Physics Q & A - Sound transmits farther at night?
Cheung Kai-chung (Translation by Yip Ying-kin)    

Sound waves will be refracted to the sky in the daytime
Fig. 1  In the daytime, the air near the earth surface is hotter, sound waves are refracted to the sky.

Sound waves will be refracted to the ground in the nighttime
Fig. 2  In the nighttime, the air near the surface is cooler, sound waves are refracted to the earth surface.

Why do sound waves transmit farther at night? Is it because it is quieter at night?

If you go to the beach on vacation, during the night, you will discover that voice of people far away can be heard clearly. You may wonder, it is because it is quieter at night than in the daytime. Therefore it is easy to hear the sound far away. However, it is only one of the reasons. Actually, sound transmits farther at night may be related to refraction of sound waves! First, sound is the vibration of air, and it is a kind of wave motion. The propagation of sound wave is faster in hot air and slower in cold air. Therefore regions of air at different temperatures have different refractive indices, just like media with different optical densities. When sound wave propagates in air whose temperature changes with altitude, refraction of air happens. Sound will move towards areas with lower temperatures. In the daytime, when the sun shines the earth, the air near the earth surface is hotter than the air above. Sound waves will be refracted to the sky (Fig. 1). On the contrary, in the nighttime, the air near the surface is cooler and sound waves are refracted to the earth surface (Fig. 2).