Why does the wind drop off when a patch of cloud comes over?

I work at a UK Parachute centre and therefore spend a lot of time waiting for a breaks in the weather.

What I've noticed tends to happen on sunny days that are also wind is that wind speed tends to go from say 25kts to 15kts when cloud comes in and then back up to 25kts as soon as it clears again.



does this mean that the pressure under the clouds is higher? It doesnt seem to make sense to me.Why does the wind drop off when a patch of cloud comes over?
I dont know why that is, but i know that the wind is stronger with black/grey clouds than white fluffy clouds. I remember being taught that at sailing last week. I dont know the clever stuff behind it though :(