Dear Tom,
When a train comes through during the night, I hear the train horn. Sometimes it’s louder than at other times. It’s not wind causing this, because I am comparing the difference only on calm nights. Explanation, please?
— Willy Neuman, Batavia
Dear Willy,
The intensity of sound (such as a train horn) that you hear will vary at night, sometimes louder and sometimes softer. The explanation is the height of the “inversion” above the ground. On a clear, calm night, it is colder at the ground than higher up, and the approximate boundary between those two air masses is called an inversion. Sound travels faster in warmer air than it does in colder air. That means the sound from a train horn will bend downward when it passes through an inversion, but the height of the inversion, which varies from night to night, will determine where that downward-bent sound strikes the ground.
