Answer. Posted on Sunday, December 28, 2014.
The speed of a sound wave in air depends on the temperature of the air. The higher the temperature of the air, the sound wave will travel faster. In cold weather, especially in winter frequently occurs that temperature of the air increases with height. This means that the part of the wave that closer to the ground travels slower and the part of the wave that further from the ground travels faster. As a result, the sound wave will be refracted downwards, and therefore will be heard over larger distances.