Cumulonimbus (from Latin cumulus 'swell' and nimbus 'cloud') is a dense, towering, vertical cloud,[1] typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents.

Above the lower portions of the cumulonimbus the water vapor becomes ice crystals, such as snow and graupel, the interaction of which can lead to hail and to lightning formation, respectively.

Cumulonimbus clouds can also occur as dangerous winter storms called "thundersnow" which are associated with particularly intense snowfall rates and with blizzard conditions when accompanied by strong winds that further reduce visibility.

However, cumulonimbus clouds are most common in tropical regions and are also frequent in moist environments during the warm season in the middle latitudes.

Cumulonimbus are a notable hazard to aviation due most importantly to potent wind currents but also reduced visibility and lightning, as well as icing and hail if flying inside the cloud.

Wind shear within and under a cumulonimbus is often intense with downbursts being responsible for many accidents in earlier decades before training and technological detection and nowcasting measures were implemented.

A small form of downburst, the microburst, is the most often implicated in crashes because of their rapid onset and swift changes in wind and aerodynamic conditions over short distances.