why is sound heard more clearly in night than at day?
"On clear nights, as the ground cools, a temperature inversion forms in the lowest hundred meters or so of the atmosphere. The winds in the inversion tend to be light and turbulence levels tend to be low. Above the temperature inversion one generally finds a stiff wind known as the nocturnal jet. The temperature inversion and the nocturnal jet form a duct in which sound propagates efficiently to distances of many kilometers."
(Abstract of a talk by Roger Waxler, http://www.phy.olemiss.edu/Colloquia/0708spring.html#waxler")
What Bill K said. You can get the same effect during the day on the coastline, because of the cooling effect of the sea water. Sounds from small boats hundreds of meters offshore (e.g. people talking to each other) can sometimes be heard very clearly, though you would not be able to hear people talking (at normal levels, not shouting) hundreds of meters inland.
Also, the background noise level in the environment tends to be lower at night, so quiet sounds are more easily heard.
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