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 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 her sounds from a long way away if the night air is cold.