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WEATHER U: Hooked on clouds

Jessica Herrett
Jessica Herrett - Contributed

Clouds are so much fun. 

Over the years I’ve received photos of clouds that look like people, dragons, angels, trains, you name it. Last week I came across one of the most unique cloud photos I’ve ever seen. Jennifer posted a photo that her friend Jessica Herrett took at sunset near Springhill, N.S. These clouds that look like rolling ocean waves are known as Kelvin-Helmholz waves and are quite rare.  

Also known as billow clouds, they form when two air currents of varying speeds meet in the atmosphere, and they make a stunning sight. You also need a moist, cloudy layer of air near the ground which isn’t moving much and another layer of air just above it which is moving rapidly. The resulting turbulence near the boundary between the two can create some pretty interesting cloud formations, like the ones captured by Jessica.

As with many distinct cloud formations, billow clouds can tell us something about atmospheric conditions. The presence of this unique cloud type indicates instability in air currents, which may not affect us on the ground. It is, however, a concern for pilots as it points to an area of turbulence.

When and where to look for Kelvin-Helmholtz?

Your best chance for observing billow clouds is on a windy day because they form where two horizontal winds meet. This is also when temperature inversions – warmer air on top of cooler air – occur because the two layers have different densities.

The upper layers of air move at very high speeds while the lower layers are rather slow. The faster air picks up the top layer of the cloud it’s passing through and forms these wave-like rolls or hooks in the sky. The upper layer is typically drier because of its velocity and warmth, which causes evaporation and explains why the clouds disappear so quickly.

Kelvin-Helmholtz instability is not found on Earth alone; scientists have observed formations on Jupiter as well as Saturn and in the sun’s corona.

By the way, the name is a combination from Lord Kelvin, a Scottish baron, who along with a German physicist Hermann Helmholtz, came up with an explanation for these wonderful waves. 



Cindy Day is the chief meteorologist for SaltWire Network

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