Heat lightning is the name for lightning from thunderstorms that are far away that is not followed by thunder. This happens because the lightning is far away and the sound of the thunder stops before it reaches the person watching the thunderstorm. Although it is named heat lightning, it does not have anything to do with the heat of the lightning itself. At night, lightning can be seen from up to 100 miles away, but the thunder doesn't go that far.
Heat lightning is often seen during the summer months. In Florida, heat lightning can often been seen over the water at night from thunderstorms that formed during the day.