Here's why:

* "Nimbus" means "rain" in Latin.

* Nimbus clouds are dark, gray, and often have a layered appearance.

* They are thick enough to block sunlight.

* They form when warm, moist air rises and cools, causing water vapor to condense into droplets or ice crystals.

Different types of nimbus clouds:

* Cumulonimbus clouds: These are the most powerful storm clouds and are associated with heavy rain, thunderstorms, hail, and even tornadoes.

* Stratonimbus clouds: These are associated with widespread, steady rain or snow.

So, if you see a nimbus cloud, it's a good bet that rain or other precipitation is on the way!