[yoe]

If the wind is in the right direction, we hear a train that is about nine miles away.

[Erasmus]

Wind has a remarkable effect on the propagation of sound. I didn’t understand the phenomenon until a few years ago; before that time I was skeptical.

When wind blows, it creates a “wind shear,” which means that the air closer to the ground moves slower than the air above it. There is a continuous increase as you go up from the ground. Of course, it is disturbed by turbulence, particularly when there are obstructions.

So when the wind is blowing from a sound source towards you, the waves are travelling faster towards you at the higher altitudes than near or at the ground. This causes the sound energy that would otherwise dissipate into the sky to ‘bend’ towards the ground and be concentrated there.

Under these conditions, the inverse-square effect that would otherwise diminish the sound at distance is partially negated, so you can hear sources of sound at longer distances than under the condition of still air.

[MindBender26]

Overcast clouds can also reflect sound back to ground. In Clearwater, Florida, in the 60s, on a very quiet, high overcast cloud night, with strong east winds, you could hear the large missiles launch from the Cape 120+ miles away. Of course they are very loud to start with, but thetas a lot more then 7 miles.

[expat2]

The wind direction is not a factor (sound waves travel much faster than the air can). It is the temperature profile in the air above ground level which causes ducting of the sound energy, and is the biggest factor.