What type of clouds are classified as large and heavy, often indicating stormy weather?

Nimbus clouds are known for their thick, heavy presence and are often associated with stormy weather conditions. These clouds can produce significant precipitation and are typically dark in appearance due to the water droplets they contain. The term "nimbus" is specifically used to describe clouds that are capable of producing rain or other forms of precipitation.

Cumulus clouds, while they can grow large and develop into storm clouds under the right conditions, do not inherently indicate stormy weather unless they have developed into a cumulonimbus form. Stratus clouds tend to form in layers and typically bring overcast skies, sometimes light rain or mist, but are not associated with heavy storms. Cirrus clouds are high-altitude, wispy clouds that generally signal fair weather, not stormy conditions.