What is the ‘cap’ in weather? How this atmospheric lid affects severe storms
Meteorologists look at lots of data when forecasting severe weather. One of them is the strength of the cap, which can have a big impact on how the storms develop.
Science behind 'The Cap': Warm layer of air keeps lid on atmosphere during severe weather
FOX Weather's Ian Oliver explains the science behind "The Cap," a warm layer of air that keeps a lid on the atmosphere during severe weather.
Before thunderstorms produce severe weather comes the forecast warning of such weather.
Meteorologists spend hours analyzing copious amounts of data – from current temperatures and wind to computer models that simulate future weather conditions – to determine how the storms will likely play out.
One of the many parameters they evaluate while building their forecast is the strength of the "cap."
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A cumulonimbus hangs in the evening sky over the outskirts of Leipzig.
(Jan Woitas/dpa-Zentralbild/dpa / Getty Images)
What is the cap?
Most of the time, the troposphere – the layer of the atmosphere that we live and experience weather in – gets colder with height. This allows air that is warmed at the surface to rise, cool, condense and create clouds.
That rising air needs to be uninhibited for severe weather to occur. The cap, which is a layer of warm air sitting between 3,000 and 7,000 feet above the surface, prevents that from happening.
"Think of it like steam rising from a boiling pot of water," said FOX Weather senior data specialist Shane Brown. "If you have the lid on tight, steam can't escape the pot and rise up. But once you take the lid off, all the collected steam rushes upward."
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The cap can make or break a forecast
Any meteorologist who has been forecasting severe weather for a while can recount a time when the cap has affected their forecast. The stronger the cap, the harder it is for thunderstorms to develop.
"The cap is extremely critical in a severe weather forecast," said FOX Weather senior meteorologist Jordan Overton.
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A battle is waged between the rising air and the cap. If the air cannot overcome the cap, the risk of severe weather lowers. However, if the air can punch through the cap, storms develop. Meteorologists have to decide how likely the air will win when building a forecast.
Tornadoes are just one of the types of severe weather that can develop with storms that develop in this type of environment. Large hail and damaging wind are also possible.