American Meteorological Society Certification (AMS) Practice

Cumulus clouds play a significant role in weather formation, particularly in their capacity to indicate both fair weather conditions and the potential development of storms. Typically, cumulus clouds are characterized by their fluffy, white appearance and their formation in relatively stable air conditions. When they appear in a fair-weather setting, they suggest that the atmosphere is generally clear, and these clouds are often harmless.

However, as cumulus clouds grow vertically—transforming into cumulonimbus clouds—they indicate an increase in atmospheric instability and moisture. This vertical growth can lead to the development of thunderstorms and severe weather conditions, making their presence an essential factor in predicting potential storm activity. Therefore, cumulus clouds not only signify fair weather when small and scattered but also signal the possibility of thunderstorms when they develop into larger formations.