Why are fast bowlers called to be in a general advantage on a rainy day. Why does the ball swings more on such a day?
As NASA scientist, Dr Rabindra Mehta, who has studied the aerodynamics of cricket balls for three decades, remarks here:
The reason why it is believed weather conditions matter is psychological. The idea that the ball swings under cloud is so ingrained that we remember occasions that confirm this impression and forget those that do not, and overcast conditions may also affect bowlers’ confidence.
Damp weather means the pitch is more likely to be visibly green. That implies the grass on it is alive, and its cells are turgid. That means the position of the stalks and leaves of the grass, which is essentially random, is more likely to affect the bounce of the ball, and the traction that the seam gets on the ground.
The overall effect is that the bounce and movement of the ball vary more from delivery to delivery. This is most obvious with seam bowling, but can also affect the bounce of swing bowling.
I debate the thought that a "rainy" day is more conducive for fast/seam bowling.
As @maskedman pointed out, the air conditions have little effect on the ball, rather the variation in the ball's themselves (hardness, uniform curvature, lacquer thickness/robustness, seam strength) have a muhc larger impact on swing (along with bowler ability).
Relating to the "rainy days" idea, this actually makes fast bowling much harder, for 2 main reasons:
- Firstly, the bowling crease becomes "greasy" and landing the front foot is extremely risky and dangerous as is, let alone when wet. The potential for the foot to slide and tear the achilles, hamstring, or hyperextend the knee is extremely dangerous.
- The other difficulty is that the ball also becomes slippery. This makes control of length very difficult for a seamer, hence why see bowlers in wet conditions often bowling unusually high full-tosses or even beamers.
I hope this relates to your idea that rainy weather is good for fast-bowling and helps to eliminate some preconceived notions.