Refraction of sound waves, why you can hear your neighbors across the lake better at night.

MANKATO, Minn. (KEYC) - Suppose you rent a cabin on a lake which is not too wide, say about half a mile across. During the day you can see other cabin goers across the lake enjoying the afternoon sun, but you can not hear them. As night time falls, you can still see those across the lake but now you can hear them as if they are standing right next to you. This phenomena is due to the refraction of sound waves and temperature plays a key role.

Sound wave speed is dependent on air temperature. Sound waves move faster in warmer temperatures and slower in colder temperatures. During the day, temperatures are warmest near the surface and cool off with height. This is known as a lapse rate. When you shout something to someone across the lake, the sound waves near the surface end up moving faster than the sound waves higher up. Since the speed is greatest near the ground, sound waves bend upward, causing the audio to be just out of ears reach.

During the night our atmosphere ends up producing a temperature inversion. This means temperatures increase with height. Therefore when you shout across the lake, sound waves higher up are now traveling faster than those near the surface. The faster speeds aloft cause the sound to bend downwards, making it easy to hear what others are saying.

Go ahead and give this experiment a try the next time you are out on the lake. Now this only works if the lake is small enough to see across.