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?
I just found out that the idea of clouds and overcast conditions assisting swing is essentially an Urban Legend, and also the result of Confirmation Bias.
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.