Blue Sky Science: How do temperature and wind affect traffic noise?
Q How do temperature and wind affect traffic noise?
— Lauren Singleton, Omaha, Nebraska
A Scott Lindstrom, atmospheric scientist with the University of Wisconsin-Madison’s Space Science and Engineering Center:
In terms of temperature, sound waves move faster in warm air and slower in cold air. So as sound moves through the atmosphere, some parts of the wave will be moving faster than the rest.
For example, if the air at the surface is cooler than air higher in the atmosphere, the top of a sound wave will be moving faster than the bottom. The wave will tilt and refract back toward the ground.
In this case, sound would leave an interstate or highway, go up into the atmosphere and then be bent back down to the surface toward the cooler air. This means the sound would be louder and you could hear the traffic noise from farther away.
During the daytime it’s hot at the surface, and sound refracts the other way, up into the atmosphere and away from our ears.
If you’ve ever seen a thunderstorm come in on a hot summer day, when it’s very hot at the surface, the refraction takes the sound up away from you so you don’t hear the thunderstorm until it’s practically on top of you. But at nighttime, when it’s cold and the refraction is back down toward the surface, you’ll hear the thunder from a long distance away.
Wind’s effect is more about distortion of sound. As a sound wave propagates, or moves, through the air, some pieces are accelerated and others slowed down as it meets wind. On a windy day, sound waves won’t propagate as one distinct wave, so it will be indistinct when it reaches your ear.