Problem
Why are distant sounds heard much more clearly at night than during the day? Think about the refraction of sound waves in air with a temperature gradient.
Solution
The key is sound refraction. The speed of sound increases with temperature, so sound bends towards the colder air layers (where it travels more slowly).
- During the day the ground, heated by the Sun, warms the low-lying air: it is warmer near the ground and colder higher up. Sound is bent upwards, moving away from the ground: at a distance it can no longer be heard.
- At night the ground cools and creates a temperature inversion: cold air below, warmer air above. Sound is bent downwards, stays close to the ground and reaches distant ears.
Lower night-time background noise also contributes, but the dominant physical effect is refraction due to the temperature gradient.
Links
Topics: Onde Concepts: Rifrazione e legge di Snell