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Silent thunder bends up
Q: I see lightning but often don't hear thunder. Why is there no thunder? -Arno R., Albuquerque, New Mexico
A: You hear no thunder, if you are more than fifteen miles away, because the sound bends upward and misses you. Also thunderstorms
are chaotic maelstroms that disorganize and dissipate sound waves before the thunder sound can reach you.
Sound waves bend when parts of the wave fronts travel at different speeds. This happens when the sound travels through air of different
temperatures or in uneven winds. Thunderstorms tower up to fifteen miles high and reach through a gradient of winds and temperatures.
The speed of sound is faster in warmer air. Usually air is warmer near the ground and therefore sound travels faster there. Consider just
one sound wave. The part near the warmer ground outruns the part higher in the cooler sky and the wave bends up. (See the figure.)
Uneven winds also bend sound waves like uneven temperature does. The sound of thunder always moves against the wind because winds
blow toward low pressure and, therefore, toward the storm. Normally winds are stronger higher up. The part of the sound wave near the
ground encounters weaker wind and consequently outruns the part higher in the sky. The wave bends up.
Uneven temperatures and winds work together to rob you the sound of thunder.
(Answered by April Holladay, science correspondent, January 30, 2002)
Further Reading:
Hewitt, Paul, Conceptual Physics, Little, Brown, and Company, Boston (1998)
Walker, Jearl, The Flying Circus of Physics, John Wily & Sons, New York (1977)
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