Final answer:
Nimbus clouds are typically the ones that indicate an impending storm, with cumulonimbus clouds being a specific type that can signal severe weather like thunderstorms.
Step-by-step explanation:
The type of cloud that usually indicates a storm is approaching is the nimbus cloud. Nimbus clouds are dense and dark and can produce precipitation such as rain, snow, or hail. Cumulus clouds, mentioned in your reference material, are typically associated with fair weather when they are in their standard form.
However, when cumulus clouds continue to grow vertically, they can develop into cumulonimbus clouds which are a type of nimbus cloud, and these are often harbingers of severe weather including thunderstorms. The rising of water vapor due to convection, which is noted about cumulus clouds, can lead to the formation of these storm clouds when enough instability and moisture are present in the atmosphere.