Seen from an aeroplane at cruise altitude the clouds below can often look a bit like the ocean - like the view I snapped here while on a flight to Sicily for this story last week - but it's rare indeed that the clouds look like ocean waves from the ground.
Yet, that's just what folks in Birmingham, Alabama were treated to recently when the rare atmospheric phenomenon pictured above was visited upon them, causing quite a stir (making waves even) in the news media.
Called a Kelvin-Helmholtz instability, the bizarre effect results in what looks like slow-moving waves traversing the skies. It's caused when the wind is moving at very different speeds at different altitudes - so-called velocity shear. It means that upper parts of cloud strata are moving faster than those nearer the Earth's surface (they don't have to be on the ground) with the resulting unstable motion producing a wave-like effect. The gassy stratification effect is not confined to the Earth however - here it is for instance in action on Saturn.
If you're a fan of cloud formations, by the way, you may enjoy an extended gander at this: the online gallery of the Cloud Appreciation Society.
http://www.newscientist.com
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