Sound levels effected by wind direction

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I have noticed when out walking that the sound level of (say) distant motorway traffic or PA system is very significantly effected by my being upwind or downwind of the source. Bearing in mind that the wind speed might be as low as 10mph, and the speed of sound is over 700mph, I was wondering why this should be.
 
Not only wind direction, but humidity as well.

I now live in the city, but for 40 years in the yard of my rural farm house, I could either hear quite clearly, or barely hear, the interstate highway two miles away or the railroad tracks five miles away. All depended on the weather.

I recall physics class, one of the problems we had to solve was determine the speed of an object by the Doppler shift in its sound. That is where your speed of sound factor comes in. The fact the sound pressure ripples have to fight to overcome a head wind is separate.
 
Ton Danley notes outdoors PA systems are affected by wind, unless point source :) Q&A: Tom Danley | audioXpress

" The concert systems that evolved for that market may be the best they know how to make, but they radiate an interference pattern you can hear if the wind blows. Every location has a different sound and radiate sound in the wrong directions. When you can eliminate all that interference, radiates enough acoustic power with one single source, and have more directivity, then it becomes a pretty obvious sonic choice. It’s as close to hi-fi as you can get on a grand scale.

The familiar large sound systems are a huge array of sources, which depend on DSP, and sound worse the larger they are. The Synergy horns work because the source is acoustically correct at the origin."
 
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