At night the ground cools quickly. The higher air is warmer than the air close to the ground. During the day the sound travels faster near the ground. The sound further from the ground travels faster at night causing the sound wave to refract back towards the earth.
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Answer:
At night the ground cools quickly. The higher air is warmer than the air close to the ground. During the day the sound travels faster near the ground. The sound further from the ground travels faster at night causing the sound wave to refract back towards the earth.