The total effect of these modifications of the sound wave is to further alter the volume, pitch and character of the sound heard. The scattering and attenuation processes in the atmosphere alter the total sound package reaching the listener, mostly by weakening the higher pitched frequencies in the sound packet. Thus, by the time the thunder wave reaches a listener several miles from the lightning stroke, the predominant sound will be a low-pitched rumble. Because the temperature of the lower atmosphere usually decreases with height and sound travels faster in warm than cold air, sound waves moving through the lower atmosphere curve upward. Thunder propagating from the lowest portions of the lightning bolt, therefore, may not be heard as refraction bends the sound wave upward, causing it to pass over the head of the listener on the surface.
Wind can have two different effects on thunder.
First, it may increase or decrease the speed at which the sound wave moves through the air. Sound moves faster downwind that it does upwind. Second, the variation of wind with height may refract sound waves similar to the effects of vertical temperature gradients. If wind increases with height as is usually the case near the earth's surface, the sound wave will refract upward, bending away from the surface-based listener. The combined effects of scattering, attenuation, refraction and, in most cases, reflection, limit the distances at which thunder may be heard by a ground-based observer.