Cumulonimbus clouds develop from Cumulus clouds, specifically, Cumulus Congestus. These clouds are massive, up to 35000 ft. in altitude. The cause of the familiar "anvil" cloud or "thunderhead" shape is the the cloud contains such great convection that its tops reach to the top of the troposphere at which point it levels off and begins to spread out. At some points, the rising air currents can reach speeds of 30 miles per hour, although 20 mph is more common.
The lifecycle of a typical cumulonimbus cloud begins in the early morning with cumulus humilius, the smallest form of cumulus clouds. With favorable conditions (as are often found on a warm summer day), this small cloud progresses through the phases of cumulus mediocris and cumulus congestus. Provided that there is some instability in the atmosphere, the cloud will then develop into the first phase of a thunderhead, cumulonimbus calvus.
Cumulonimbus Calvus clouds, though still the early stage of a thunderstorm, cause a great deal of turbulence because of the major convection currents. It usually contains rain and strong winds, sometimes quite strong, though the rain is usually only in the form of virga in less humid climates. In some cases, before it develops into a full fledged Cumulonimbus Incus cloud, it may develop a cloud phenomena called Pileus.
"Pileus" is the Latin word for "cap". Pileus is what occurs in storms whose updrafts exceed 20 mph. These strong updrafts cause a section of air to rise above the rest of the cloud, and in the cooler air above the other cloud, the moisture in the section of air condenses, forming a "cap." The Pileus cloud is eventually swallowed up by the expanding mass of the cloud. By this time, the cloud has generally acheived the status of Cumulonimbus Incus.
"Incus" is Latin for "anvil." Although typically only 35,000 to 40,000 feet in height, in tropical areas, the height can be as much as 60,000 feet. The cloud has the distinctive anvil shape, that most people recognize from the summer days. The shape is caused by the sheer height of the clouds, reaching into the lower part of the stratosphere. Here, temperatures actually begin to rise from the chilly -70 degree temperatures (farenheit). This stops the updrafts from actually bringing the cloud up further, and has the effect of spreading it along the boundary of the stratosphere. These types of cumulonimbus clouds contain not only heavy rain, but also hail, lightning, tornadoes, and even damaging microbursts.
One formation that can indicate a particularly severe thunderstorm is mammatus clouds. These appear on the underside of the cloud's anvil, and appears to be a large group of bumps hanging from the cloud. Instability within the anvil, caused by the updrafts reaching the stratopshere, leads to air moving downward, creating this formation. These clouds usually indicate severe turbulence, as well as being a precursor to tornadic activity.
And, on a weather map, a cumulonimbus incus cloud indicated on a weather map with a symbol that looks like this:
, ' ' ,
And a thunderstorm is indicated with this symbol:
With an arrow on the bottom of the slanted part, in order to indicate lightning