|The Provincetown breeze forms a series of rolling waves in the clouds as the sun sets over the rooftops.|
I happened into a crowd that had gathered spontaneously in front of the Aquarium Marketplace, at the beginning of Provincetown's West End, where people were looking up over the rooftops toward the western sky, exclaiming over the sight they were seeing as they fumbled for their cameras. The wind was twirling the clouds into neat little curlicues as it blew them along the horizon. I heard someone say there was a name for this phenomenon, but no one seemed to know what it was.
You might think these clouds have been "photoshopped" into the picture, but there is no trickery here. This is simply Mother Nature doing what she does… turning the natural elements around us into stunning, elegant little displays that magically appear wherever you go in Provincetown.
UPDATE: My thanks to a reader who e-mailed me a name for these clouds, enabling me to look up some information on them. It turns out this sort of cloud formation sometimes results when there is more than one layer of air above us, moving at different speeds on a windy day. A thinner layer of air might move more quickly over a denser, heavier layer, rolling a bank of clouds into a shape that resembles a series of cresting waves.
These cloud formations are known by a number of names around the world, and are often called "Kelvin-Helmoltz" clouds or billows. They were named for Lord William Thomas Kelvin, a Scottish physicist and mathematician, and for Hermann von Helmholtz, a German physicist, physician and philosopher (say that five times really fast,) both of whom were born in the 1820s.
"Shear-gravity clouds" is another monicker for these whimsical swirls of condensed water vapor that most often indicate instability in the atmosphere, and are a predictor of likely turbulence for airplanes. And they make a dandy bit of sculpture in Mother Nature's sky.