Raising Clouds, how'd they do that?

[Will Schmidt]

I have been a fan of the BBC show "Top Gear" for a while now and every so often the use a rather nifty trick. During the car testing at the track they somehow make the clouds above the test track raise and lower. A good example of this is in season 3 episode 9 "No Money Left" towards the end somewhere around minute 44:16. I have been wondering how they do this, is it an post production tweak by splicing in two different shots if so how do you make the clouds appear to raise in the one shot? I first thought it was just a sort of time lapse trick but they seem to be able to reproduce it far to regularly for that to be the case. Anyone know how this is done?