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Understanding CAPE, the fuel that powers a thunderstorm

Monday, August 15th 2022, 11:07 am - A mighty thunderstorm requires a powerful updraft in order to rage across the horizon. Here’s a look at how we measure a storm’s potential using instability.

Nothing shows off our atmosphere’s awe-inspiring power more than a raging thunderstorm. But even the mightiest storm that towers to the edge of the sky starts as the most unassuming cloud. It takes an immense amount of rising air to create these fierce storms, and meteorologists measure the instability that fuels a growing storm using CAPE.

CAPE MEASURES THE AIR’S INSTABILITY

CAPE, an acronym for Convective Available Potential Energy, is a measurement of the amount of instability present in the atmosphere.

CAPE Definition


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We measure CAPE using joules per kilogram (j/kg), the calculation of which involves some equations and fun charts. The end result, though, shows us just how unstable the air is at any given point.

CAPEGeneric

Higher CAPE values indicate higher instability, with the potential for stronger storms to develop. Maps showing CAPE (sometimes called “thunderstorm energy”) show higher CAPE values using warmer colours.

WHAT SPARKS A STORM

A storm needs moisture, instability, and a trigger in order to form. Triggers are usually boundaries like cold fronts, lake breezes, troughs, and even the “urban heat island” effect of hot city centres.

While triggers set a storm into motion, instability is the bread and butter of a thunderstorm’s existence.

On a typical sunny day, temperatures are warm near the surface and steadily cool with altitude into the atmosphere. A trigger such as a lake breeze can nudge that low-lying warm air skyward, allowing it to rise through the cooler air thousands of metres above the surface.


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The temperature difference between air at the surface and air high in the atmosphere determines how much instability is present.

A sharper temperature gradient between lower altitudes and higher altitudes creates more instability, allowing air to rise faster and faster as it climbs several kilometres into the atmosphere.

Weather models and weather balloon observations give meteorologists a great idea of how much instability is available for thunderstorms to work with, which can tell forecasters how strong those storms could grow.

As a general rule of thumb, lower CAPE values result in showers and weak thunderstorms, while very high CAPE values can support supercells that generate large hail, destructive winds, and even tornadoes.

INSTABILITY DOESN’T ALWAYS RESULT IN AN EXPLOSIVE STORM

There are some caveats, of course.

“Potential” is the key word in “Convective Available Potential Energy.” We often see unstable days with mammoth CAPE values fail to produce any thunderstorms at all. There are a few culprits behind a high-CAPE day that fails to deliver any raucous thunderstorms.

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Capping frequently thwarts any storms that attempt to form. A cap is a temperature inversion, or a layer of warmer air sandwiched between layers of colder air aloft. This warm air can act like a ceiling—or a cap—that stops rising air in its tracks.

Vivacious thunderstorms can result from rising air breaking through a weak cap. However, a strong cap can turn a once-promising thunderstorm potential into little more than a partly cloudy day.

Caps are often the reason you’ll hear meteorologists talk about a “conditional” risk for thunderstorms. Sometimes the trigger is enough to break through the cap and allow storms to blossom, but sometimes there’s just enough of a lid on the atmosphere to keep down even the most unstable environment.

Thumbnail courtesy of Unsplash.

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