Sound is heard better downwind due to refraction caused by the change in viscosity with altitude. Wind speed near the ground is slower, causing sound waves to slope downwards, making them more audible. When moving against the wind, waves are refracted upwards.
You may have noticed that you can hear a sound better if it comes from downwind rather than upwind. We assume that this is due to the fact that the wind “pushes the noise”. Unfortunately for our intuitions, this force can easily be shown to be too small to explain the observed effect.
The speed of sound in air is approximately 760 miles per hour (1,223 km/h). If a typical wind blows at 30 mph, that’s only 4% of the speed of sound, meaning that the wind can only reduce or increase the distance a given sound has to travel by that amount. The difference would be too subtle for the human ear to detect, so obviously this doesn’t reveal the origin of the phenomenon.
The actual solution is related to a property that physicists call viscosity. Due to viscosity, wind speed near the ground is actually slower than speed at higher altitudes. Collisions between air molecules and the ground give rise to turbulence effects which prevent the transmission of waves along this air level as rapidly.
If the air has a uniform temperature, the change in viscosity with altitude causes a sound wave to accelerate along the upper layers of the air. This causes the wave to slope downwards, which makes it more audible to a human listener. This redirection phenomenon is called refraction. When the wave moves against the wind, it is refracted in the opposite direction, upwards. Indeed, if you were above the ground in an area upwind of the source, you would hear the sound quite clearly, due to the reflection of the waves in your direction.
In an area of uniform temperature and no wind, sound waves always travel outward at the same speed from the source. As we have seen, this is not always the case.
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