Q: The noise from distant roads, railways, and so on is heard much more
clearly when the wind is blowing from the sound source towards us, than when
it is blowing against the sound. If you shout in a gale, people downwind can
hear, those upwind can鈥檛. The speed of a gale-force wind is only a few tens of
kilometres per hour. That is almost negligible when compared with the speed of
sound. So, as sound is propagated by vibration in the air, how does the
relatively slow air movement of the wind have such a noticeable effect when
carrying sound downwind or upwind?
A: When the wind blows, friction between the air and the ground means that
the wind speed increases with height. Any sound wave moving obliquely upwards
along the direction of the wind will therefore increase its speed as it
travels. However, sound is a wave and any increase of speed will result in a
change of direction (refraction). So if you stand downwind of a sound source,
some waves which had started off aimed above your head will be refracted down
so that you can hear them.
The opposite thing happens if you stand upwind. Sound waves aimed above
your head are now slowing down as they travel up, causing them to be refracted
upwards. So as the sound travels from slow moving air to fast moving air, its
direction swings up.
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A similar thing occurs on still nights. As the ground cools, a temperature
gradient is set up in the air. Sound waves moving obliquely up through this
gradient are refracted as they move from (slow) cold air to (fast) hot air.
They are refracted downwards, back towards the ground. This is the reason why
sound travels much further at night.