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You may have noticed that you can hear a sound better if it comes from downwind rather than upwind. We assume that this is because the wind "pushes the noise along." Unfortunately for our intuitions, it can easily be shown that this force is too small to account for the observed effect.
The speed of sound in air is about 760 miles per hour (1,223 kph). If a typical wind is blowing at 30 mph, this is only 4% of the speed of sound, meaning that wind can only shorten or increase the distance a given sound needs to travel by that amount. The difference would be too subtle to be detected by the human ear, so obviously this does not uncover the source of the phenomenon.
The actual solution is attached to a property physicists called viscosity. Due to viscosity, wind velocity near the ground is actually slower than velocity at higher altitudes. Collisions between air molecules and the ground give rise to turbulence effects that prevent waves from being transmitted along this air level as quickly.
If air has a uniform temperature, the change in viscosity with altitude causes a sound wave to accelerate along the top layers of air. This causes the wave to tip downwards, which makes it more audible to a human listener. This redirecting-phenomenon is called refraction. When the wave is moving against the wind, it is refracted in the opposite direction — upwards. In fact, if you were hovering above the ground in an area upwind from the source, you would hear the sound quite clearly, because of the reflection of waves in your direction.
In an area with uniform temperature and no wind, sound waves always travel outwards at equal speeds from the source. As we have seen, this is not always the case.