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During the day, the ground is warmer than the air above. the wave is therefore retracted upwards. Under this condition, sound waves are not heard over a long distance on the ground.
At night, the ground is cooler than the upper atmosphere. The upper part of the wave travels faster and the wave is retracted towards the ground. Therefore sound is heard over a long distance.
chrispohmomanyi answered the question on October 2, 2017 at 11:13
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