Here's why:

* Low: Cumulus congestus clouds form relatively low in the atmosphere.

* Large: They can grow quite large, often towering thousands of feet.

* White and Gray: They are bright white on top, but the base can be gray, especially as they develop.

* Globular Masses: Cumulus congestus clouds have a rounded, puffy appearance, with the distinct "cotton ball" look of cumulus clouds.

* Rolls: The "rolls" you describe are likely the result of strong updrafts within the cloud. These updrafts can cause the cloud to have a more rounded, bulging appearance.

Important Note: While cumulus congestus clouds are usually harmless, they can sometimes indicate the potential for thunderstorms. If you see them growing rapidly or developing a dark base, it's a good idea to be aware of the weather conditions.