Cumulonimbus (Cb) clouds, in which Thunderstorms are found, form when three conditions are met:
- There must be a deep layer of unstable air.
- The air must be warm and moist.
- A trigger mechanism must cause the warm moist air to rise:
- Heating of the layer of air close to the surface.
- Rising ground forcing the air upwards (orographic uplift).
- A front forcing the air upwards.
A Cumulonimbus Cloud develops in three distinct phases:
- Building Phase. A pocket of warm air begins to ascend as a result of one of the triggers mentioned above. As the moist air rises, it becomes saturated, cloud forms, and the latent heat released as the moisture condenses further warms the air and it continues to rise. The air within the cloud is warmer than the air outside it and more air is drawn into the cloud from the base as well as the sides. The cloud grows in height rapidly, faster than many aircraft can climb, extending from the surface to a great height, sometimes as far as the Tropopause. as the temperature of the rising air drops below freezing point, the water droplets become super cooled and join together to become larger and larger.
- Mature Phase. As the top of the cloud reaches great heights, precipitation begins to fall. The falling rain, snow and/or ice (Hail) cools the surrounding air creating downdrafts. The friction between ice particles descending through the cloud, and ice particles being carried aloft by the updrafts, creates a static charge in the cloud with the top of the cloud having a positive charge and the bottom of the cloud having a negative charge. Eventually the difference in potential is so great that powerful electrical discharges (Lightning) occur, accompanied by Thunder. The top of the cloud begins to flatten out and Cirrus like cloud, consisting of ice crystals, spreads out creating a distinctive anvil shape.
- Dissipation Phase. The cooling effect of the downdrafts on the air beneath the cloud reduces the strength of the updrafts and until the the updrafts eventually stop and the lower cloud begins to dissipate. the upper cloud will then linger for some time afterwards.
The active cycle of the Cb cell lasts little more than an hour but many Thunderstorms contain several active Cb cells in various stages of development meaning that a storm can last several hours and extend over a large area. The active cells are often embedded in a larger cloud mass consisting of the remains of decayed cells as well as other cloud types at various levels. This can make the active cells very difficult to detect visually and appropriate use of Radar is required to safely avoid active weather.