What physical characteristic is associated with nimbus clouds?

Nimbus clouds are thick and dark because they are associated with precipitation. These clouds accumulate moisture and are typically dense, which contributes to their darker appearance compared to other types of clouds. The presence of water droplets or ice crystals within these clouds scatters light, making them appear darker. Nimbus clouds often indicate overcast skies and can lead to rain, snow, or storms, which is why they have a more substantial and foreboding look compared to the lighter and fluffier clouds that might represent fair weather. This characteristic helps meteorologists identify and predict weather patterns, particularly those that involve significant rainfall or storms.