How do dark clouds form?

White clouds and dark clouds are all made up of small water droplets. There are some differences between white clouds and dark clouds in "water content", which can be understood as the water content of the whole cloud or as the water content of a single water drop.

Judging from the formation process of clouds, if they don't float from other places, then the dark clouds must come from white clouds. White clouds can not only come from other places, but also change from dark clouds, and can also be suddenly "created" on the background of clear skies in Wan Li. In the process of studying solar energy, scientists pay great attention to the changes of clouds in the sky. They have seen many times that the blue sky background can change from blue to pink blue in the eyes of scientists, and then into faint white clouds with blurred edges, or even white clouds with clear edges. I've never seen dark clouds suddenly appear against a blue sky background. He also noticed that most of the white clouds turned into dark clouds before it rained, and most of the dark clouds turned into white clouds after it rained.