In Juneau, years can pass without a thunderstorm. Why are they so rare?

Lightning strikes over Juneau, June 17, 2013. (Photo by Mikko Wilson)

Last Tuesday, Juneau climatologist Rick Fritsch was getting ready for his afternoon shift when the sky darkened suddenly, and the wind picked up. 

“It was rocking the trees, something furious. And my birdhouses were flying, you know, seven ways till Sunday,” Fritsch said. “So I kind of knew that something was up.”

A few minutes later, when he got to the National Weather Service office, a thunderstorm was already in full force. 

“And the office was just buzzing alive with activity,” he said. 

It was an exciting day for Juneau’s meteorologists because thunderstorms rarely happen here. On average, they only happen once about every two years. But on June 4, all the right ingredients came together. 

Earlier that day, Fritsch’s colleagues had been tracking an unseasonable cold front that was hovering high in the atmosphere just south of Juneau. Meanwhile, on the ground, unsuspecting Juneauites were enjoying a brief moment of sunshine and warmth. 

Now, imagine boiling a pot of water. Cooking up a thunderstorm works much the same way. 

“When it’s a sunny day, and the ground heats up, that’s the element on the stove that’s heating the pot of water from below,” Fritsch said. 

As the water heats up, bubbles of water rise to the top of the pot, while cooler water sinks from the surface. It’s a process called convection. Air does the same thing. 

On warm, sunny days, bubbles of hot air form near the ground and start rising. As they climb higher in the atmosphere, temperatures cool. Especially when there’s a cold front like the one we had last week.

When the air bubbles cool off, water particles inside them condense and form clouds. This process happens over and over again. Each newly formed cloud pushes the one that came before higher and higher into the atmosphere. 

“And they keep going and going and going,” Fritsch said. “That’s how you build a thunderstorm.”

The clouds stack and combine until, eventually, they form towering, dark gray cumulonimbus clouds. 

These storm clouds are powerful. When they burst, they release a deluge of rain, gusty wind, occasional hail storms — and of course, thunder and lightning.

But the chaos is usually brief. On June 4, it lasted just about 30 minutes. That’s because a thunderstorm is its own worst enemy. 

“Basically it kills itself because then it rains out, and it cools the ground underneath it,” Fritsch said. “You take away the heat source. You take away the fuel, if you will, for the thunderstorm.”

And in Juneau, thunderstorms have a couple of other adversaries. 

The first is the ice field. It stands between Juneau and British Columbia to the east, where hot stretches in the summer create massive thunderstorms. Lightning from these storms has sparked some of the Canadian wildfires that have become so prevalent.

But these powerful storms usually can’t make it to Juneau. 

“These thunderstorms grow and grow and grow and they get really exciting. And meteorologists get all tripped out about it, like, ‘Oh, this is really cool,’” Fritsch said. “And then it hits the icefield, you take away the heat, the fuel source, and it just peters out.” 

The storm deflates. 

“All of a sudden it just turns into a rainstorm, a rain shower,” Fritsch said. 

The other adversary, Fritsch said, is Juneau’s northern latitude. 

“We live at 58 (degrees) north,” he said. “And physically, the atmosphere is thinner the closer you go to the pole.”

At lower latitudes, there is a thicker layer of atmosphere separating the surface of the Earth from space. Which means thunder clouds have plenty of room to grow. Near the equator, they can get up to 50,000 feet, or nearly ten miles, tall. 

In Southeast Alaska, the troposphere — that’s the part of the atmosphere where most weather forms — is only 15,000 to 20,000 feet thick.

“There’s not as much potential or ability for the thunderstorms to really grow to the point where they can manifest, like something in Kansas or Iowa,” Fritsch said. 

So even when thunderstorms do form, they’re typically milder than they might be in the Lower 48. In fact, Southeast Alaska has only had one severe thunderstorm warning in Fritsch’s 18 years forecasting here — in Misty Fjords, back in 2019.

“The only time I’ve ever seen it. As a matter of fact, the only time since our weather station has been in existence,” Fritsch. “And that goes back to the 1890s.”

So if you missed last week’s light show, you might have to wait a while for the next one.

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