As a general rule, the more overcast it is, the damper the air usually is. It doesn't always work that way, but that's why cloud cover is talked of. Damp and humid are the same thing as far as a cricket-ball is concerned - the damper the air, the more swing you'll get.
I've always said there's really no such thing as "swing-friendly" though, not insomuch as one thing allows swing, another doesn't - good bowlers can swing a good-quality cricket-ball in the correct condition under any circumstances. That's what the beauty of swing is - it takes "reliance" on other matters out of the equation if you can do it well. Trouble is, a great many bowlers can't - many bowlers have actions that sometimes mean the seam comes out right and sometimes doesn't; some cricket-balls are poor quality; and sometimes you just don't bowl well. The damper the air the better, though, obviously - and aside from the cloud-cover, time of day also impacts on this. A good breeze never hurts either, though this aspect is overrated, especially with the quickest bowlers - once you go over about 70mph the ball's going too fast for the wind to have any real effect.
The best swing-bowlers with a good cricket-ball and a good fielding team who shines the ball as they should will achieve swing anywhere and under any circumstances. But obviously, there's a preference for the UK, at night, under cloudy skies. That will cause chaos. And if you can swing the ball at 90mph under those circumstances your chances of wrecking a batting-line-up are considerably higher than your chances of not doing.
FFS, that means zero practice matches IF he manages to make the First Test at all.
