What are cumulonimbus clouds?

Cumulonimbus Clouds (thunderclouds)
Cumulonimbus is a type of cloud that is tall, dense, and usually dark. These clouds form thunderstorms, and can create severe weather conditions, such as heavy rain, damaging winds, hail, lightning, and tornadoes. A cumulonimbus cloud that progresses to an intense, long-lived form is known as a "supercell". Groups of these storms are associated with damaging straight-line winds, or derechos.
The clouds can form alone, in clusters, or along a cold front in a squall line. During the spring and summer, cumulonimbus clouds are more likely to form in the afternoon, due to the heating of the Earth's surface. However, they can also form along a cold front when the warm buoyant air is forced upward by the heavier cold air mass that cuts under the warmer air like a wedge. This can happen at any time of the year, as demonstrated by thunderstorms that happen in conjunction with snowstorms in the winter. Cumulonimbus clouds are also a constituent of tropical cyclones.

Formation
Three ingredients are needed for the formation of a perfect cumulonimbus cloud:

• Plenty of moisture.
• A mass of warm unstable air.
• A source of energy to lift the warm, moist air mass rapidly upward.

Appearance
Cumulonimbus clouds usually form from cumulus clouds at a much lower height, thus making them, like cumulus clouds, grow vertically instead of horizontally. This gives the cumulonimbus its mushroom shape. The base of a cumulonimbus can be several miles across, and it can be tall enough to occupy middle as well as low altitudes : though formed at an altitude of about 3,000 to 4,000 meters (10,000 to 12,000 feet), its peak can reach up to 23,000 meters (75,000 feet) in extreme cases. Typically, it peaks at a much lower height.
Cumulonimbus are also characterized by a flat, anvil-like top (the anvil dome), caused by straight-line winds at the higher altitudes (which shear off the top of the cloud), as well as by an inversion over the thunderstorm caused by rising temperatures above the tropopause. This anvil shape can precede the main cloud structure for many miles, causing anvil lightning.