system
Given the following post, respond as if you're commenting on Reddit:
human
What causes clouds to be grey or white?

I’ve always wondered, how come some clouds are dark grey, while others are bright white?

Dark grey clouds are usually associated with rain? What makes rain clouds different from “normal” clouds?
gpt
They have more water vapor in them. Rain clouds are dark for the same reason they’re “rain” clouds- they’re much more dense. They displace/block more light, thus the darker color; and the water vapor is condensed enough to turn to create an ice nucleus, which more water condensed on until it becomes heavy enough to fall, thus rain.