system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is a cloud gray when raining , but white if not?
gpt
Clouds are just little pieces of water in the air, that have grouped up enough to start reflecting light from the sun. At first clouds are white, because they aren't very dense. Light moves through them pretty easily. As clouds gain more water mass, they start blocking some of the light, making the cloud grow darker. Then once heavy enough, the water in the cloud collects enough to rain. The cloud remains darker.