Air molecules move faster in hot and humid environments due to their increased thermal energy. Since sound relies on molecules bumping into one another to create compressions and rarefactions, increased speed of molecules makes the sound waves move faster. The speed of sound increases by 0.6 meters per second for every degree Celsius increase in temperature. Water vapor in the air also increases the speed of sound because water molecules are lighter than oxygen and nitrogen molecules. Therefore, on hot and humid days, sound travels faster than on cool, dry days.

The following formula gives the speed of sound in air: Speed of Sound = (331 m/s) (1 0.6 T) where temperature, T, is measured in Celsius.