I'm repeatedly told it's the wind direction, but I fail to see how a 10 kt
breeze in any direction can have any significant effect on sound travelling at
600kts, or so.
All to do with wind gradient. Downwind the air above the surface moves faster, and a pressure wave is refracted downwards, in effect the sound 'hugs the ground'. Upwind the reverse effect, sound waves are deflected upwards.