Answer:-An echo is heard sooner on a hot day because the speed of sound in air increases with temperature. So the speed of sound in air is more on a hot day, and an echo is heard sooner. The velocity of sound in a gas is directly proportional to the square root of its absolute temperature (v=MρRT ). Since, temperature of a hot day is more than cold winter day, therefore sound would travel faster on hot summer day than on a cold winter day. A reduction in pulse-echo amplitude was observed at all temperatures in the range and was found to be proportional to temperature increase. Sound wave speed is dependent on air temperature. Sound waves move faster in warmer temperatures and slower in colder temperatures. During the day, temperatures are warmest near the surface and cool off with height. This is known as a lapse rate. Due to the high energy of atoms on a hot day, sound travels faster compared to a cold day, where atoms don't have much energy to vibrate freely. Therefore, an echo will return more quickly on a hot day than on a cold day.