Warmer or damper air

The idea

The speed of sound in air increases as the temperature or humidity of the air increases.

Morning and noon

Early in the morning the air is cool. At noon the same air is warmer. A friend shouts across a field both times.

How do you think the speed of sound in the air changes from morning to noon?

• It is higher at noon
• It is higher in the morning
• It does not change

Heat it, wet it

Choose a temperature and the humidity of the air.

Warmer, damper, faster

In dry air, sound travels at about 331 m s⁻¹ at 0 °C and nearly 344 m s⁻¹ at 22 °C. So warmer air carries sound faster. More humid air does too.

The speed of sound in air increases as the temperature or the humidity of the air increases.

Notes

The speed of sound in air increases as the temperature or humidity increases.

Check yourself

Dry air is warmed from 0 °C to 22 °C. What happens to the speed of sound in it?

The air becomes more humid. What happens to the speed of sound in it?

The speed of sound in dry air is about 331 m s⁻¹ at 0 °C and nearly 344 m s⁻¹ at 22 °C. How many m s⁻¹ greater is it at 22 °C?

Answer: 13 m s⁻¹

344 − 331 = 13 m s⁻¹.

Which air gives the highest speed of sound?

• It decreases. The speed of sound in air increases as the temperature increases.
• It increases — correct. Yes!
• It stays the same. In dry air the speed is about 331 m s⁻¹ at 0 °C and nearly 344 m s⁻¹ at 22 °C, so it changes.
• It decreases. As the humidity increases, the speed of sound in air increases.
• It stays the same. The speed of sound in air depends on both the temperature and the humidity, and increases when either increases.
• It increases — correct. Yes!
• Warm, humid air — correct. Yes!
• Cold, dry air. Higher temperature and higher humidity both raise the speed of sound, so cold, dry air gives the lower speed.
• The speed does not depend on the air. The speed of sound in air increases as the temperature or the humidity increases.