(Image source: WBMA )

 

BY STEVEN SPARKMAN

 

Weird weather had an entire city staring at clouds during their morning commute.

 

Last Friday, giant waves rolled past Birmingham, Alabama. But it’s no tsunami -- these are clouds. This rare event is a picture-perfect example of what meteorologists call a Kelvin-Helmholtz instability. (Image source: WBMA )

 

Videos and photos of the curious clouds went viral this week. A meteorologist for KTTV explains what we’re seeing.

 

“This type of instability is caused by velocity shear, which is a difference in wind velocity across two different layers of air, causing these wave-like formations. Another example: two layers, one being air, one being water, wind blowing over calm water. Small ripples form on the smooth water surface, and will amplify into small waves and white caps.”

 

So you’ve seen the same effect any time you’ve watched waves form over a windy lake. But it’s rare to see it in clouds along the ground, especially near a big city. A writer for Life’s Little Mysteries explains why.

 

“Most of the time, the difference in wind speed and temperature between two layers of the atmosphere is small, and so the fast-moving air on top ‘simply slides smoothly over the slower-moving air’ … At the other extreme, if the wind-speed difference is too large, the interface between the two layers breaks down into random turbulence. Kelvin-Helmholtz waves form when the difference in the temperature and wind speed of the two layers hits a sweet spot.”

 

These instabilities are part of the physics of fluid dynamics, and scientists have known about them since the 1800s. So you probably won’t be surprised to learn they’ve seen even more extreme examples.

 

Some of them are out of this world, like the ripples around Jupiter’s Great Red Spot . The waves have also been seen in the gas layers of Saturn and even in the atmosphere of the Sun . (Image source: NASA)

 

Since you can’t let rare weather go to waste, an NPR blogger engages in the time-honored tradition of seeing shapes in the clouds.

 

“They sure look like either some amazingly huge waves or a line of Loch Ness monsters marching across the horizon.”