Ever wondered why clouds can be white and fluffy like cotton on bright sunny days and dark and gloomy when it’s gonna rain?
My mum used to tell me that was cos the clouds absorbed all that dirt in the skies, turning it really dark and dirty. (That’s also why she says it’s not a good idea to get yourself wet standing in the rain)
So are the clouds dirty just before it rains? Well, not really.
The truth is, when you look at clouds, the extent of their brightness (or greyness) depends on how much light is reflected into your eyes. The more water droplets there are in the skies, the more light is absorbed such that less light can reach your eyes –> hence darker clouds. When there isn’t a lot of water droplets in the skies, most of the light can be reflected into your eyes and you see the the clouds are white.
When there is a lot of water droplets in the skies, they block off a lot of light, hence causing the clouds to look dark. And because there is a lot of water droplets in the skies, it is more likely that dark clouds signify that it’s going to rain!
I found a more scientific explanation in this site here: do read it for better insight!