Well, there is no such thing as lightning that is caused by heat. Lightning is caused by a charge differential in a thunderstorm cloud. Scientists are not 100% sure of the exact way that the charge differential forms, but the latest and most popular is by ice crystals rubbing against each other.
With that said, there are times that lightning can be seen, but is far enough away that the thunder cannot be heard, as the sound waves are absorbed before they can reach the observer. Many times thunderstorms occur in hot and humid air and thus the observance of lightning and not hearing thunder happens a lot when it is hot out.
Hope this makes sense.
-John
thank you very much John for this site 