The distinction between a large Cumulus (congestus) or Cumulonimbus (calvus) cloud can sometimes be very difficult when viewing only its appearance. However if thunder is heard (or lightning is seen) the cloud is by convention called Cumulonimbus (Cb). This cloud has the largest vertical extent: its base mostly found between 600-1500 m and its tops usually up to the tropopause level which varies with the season, in the warm season this may reach 16 km. Besides lightning and thunder, Cb clouds are always accompanied by precipitation, in most cases reaching the Earth's surface as so called "showers".

Showing the intra-cloud lightning of a Cb calvus, the picture was taken during twilight period when an active low topped supercell thunderstorm passed by.