Cumulonimbus clouds are a type of cumulus cloud associated with thunder storms and heavy precipitation. They are also a variation of nimbus or precipitation bearing clouds. They are formed beneath 20,000 ft. and are relatively close to the ground. This is why they have so much moisture. Cumulonimbus clouds are also known as thunderheads due to their unique mushroom shape.
These clouds often produce lightning in their heart. This is caused by ionized droplets in the clouds rubbing against each other. The static charge built up create lightning. Cumulonimbus clouds need warm and humid conditions to form. This gives them the moist warm updrafts needed to produce them. In some instances a Thunderhead with enough energy can develop into a supercell which can produce strong winds, flash floods, and a lot of lightning. Some can even become tornadoes given the right conditions.
Despite the heavy rainfall these clouds produce, the precipitation normally just lasts for around 20 minutes. This is because the clouds require not only a lot of energy to form but also expend a lot energy. However, there are exceptions to the rule. There are also dry thunderstorms which are cumulonimbus clouds whose precipitation does not touch the ground. This type is common in the Western United States where the land is more arid. It is often cited as a cause of wild fires.
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An overlooked result of Cumulonimbus clouds are flash floods. This was proven recently in Atlanta, Georgia area of the United States. The state had gone through a two year drought and water supplies such as creeks and rivers were low. However the fall season brought with it the end of the drought and a lot of Thunderstorms. Even though Atlanta is not near any major waterways, the resulting flash floods were on a scale seen only with areas near major rivers with wide flood plains. This demonstrates how much precipitation that Cumulonimbus clouds can produce even in a short amount of time.
Cumulonimbus clouds are a perfect example of how difference in altitude can affect the formation of clouds. Cumulonimbus clouds form in the lower part of the troposphere, the layer of the atmosphere closest to the surface of the Earth. This region due to evaporation and the greenhouse effect produces alot of the warm updrafts that make creation of cumulus and cumulonimbus clouds possible. The turbulence created by the friction between air and the surface of the Earth combined with stored heat from the sun helps to drive the majority of weather.
If you enjoyed this article there are others on Universe Today that you will be sure to enjoy. There is a great article on cloud types and another on the composition of the Earth’s atmosphere.
There are also great resources online. USA today has a great article on cloud types. You can also check out the cloud types website for the University of Illinois.
You can also check out Astronomy Cast. Episode 151 is about atmospheres.