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Matt:

Maybe I'm wrong about this, but it seems that when I'm at the beach on a windy day, I can hear sounds that are being blown toward me, but I can't hear sounds that are downwind. Why would that be?

-- Beachcomber, Encinitas

The elves finally got their science hats back from the dry cleaners, so we're all set to tackle this one. Sound is just bands of compressed air. They don't go up and down like ocean waves, more like a flat line. Speed and direction depend on, among other things, air movement and air temperature. Because of friction against Earth's surface, wind moving near ground level moves more slowly than wind higher up. So as sound moves through these speed layers, it's direction is refracted. Sound moving into wind is refracted upwards. So is sound moving from warmer air into cooler air. If you're upwind from that sound, it will be very faint or you won't hear it at all. Downwind, vice versa.

Ever notice how on nights with low clouds and a temperature inversion, distant sounds seem very loud? (E.g., distant freeway sounds seem much louder than usual.) Sound moving up from cooler air to warmer air is reflected downward.

Teachers trying to pound this stuff into our heads have a favorite story to show how this works. The outcome of several Civil War battles was changed when field commanders were told to begin their attacks on one flank of the enemy when they heard the sound of gunfire as their compatriots attacked the other flank. Because of wind speed and direction, the sound from the first attack never reached the second group, and the battle was lost. Hardly a day at the beach, but the same principle.

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