More commonly known as Wave Clouds or more ominously, Tsunami Clouds, Kelvin-Helmholtz Clouds are a rare and often spectacular weather phenomenon whose rolling, curling, wavy procession across the sky can be both beautiful and frightening.
Catch a Wave!
(image via: Dusky’s Wonders)
Kelvin Helmholtz… wasn’t he that nerdy kid two desks over who later founded PeoplePages, a groundbreaking, game-changing social media platform that made him a mega-billionaire after the company’s IPO? Perhaps, but we’re not here to talk about him.
Kelvin-Helmholtz Clouds, or more precisely clouds which visually manifest Kelvin-Helmholtz Instability, are another thing entirely. Those seeking to boost their geek cred to near-stratospheric heights need only to drolly point up at the wispy waves and declare as smugly as possible, “Kelvin-Helmholtz, meh”.
(image via: BBC)
The phenomenon, which can be expressed in a variety of ways, is named for British mathematical physicist Sir William Thomson Kelvin (below, left) and German physician and physicist Hermann von Helmholtz (below, right).
Kelvin-Helmholtz Instability occurs at the boundary between two different layers moving at different speeds. The shearing effect where the layers meet – known not surprisingly as “velocity sheer” – causes one of the layers to distort the other in a regular pattern: the classic wave shape in repetitive mode. Though the vast majority of us have only seen Kelvin-Helmholtz Instability in the form of waves, it can also be seen (albeit much more rarely) in clouds.
Surf’s Up… in Alabama?
Air and water in conjunction can (and usually do) move at vastly different speeds, which is why ocean waves are the norm instead of the exception. Different layers of air, on the other hand, don’t usually maintain their heterogeneity when they come into contact over a wide area.
(images via: The Weather Channel)
When the two air masses are moving at contrasting speeds regardless of their directions, and even more so when the air layers differ markedly in temperature, conditions are ripe for the formation of Kelvin-Helmholtz Clouds.
(image via: Xinhuanet)
Such was the case on Friday, December 16th, 2011 when observers in and around Birmingham, Alabama, were shocked to see a series of tall, crested waves moving slowly in unison across the morning horizon. Worried callers to local newspapers and weather stations wondered what these strange “Tsunami Clouds” were… and what they meant. The single isolated “wave” in the image above looks like a cobra about to strike, no?
“There is probably a cold layer of air near the ground where the wind speed is probably low. That is why there is a cloud or fog in that layer,” explained Chris Walcek, a meteorologist from the Atmospheric Sciences Research Center at SUNY Albany. “Over this cloudy, cold, slow-moving layer is probably a warmer and faster-moving layer of air.”
Here’s a short video taken as Birmingham was being swamped by a line of magnificent yet barely substantial waves: