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Sound waves controlled by temperatures

In a Sept. 17 Since You Asked column, we stunned our readers by admitting to be somewhat at a loss to explain why noise coming from Interstate 5 seems so much louder in the morning. We and others speculated that it could be because there's a lot of going-to-work traffic at that time of day and even wondered whether tire pressure might be lower in the cooler temperatures. Well, it apparently does have something to do with temperature, but not because of its effects on tires. John Casad, a meteorologist at the National Weather Service for 30-plus years, came up with a much more logical explanation:

"The movement of noise (sound waves) from the freeway is controlled by the temperature of the air in which it is traveling. In general, during the day the warmest air is near the surface and the air aloft is cooler. At night the cooler air is near the ground and the air aloft is warmer.

"Noise (sound waves) travels faster in warm air and slower in cooler air. When a sound wave moves through layers of air that are at different temperatures the sound wave is bent toward the cooler air. So during the night and early morning hours, noise from the freeway is bent downward and it travels along the ground and the freeway sounds louder as if you were standing near it.

"After sunrise the air near the surface becomes warmer than the air aloft and the sound waves are bent upward. We no longer hear the freeway noise even when the freeway is only a short distant away. This also explains why you may hear the early morning/night flights taking off at the airport and not flights during the day even when you are at the same location.

"I am sure that traffic volume/patterns play a part in the freeway noise being heard or not being heard, but the atmosphere is also a player."

So now you (and we) know. Thanks, John.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com.