Does sound travel better when the air is cold?

Answered by Dr. Steve Shropshire, Physics Professor, Idaho State University

SoundWave5If conditions are right sound can travel farther and clearer when it is cold. Sound travels faster in warmer air due to the greater average speed of gas molecules with higher temperature. There is also a very slight increase in sound speed with increasing humidity because the mass of a water molecule is less than that for molecules of nitrogen, oxygen, and most other molecules in air. This increase is only a few tenths of a percent though.

According to Stokes’s Law of Sound Attenuation*, how much the intensity or volume of sound decreases with distance depends on sound speed, the frequency of the sound, air density, viscosity, and, of course, distance. Stokes’s Law predicts that sound attenuation is greater in colder air, so sound intensity will be less at greater distances than would be the case in warmer air.

SoundWave1However, if the air is very cold and still there is frequently an inversion where cold air is trapped near the ground by warmer air above. In this case sound does not penetrate the warmer air as well and is reflected or bent back down. This has a large effect on how well sound carries, so that sound is LESS attenuated and travels further and clearer in cold air with an inversion than would be the case in warmer air.

What is an inversion?
Inversions occur during the winter months when normal atmospheric conditions (cool air above, warm air below) become inverted. Inversions trap a dense layer of cold air under a layer of warm air. The warm layer acts much like a lid, trapping the cold air near the valley floor. The surrounding mountains act like a bowl, keeping this cold air in the valleys. The snow-covered valley floors reflect rather than absorb the heat from the sun, preventing the normal vertical mixing of warm and cold air.

Sound Refraction
Sound travels in waves. The wave moves and transports energy from one place to another through a medium like air.

Refraction or bending of sound waves happen when the waves enter a medium that changes the speed of the wave. To illustrate, roll a toy car across a smooth floor towards a rug. As the car moves from the smooth floor, the rug slows the tires’ speed and the car changes direction.

Light also travels in a wave and can be refracted.

Disappearing Coin
Materials: coin, clear drinking glass, saucer, water

  1. Set a coin on a flat surface like a table or counter.
  2. Place the base of the drinking glass over the coin.
  3. Cover the mouth of the glass with a small saucer. Looking in through the side of the glass, you can still see the coin.
  4. Now, remove the saucer and fill the glass with water.
  5. Once you’ve filled the glass, replace the saucer. Can you still see the coin through the side of the glass? It has disappeared!
  6. Take the saucer off of the mouth of the glass. Peer straight to the bottom of the glass through the water. It’s back!

SoundWave4When light waves travel through air, they experience little or no refraction. That’s why you can still see the penny through the side of the empty glass.

When you poured water into the glass, it was as though the penny had disappeared, but the light waves were actually being refracted by the water. After traveling through the water and the side of the glass, none of the waves were able to reach your eyes. Gas molecules in air are spread out. and little to no refraction occurs. However, when light rays pass through a substance such as water, the refraction is greater because the molecules are closer together.

The light waves bend the image near the top of the glass. You would be able to see it if the saucer were not strategically placed on top of the glass.

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