Humidity is one of several atmospheric factors affecting the propagation of sound. Heavy, muggy air actually carries sound better.
Bone-dry air greatly attenuates sound. But humidity is not as prominent a factor in affecting low-frequency sound as the bending of sound rays (or sound waves going in the same direction) by wind and the temperature profile of the air.
A steady wind blowing toward you from the sound source increases the sound, and one blowing away from you decreases it. The rays that would otherwise go out to space are bent down toward you if the wind is toward you, up if the wind is away.
With some loud sounds, like artillery fire, people very close hear the sound, then there is a dead zone, and then people farther away hear it again. This lensing effect leads to dangerous situations at unguarded railroad crossings because the whistle of a high-speed train may not be heard until it is quite close to the road.
The other lensing effect leads to dangerous situations at unguarded railroad crossings because the whistle of a high-speed train may not be heard until it is quite close to the road.
The other lensing effect occurs in a temperature inversion, the weather pattern that traps smog in California. In a normal atmosphere, as you go up in altitude, the temperature drops.
In a temperature inversion, as you go up, the temperature rises, rather than falling, and this type of temperature profile bends rays down toward the receiver.