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The answer is blowin' in the wind

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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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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4S Ranch Allied Gardens Alpine Baja Balboa Park Bankers Hill Barrio Logan Bay Ho Bay Park Black Mountain Ranch Blossom Valley Bonita Bonsall Borrego Springs Boulevard Campo Cardiff-by-the-Sea Carlsbad Carmel Mountain Carmel Valley Chollas View Chula Vista City College City Heights Clairemont College Area Coronado CSU San Marcos Cuyamaca College Del Cerro Del Mar Descanso Downtown San Diego Eastlake East Village El Cajon Emerald Hills Encanto Encinitas Escondido Fallbrook Fletcher Hills Golden Hill Grant Hill Grantville Grossmont College Guatay Harbor Island Hillcrest Imperial Beach Imperial Valley Jacumba Jamacha-Lomita Jamul Julian Kearny Mesa Kensington La Jolla Lakeside La Mesa Lemon Grove Leucadia Liberty Station Lincoln Acres Lincoln Park Linda Vista Little Italy Logan Heights Mesa College Midway District MiraCosta College Miramar Miramar College Mira Mesa Mission Beach Mission Hills Mission Valley Mountain View Mount Hope Mount Laguna National City Nestor Normal Heights North Park Oak Park Ocean Beach Oceanside Old Town Otay Mesa Pacific Beach Pala Palomar College Palomar Mountain Paradise Hills Pauma Valley Pine Valley Point Loma Point Loma Nazarene Potrero Poway Rainbow Ramona Rancho Bernardo Rancho Penasquitos Rancho San Diego Rancho Santa Fe Rolando San Carlos San Marcos San Onofre Santa Ysabel Santee San Ysidro Scripps Ranch SDSU Serra Mesa Shelltown Shelter Island Sherman Heights Skyline Solana Beach Sorrento Valley Southcrest South Park Southwestern College Spring Valley Stockton Talmadge Temecula Tierrasanta Tijuana UCSD University City University Heights USD Valencia Park Valley Center Vista Warner Springs
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