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Wave clouds

A wind shear between a cooler, denser air mass below and a warmer, less-dense air mass above scoops up clouds into shapes that look like waves.

In 1972, Don McLean released a guitar ballad that flowed like a river called "Vincent," which could be heard across the radio dial that summer as a tribute to Vincent van Gogh.

Many radio listeners, like myself, knew it as "Starry Starry Night," the opening line of his song about van Gogh’s famous painting "The Starry Night."

That beautiful painting of the night sky contains "billow, shear-gravity, fluctus or wave clouds" that mimic breaking waves on the beach.

Billow cloud formations are officially known as Kelvin-Helmholtz instabilities, named after two renowned 19th-century physicists, Lord Kelvin and Hermann von Helmholtz.

They were early pioneers in the study of vortex dynamics, or the disturbance caused by fluids of different densities moving at different speeds and directions interacting with each other.

You see, the atmosphere is a fluid of air made of varying air velocities and densities.

Do Kelvin-Helmholtz instabilities form along the Central Coast? They do, but they are uncommon, and here is why:

We experience severe temperature differences in the horizontal plane from one Central Coast location to another; for example, last year on July 25, Diablo Canyon Power Plant reported a temperature of 55 degrees Fahrenheit, while just a few miles away, the PG&E Energy Education Center in Avila Valley was at 108.2 degrees. That's a 53-degree temperature differential.

But we also see large temperature gradients in the vertical direction, a condition called a temperature inversion — when a warmer, less dense air mass covers cooler, denser air at the surface, which we often see as the marine layer.

On infrequent occasions when there is just enough wind shear — not too strong or too weak — on the horizontal plane along the boundary of these two dissimilar air masses, the warmer less-dense winds scoop the heavier marine air or stratus clouds upwards, but gravity pulls the crest of the cloud wave back to earth.

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That condition can produce the evenly spaced rolling vortexes we call Kelvin-Helmholtz instabilities.

Think of locally generated seas and waves. As the wind blows over the ocean, or even a cup of coffee, the moving air along the surface of denser water can carve out small ripples in your mug or large cascading waves on the ocean, like your hand sweeping across a half-filled sink.

Higher in the atmosphere near the jet stream, Kelvin-Helmholtz instabilities are more likely to develop as air masses of different temperatures, densities and velocities that are more likely to interact with each other and produce cirrus or "curl of hair," cumulus or "heap or pile" and stratus or "layer or sheet" clouds and the chance of awe-inspiring waves in the sky.

By the way, “fluctus” is the Latin word for wave, billow or disorder. Unfortunately for photographers, Kelvin-Helmholtz instabilities, like the green flash, tend to be short-lived.

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If you’re flying and see such clouds, it can often indicate severe turbulence.

Vincent van Gogh may have seen those clouds while in the South of France, which may have his inspiration for “The Starry Night.”

Van Gogh died about a year after completing “The Starry Night” at only 37 years of age. Over the last two years of his life, he completed more than 800 oil paintings.

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Be prepared for emergencies. Spend time this weekend getting your emergency plan and supply kit together.

Make sure you and your family are prepared for earthquakes and other natural disasters and dangerous events. Visit www.pge.com/beprepared for information.

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John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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