Why is there sometimes lightning without thunder?

Thunder is the sound of air exploding outward from the lightning channel, and it is produced by every stroke without exception. What varies is whether the sound reaches you. Thunder attenuates with distance, and the atmosphere refracts it, bending sound upward as it passes through warmer air near the ground, so thunder is rarely audible beyond roughly ten miles and often less. Lightning, by contrast, is visible from far greater distances, especially when reflected off high clouds at night. The result is a flash you can see from a storm you cannot hear.

Heat lightning is the popular name for this, and it is a misnomer. There is no distinct type of lightning caused by heat; the term describes ordinary lightning from a storm over the horizon, common on summer evenings when storms form in the distance and the clear humid air makes the flashes visible for a hundred miles or more. The association with heat comes from the season, not from any mechanism.

The practical rule follows. If you can hear thunder, the storm is within range to strike you, and lightning has struck people more than ten miles from the parent storm. If you see lightning but hear nothing, the storm is far enough away for the moment, but storms move, and a distant flash is a reason to monitor rather than to relax. Counting seconds between flash and thunder and dividing by five gives the distance in miles, which is the same physics applied in the other direction.