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"Watching" Thunder

© Keith C. Heidorn

Atmospheric Effects on Thunder

My previous piece on thunder looked at the source of the thunder. However, it assumed thunder's sound wave moved through a uniform atmosphere. But, the real atmosphere varies in vertical density and has winds usually blowing at stronger speeds with rising altitude, and thus sound waves may be scattered, attenuated, or refracted on their way to an observer. The total effect of these sound wave modifications will alter their volume, pitch and character.

Scattering and attenuation in the atmosphere alter the thunder we hear by softening its volume and filtering the high-pitched frequencies from the sound packet. Thus, by the time a thunder wave travels more than several miles from the lightning stroke, its predominant sound has become a low-pitched rumble. Furthermore, if the lightning flash has low energy, it produces mostly higher pitched thunder, and no audible thunder will be heard except close to the lightning channel.

Vertical variations of both air temperature and wind will refract, or bend, sound waves from a straight line path. Because temperature in the lower atmosphere generally decreases with height and sound travels faster in warm air than cold, sound waves will bend upward. Therefore, thunder propagating from the lowest portions of a lightning bolt may pass overhead and not be heard at the surface.

Wind variation with height refracts sound waves in a similar to vertical temperature gradients. When wind increases with height, the usual case near the surface, the sound waves bend upward, curving away from the surface.

Varying with temperature gradient, wind speed and the height of the lightning flash, the combined effects of scattering, attenuation, and refraction limit the distance over which thunder may be heard on the ground. Thus, thunder will not generally be audible further than 10 to 25 kilometres (6 to 15 miles) from the lightning bolt.

A common example of this is heat lightning, that visible lightning from distant thunderstorm cells (seen as in-cloud or between-cloud flashes) for which no thunder is heard. Such events are common when thunderstorms are widely scattered across a region, for example, during hot humid weather.

Topography and buildings can also alter the thunder we hear, often reflecting the sound waves and thus allowing them to echo across the countryside or city.

Hints for Observing Thunder

We can use thunder to give us a better insight into the lightning that spawned it using no more sophisticated equipment than a watch capable of measuring seconds and a critical ear.

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