?
There is no such thing as heat lightning. I know that contradicts what a favorite aunt, uncle, mother, father, grandmother or grandfather may have told you...but I'll say it again. There's no such thing as heat lightning.
There are two explanations for lightning that strikes where rain isn't falling. The first is that lightning can exit a cloud at an angle, instead of straight down. As a result, lightning can strike as much as 10 miles beyond where rain is falling from a thundershower. The second is that occasionally the air is so dry beneath a thundershower, that the rain evaporates before it reaches the ground-yet of course the lightning can still strike the ground. This, by the way, is a very dangerous situation when the risk of fire is high.