Weathercatch: Why the great ‘Cloud 9’ filled our region’s skies late last week

On two days last week, the king of clouds reigned the skies above parts of the Inland Northwest, evoking an eerie beauty and spewing impressive thunderstorms.

Cumulonimbus clouds, also known as thunderheads, unleashed everything from downpours and golf ball-sized hail to thunderous booms and spectacular lightning. Hardest-hit areas included east Spokane County and portions of North Idaho extending as far south as Cottonwood. Hailstones reaching 1.5 inches in diameter fell in the community of Plaza, 27 miles south of Spokane, according to the National Weather Service’s Storm Prediction Center.

The most notable storms occurred last Thursday, although plenty of majestic cloud formations still hovered on Friday. Besides producing bursts of chaotic weather, the billowing clouds were the subject of numerous photographs posted by social media users.

Cumulonimbus can form alone, in clusters or as a wall of clouds and are capable of producing huge downpours, hailstones, lightning and thunder. They can also spawn tornadoes.

They can grow so large that they resemble an explosion, extending more than 6 miles into the sky and harnessing as much energy as 10 Hiroshima-sized bombs, according to Gavin Pretor-Pinney, author of “The Cloudspotter’s Guide” and founder of the Cloud Appreciation Society.

Whether a towering white cauliflower shape or a sprawling canopy of deep gray, cumulonimbus clouds commonly appear in areas with high humidity levels such as the tropics, the Great Plains and southeastern states.

But our region is dry, not humid. So why were the clouds so prevalent here late last week?

Because three weather conditions came together to form them. Upper-level winds carried monsoonal moisture from the Desert Southwest into our region. That moisture met an unstable air mass created by an upper-level trough off the Pacific Northwest coast. Finally, our hot temperatures generated an upward force of rising air and condensing water vapor high in the atmosphere, where the clouds kept growing, layer upon layer.

One last thing about this great cloud type: You know the happy idiom about being “on cloud nine”? In September 1896, cumulonimbus was classified as Cloud 9 in the International Cloud Atlas, the first global reference book for identifying clouds, according to the Cloud Appreciation Society website. To be on cloud nine is to be floating on one of the tallest clouds of all, it states.

Floating on a cumulonimbus is fine, but you’ll want to avoid getting caught beneath it when the cloud unloads its fury.

Nic Loyd is a meteorologist in Washington state. Linda Weiford is a writer in Moscow, Idaho, who’s also a weather geek. Contact: ldweiford@gmail.com.