Question

Why do distant sounds such as those of traffic and loudspeakers become louder during the night time than during the day time?

Answer 1

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There may be less background noise at night, however another reason is because sound waves travel differently at night than during the day. There is a phenomenon called refraction that affects the direction of sound propagation. ... To put short, sound bends towards cooler air (I won't go into the physics of why).

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Answer 2

There may be less background noise at night, however another reason is because sound waves travel differently at night than during the day. There is a phenomenon called refraction that affects the direction of sound propagation. During the day, the sound bends away from the ground; during the night, it bends towards the ground. Hence at night you have additional "sound" reaching you, making it louder. This is not a perceived effect because there is less other noise around - the sound IS actually louder.

So what governs the direction the sound bends ins? It is governed by the speed of sound in the medium. In air, a change in temperature affects the speed of sound propagation, so if we have air at different temperatures, we can consider this two different mediums.

During the day, the ground absorbs heat, so the temperature is higher closer to the ground; at night it starts releasing heat, and since hot air rises, the air closer to the ground is cooler. To put short, sound bends towards cooler air (I won't go into the physics of why).

So to summarise, sound is louder at night due to the change in the direction of sound refraction, which is caused by the reversal of the temperature gradient from day to night.