Physics Q & A - Sound transmits farther with the wind?
Cheung Kai-chung (Translation by Janny Leung)    

Sound wave will be refracted to the ground when traveling with the wind.
Fig. 1  Sound wave will be refracted to the ground when traveling with the wind.

Sound wave will be refracted upwards when traveling against the wind
Fig. 2  Sound wave will be refracted upwards when traveling against the wind.

Why can a distant sound be heard easier when it travels with the wind? Why does it become weaker if it travels against the wind?

Do you have such an experience? When you are standing against the wind, no matter how loud you shout, your distant friend cannot hear you at all, but you can hear him very well. Does wind scatter sound wave? Surprisingly, this phenomenon is related to refraction! Sound wave travels faster in the air when it is with the wind. Generally speaking, wind speed is lower near the ground because of the presence of blockages, and so it increases with height (Fig. 1). Therefore, when traveling with the wind, sound wave farther from the ground travels faster. Refraction is resulted from the difference in speed of sound in the upper and lower region, just like how sound wave travels in different mediums. Sound wave will be refracted towards the region with a lower speed of sound, thus sound wave will be refracted downwards when it travels with the wind, and hence it can be transmitted farther away.

In contrast, when sound wave travels against the wind, that is, when it travels in the opposite direction with the wind, the speed of sound will be reduced by the wind speed, resulting in a lower speed in the upper region. According to the principle mentioned above, sound wave will be refracted upwards, and hence it cannot easily reach a distant person (Fig. 2).