A man stands on the ground at a fixed distance from a siren which emits sound of fixed amplitude. The man hears the sound to be louder on a clear night than on a clear day.

Explanation

During the day, the temperature of air in maximum near the ground and it progressively diminishes upwards Therefore the velocity of sound is greater near the ground and it decreases upwards `(cpropsqrtT)`. The plane wavefronts, initially vertical, are turned upwards, so the sound rays curl up during the day. At night the conditions are reversed, so sound rays curl down, producing better audibility of distant sounds.