From AMS Glossary
A category of cumulus cloud that can distribute air pollutants from the atmospheric boundary layer to the free atmosphere.
These clouds have reached their level of free convection, allowing latent heat released during water-vapor condensation to contribute to the positive buoyancy of the cloudy air. These clouds are usually first produced by thermals, but can eventually decay into passive clouds before disappearing completely. Corresponding morphological species include cumulus mediocris, cumulus congestus, and cumulonimbus.