Thunder
When a return stroke occurs in a lightning bolt, the air is heated almost instantaneously to a temperature of several thousand degrees Celsius. The heating causes the air to expand with great force, generating a shock wave that we hear as thunder. The sound can be heard for varying distances, depending on the wind direction and speed, the presence or absence of favorable sound propagation conditions, and the amount of background noise. Usually, thunder is audible up to about 8 km (5 mi) from the point of strike.
If the lightning flash can be seen, the thunder is heard somewhat later. Sound travels through air at a speed of approximately 335 m/s, which is roughly 1 km per 3 s or 1 mi per 5 s. You can determine the distance to a lightning flash by counting the number of seconds it takes for the thunder to arrive. Divide the number of seconds by 3 to get the distance in kilometers, or by 5 to get the distance in miles.
The rumbling, or booming, noise of thunder puzzles some people. A lightning flash has short duration, but thunder seems to last for several seconds. This occurs for two reasons: echoes and propagation delays. In hilly or mountainous terrain, or in cities with many tall buildings, the acoustic noise from a lightning stroke gets a chance to bounce around. We hear not only the original thunder, but its echoes. This creates a prolonged rumble. In flat, open country, or on a lake or at sea, there are no objects to cause the echoes, and the rumbling is less pronounced. Propagation delays also contribute to the rumbling effect. Suppose you stand 1 km from a stricken object, and the lightning flash occurs vertically from a cloud base 1 km high. You are about 400 m closer to the bottom of the lightning bolt than you are to the top, and the sound is therefore spread out over a time interval of slightly more than 1 second.
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