Summer evenings lightning is often seen in the distance and thunder is not heard. Some of my most vivid weather memories of youth when growing up in Loveland, Ohio are of heat lightning and the awe we experienced at something so exotic. But it is not exotic, heat lightning is lightning, in a thunderstorm, some distance away, only you cannot hear the thunder. Adulthood thus ends another fantasy of youth. However the facts are more exciting!
Because temperature decreases (most of the time) with altitude in the atmosphere the shock waves created by lightning are refracted upward, beyond a certain distance you cannot hear thunder the shock wave travels above you.
Sometimes an inversion exists, meaning for a variety of possible reasons temperature increases upward in the atmosphere. The sound of thunder is then bent downward, and in some cases it can be focused, leading to an intense clap, or several claps of thunder. Thunder can also follow a duct, that is it is trapped between two inversions and can propogate a very long distance.
Take a stormy night with many lightning flashes in all directions, bouncing off layers in the atmosphere, buildings and terrain features and almost any sound combination can be heard.
© 2008 Steven L. Horstmeyer, all rights reserved