Sound transmission in cold air

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Dear Tom,

I lived in Fairbanks, Alaska, for a few years and I remember being able to hear people’s conversations as if they were standing next to me, when in fact they were a few hundred feet away. Temperatures were at least -35 degrees when this occurred. Can you explain that?

Tommy Jefferies,

Dear Tommy,

Sound waves normally travel in all directions from their source. In bitterly cold air, a strong temperature inversion — warmer air on top of colder air — forms, resisting the usual upward movement of sound waves. The density of extremely frigid air is great and acoustic waves travel well in such air. In addition, when there is no wind, background noise from air blowing through trees, around buildings, etc., is practically non-existent. Distant conversations travel unusually well.

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