This dramatic example of Kelvin-Helmholtz clouds was taken near the Galapagos Islands last week. The shark-fin-like clouds are the result of two air layers moving past one another. The velocity difference at their interface creates an unstable shear layer that quickly breaks down. The resemblance of the clouds to breaking ocean waves is no coincidence – the wind moving over the ocean’s surface generates waves via the same Kelvin-Helmholtz instability. In the case of the clouds above, the lower layer of air was moist enough to condense, which is why the pattern is visible. Clouds like these don’t tend to last for long because the disturbances that drive the instability grow exponentially quickly, leading to turbulence. (Image credit: C. Miller; via Washington Post; submitted by @jmlinhart)

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