A lot of mis-information about this. Journalists and commentators don't really understand it so often make things up (not that they'd normally do such a thing of course
). Most cricketers also don't really understand swing bowling, which is why there are so many old wives tales about it.
Overcast supposedly helps normal swing. Reverse swing is supposedly helped by hot and dry conditions. Actually it's all determined by the ball speed. Humidity or lack of it probably has little actual effect. It might affect the speeds at which swing happen perhaps.
Here is a link to an article which explains it all (except any humidity effect) very well.
http://content-uk.cricinfo.com/ci/co...ry/258645.html
Basically it's to do with the fact that airflow remains attached longer if the boundary layer is turbulent. If the separation is asymmetric the ball will swing. Turbulence is created by the seam and/or the rougher side of the ball. Normally the side towards which the seam is angled has later separation, so the swing is in that direction. With reverse swing the seam actually causes flow separation and the swing is then in the opposite direction.