Thunderstorms are generally associated with towering clouds known as cumulonimbus. There are two required conditions for the formation of a thunderstorm:
- unstable air, and
- a mechanism for causing air to rise
Air is said to be unstable when a 'parcel' of air continues to rise of its own accord after being given an upward impetus. This instability is the result of a rapid fall of temperature with height as well as a large amount of moisture. The mechanism may be provided by a sufficiently warm surface; the air near the surface being forced to rise over higher ground; or instability in the large-scale ascent within a front.
For example, on a summer's day, the land surface is warmed by the sun, and as the air just above the surface becomes warmer it starts to rise. As it rises it cools, and, if cooled sufficiently, cumulus clouds form at the condensation level. These small, white puffy clouds grow larger and larger as the temperature of the ground increases, causing more warm air to rise.
After a while, the top of the cloud turns to ice (usually below a temperature of -20°C) and streams away in the winds at the level of the cloud top, giving the characteristic anvil shape of a cumulonimbus cloud.
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