What are cumulonimbus clouds and what type of weather do they suggest?
The cloud with the greatest vertical development.
The most unstable cloud and the cloud necessary for the most violent weather (thunderstorms, hail, tornadoes), and also the clouds that make up a hurricane.
These clouds start as a cumulus cloud, looks like popcorn. As more heat and water vapor enter the cloud it rises, til eventually it turns into a cumulus that is at least twice as tall as it is wide (towering cumulus). At this point it may produce some rain showers, but if it is still unstable enough it will grow until it is many miles across and the top has reach a point where it no longer forms liquid cloud droplets but is an ice cloud. This gives the cloud it's characteristic "anvil" look. Particularly large cumulonimbus clouds will have tops into the tropopause or even the stratosphere.