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Jul. 29, 2015

Picture of the Week: Kelvin-Helmholtz Clouds

by Chau Tu

Click to enlarge images
Maybe you’ve seen tsunami-shaped clouds like these, rolling through Earth’s atmosphere. These repeating curls result from a flow of air hitting a layer of stagnant or slower-moving air below it. The turbulence is known as Kelvin-Helmholtz instability, named after its discoverers, William Thomson—better known as Lord Kelvin—and Hermann von Helmholtz.
 
“Basically, if you have two fluids or two gases that are streaming at different velocities, then at the interface between the two, that interface is unstable,” explains Joachim Raeder, a physics professor at the University of New Hampshire. “Waves can form, and that’s what we call a Kelvin-Helmholtz wave.”
 
The clouds in this photograph are especially eye-catching because they’re “breaking,” just as ocean waves do. But Kelvin-Helmholtz clouds, as they’re known, can also appear as bumps or rolls. “They start out as very small fluctuations,” says Raeder, “and then the altitude grows, and eventually they turn over.” To spot them, look for a regular and repeating pattern in the clouds. 
 
Kelvin-Helmholtz instability occurs throughout nature—even in outer space, such as in the atmospheres of Jupiter and Saturn. Raeder studies Earth’s own magnetosphere, and recently found that solar winds blowing plasma past our planet cause more Kelvin-Helmholtz waves in the magnetosphere than previously thought. He and one of his graduate students published their findings in Nature Communications in May.
 
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“We were able to show for the first time really how often they occur [in the magnetosphere]. People have seen them before, but it was really thought they were very rare and that they wouldn’t happen very often,” says Raeder. “We found out that they are present about 20 percent of the time.”
 
Usually, the magnetosphere should be well-shielded from the plasma of the solar wind, because the plasma can’t easily penetrate the magnetic field, Raeder explains. But Kelvin-Helmholtz waves could cause the plasma to mix with the magnetosphere, as well as rattle it, resulting in ultra-low frequency waves that can energize the Van Allen radiation belts (two swaths of space around our planet consisting of fast-moving particles). And a distrubance in the radiation belts can disrupt satellites, according to Raeder.  
 
More investigation into Kelvin-Helmholtz waves in near-Earth space could help us learn how to better predict certain space weather events. “Fifty years ago, there were very few people who were interested in space weather,” says Raeder. These days, however, we’re paying more attention.  
About Chau Tu

Chau is SciFri's web producer. She spends a lot of her time drinking coffee, seeking out street art, listening to music, and defending Los Angeles. Follow her @chaubtu

The views expressed are those of the author and are not necessarily those of Science Friday.

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