Heavy and dense cloud, with considerable vertical extent, in the form of a mountain or huge
towers.
Usually part of its upper portion is smooth, or fibrous or striated and nearly always
flattened. This part often spreads out in the shape of an anvil or huge plume.
Under the base of the cloud it is usually very dark, with frequent ragged clouds and
precipitation, sometimes in the form of virga.
Cumulonimbus is composed of water droplets and in the upper portion, ice crystals. It can
contain large raindrops , snowflakes, snow pellets, ice pellets, and hail.
Cumulonimbus can have a dark, menacing aspect when it produces thunder and lightning, intense
showers of rain, snow, or hail and by squalls or hanging protuberances (cumulonimbus mammatus.
A cumulonimbus cloud distinguishes itself from a cumulus cloud if at least part of its upper
portion has lost sharpness of outlines or appears fibrous or striated. If you cannot decide
if a cloud is a cumulonimbus or a well-developed cumulus, call it cumulonimbus if it is accompanied
by lightening, thunder or hail.
Timelapse of a thunderstorm forming. There is a beautiful CB in the middle of this
video.
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