WX 101: The Cap

Weather 101: All

“The Cap or Lid” is a layer of warm air above the ground, usually several thousand feet high, which suppresses or delays the development of storms. As air rises into this layer it becomes cooler than the surrounding atmosphere, which prevents its ability to rise further and produce thunderstorms. The cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorms can develop rather quickly.

The cap is also an important ingredient in most severe weather outbreaks. This is because it separates the warm/moist air below it from the cooler/drier air above it. With the cap in place, air beneath it can continue to warm and moisten, increasing the amount of overall instability. You can think of this scenario as a pot of boiling water with the lid on it. Once the lid is removed, the steam underneath it will explode upwards out of the pot. The cap has a similar effect on our atmosphere, which is why it can lead to explosive severe weather. Without the cap, heat and moisture isn’t able to build at the surface, therefore storms could be limited in strength.

**There could be times when the cap is too strong or warm for rising air parcels, this could prevent storms from forming at all.**

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