Dear Tom,
I can sometimes hear train horns clearly during the night when there is little wind, but on some nights not at all. Why?
Kenneth Whitcomb, Batavia

Dear Kenneth,

The intensity of sound will vary at night, sometimes louder and sometimes softer. It has to do with the height and strength of a temperature inversion just above the ground. On clear, calm nights, it is cooler at the ground than higher up. When air warms with altitude a temperature inversion exists. Sound travels faster in warm air than it does in cool air. This means the sound of a train horn will bend downward when it passes through an inversion causing sound waves to propagate farther than normal. The height of the inversion above the ground varies from night to night, and the inversion height will determine where the downward-bent sound strikes he ground.