This refers to the behaviour of cricket balls. Every cricketer in the world believes that a cricket ball "swings" more when the atmospheric conditions are cool and damp (as in the UK) as opposed to hot and dry (as in Australia). I have watched a bit of baseball and I have never heard commentatators mention atmospheric conditions affecting the flight of the ball. Once the ball has left the hand there are only two forces acting - gravity and air resistance. "Swing" refers to the ball moving horizontally to the propelled direction (as well as vertically downwards of course).