Final answer:
Refraction is the phenomenon in which waves change their path of propagation at the interface of two media with different densities. Sound waves can be refracted when there is a difference in temperature between the ground and the higher atmosphere, causing the sound to bend away from the normal and travel upward.
Step-by-step explanation:
Refraction is the phenomenon in which waves change their path of propagation at the interface of two media with different densities. When waves encounter a change in density, such as when they pass from one medium to another, they can be refracted.
When sound waves pass through air at ground level and then encounter a different temperature in the higher atmosphere, they experience refraction. Sound travels faster in warmer air, so if the temperature is higher in the atmosphere, the sound waves will be refracted away from the normal (perpendicular) to the surface of the Earth and will bend upward. This can cause sound to be heard over longer distances than if there was no refraction. The magnitude of the refraction of sound waves depends on the difference in temperature between the two layers of air.