Cumulus clouds are white clouds that are dense and large. They are known to look like cauliflower. Mostly, you can see easily that cumulus clouds have flat bottoms but the tops are always changing shape. Cumulus Cloud A Cumulus cloud forming into an anvil shape. Seeing a cumulus cloud means that the air is well mixed by up and down vertical drafts. When a cumulus cloud grows into a thunderstorm, the cloud is called a cumulonimbus cloud. This is a sign that the air is rising to the stratosphere.