The refraction of sound in hot and cold air

The speed of sound is greater in hot air than it is in cold air. This is because the molecules of air are moving faster and the vibrations of the sound wave can therefore be transmitted faster.

This means that when sound travels from hot air to cold air or from cold air to hot air it will refract.

You can notice this on a hot day or a cold night.



On a hot day the air near the ground is hot so the sound wave bends upwards from the hot air into the cold air (Figure 1).


On a cold night the air near the ground is cold and so the sound wave bends downwards. (Figure 2) This is why you can sometimes hear sounds from a long way away if the night air is cold.

I noticed a very interesting effect produced by the change of speed of sound in air of different temperatures when I was singing in a carol service in Wells Cathedral.

It was a clear starry night and the great stone walls of the cathedral were cold. At the end of one of the choir pieces the sound travelled away into the darkness of the building and a moment later I heard the echo as the sound reflected from the walls. The echo was not only quieter - it was flat. The pitch of the note had gone down.

After much thought I decided that this was because it had travelled through more cold air than hot air on its way to the walls and back and so had slowed down, therefore reducing the pitch.