During the day time, the land gets heated due to solar radiation. At the time, the air molecules closer to the land also gets heated. As the height increases, the atmospheric temperature gradually decreases. At this time, the upper layer of air acts as a denser medium and the lower layer acts as a rare medium. when the sound waves travel from the lower layer to the upper layer of the air, they bend towards the normal and finally reach high above the earth's surface and do not return back.
During the night, the land cools gradually due to the absence of solar radiation. The temperature of the lower layer of air is less than the upper layer. As a result lower layers of air act as a denser medium and upper layer as a rarer medium. when sound travels from lower to the upper layer of air, It bends always away from the normal. It refracts again and again finally returns back towards the earth by total internal reflection. So, More clear sound is heard at night than in the day time