As storm season rolls across the Prairies, weather watchers might hear meteorologists talk about a “cap” — and no, it’s not the kind you wear on your head.
Capping, also known as a “capping inversion,” is a layer of warm air higher up in the atmosphere that acts like a lid, preventing thunderstorms from forming, at least for a while.
Think of capping like a lid on a pot of boiling water. The atmosphere underneath is trying to bubble up, but that warm layer above holds it down. Storms that are capped will have a soft, flat top to it.
Under normal conditions, warm, moist air near the ground rises and cools, forming clouds and, eventually, storms. But when there’s a cap, that rising air hits the warmer layer and stops, much like steam trapped under a lid. Storms can’t grow unless something breaks that lid.
That “something” can come in the form of strong daytime heating, a passing cold front, or even upper-level winds that help lift the air high enough to burst through the cap.
When the cap breaks, all that built-up energy can release very suddenly. That’s often when we see explosive thunderstorm development, sometimes even severe weather like hail, wind or tornadoes. When meteorologist talk about a storm busting its cap, the clouds bubble over the soft flat top.
On days when there’s a cap in place, the sky might stay deceptively calm, even if it’s hot and humid. That doesn’t mean storms won’t happen, it just means the timing depends on whether that lid holds or pops. If it holds then storms will stay less severe in nature.
Capping is especially common during summer months in the Prairies, where strong surface heating and dry air can contribute to its formation. Farmers, storm chasers and anyone with outdoor plans may want to keep a close eye on forecasts, as a capped environment can flip from blue skies to intense storms in a matter of hours.
Just because nothing’s happening in the morning doesn’t mean the weather’s clear for the day. When you hear there’s a cap, it means the potential is there, and you need to stay weather-aware.