Here's why:

* Nimbus means "rain cloud" in Latin.

* Nimbus clouds are rain-producing clouds and are associated with precipitation.

* They are thick, dark, and gray, often covering the entire sky.

* You can see them in various forms, like cumulonimbus (thunderstorm clouds), stratus clouds, and altostratus clouds when they are producing rain or snow.

So, if you see a dark, gray cloud that looks like it could rain, you're probably looking at a nimbus cloud!