Today the winds are whipping fine crystals into drifts, and the world has transitioned to its winter white. While I love watching clouds of snow dance and swirl off the roof, a recent question from some Museum visitors has my thoughts drifting back to summer.
“What are those lines of foam you see on a windy lake?” the couple asked. Immediately I was transported to a vista high above a Boundary Waters lake. A stiff breeze tousled the water’s deep blue surface, which was striated with parallel lines of bubbles. The pattern was striking. Then another mental picture replaced that one: the face of my friend Sam, explaining the lines to me more than a decade ago. Unfortunately, that image no longer has a caption. What did Sam say those were called? The visitors and I laughed about the difficulty of pulling up old memories and then moved on.
A couple of weeks later, though, I saw those lines of foam in the mouth of the St. Louis River as I was crossing the Blatnik Bridge in Duluth. Determined to jog my memory, I decided that I should write about the phenomenon, in hopes that explaining it in writing will cement the details in my own mind.
Begin with a large body of water—a lake or an ocean will do. Add some wind, typically blowing between 4.5 and 7 miles per hour. The wind creates shear force on the surface of the water. The top layer experiences the most stress, but as the surface water moves, it exerts a smaller force on the layer below it, which exerts an even smaller force on the layer below it.
As the wind pushes water away from one place, more water rushes up to fill its space, causing an upwelling. Where water comes together, some of it sinks to form a downwelling. These motions converge into tubes of spiraling water that point in the direction of the wind, and circulate perpendicular to the wind. Imagine that a lake’s surface is covered in a layer of huge PVC pipes. The pipes lay lengthwise with the wind and spin perpendicular to it.
Not only do the tubes of water spin perpendicular to the wind, they also spin opposite of their neighbors. If one tube spins clockwise, its neighbors spin counter-clockwise. This creates alternate bands of upwelling and downwelling. As the surface water sinks into an area of downwelling, air bubbles and other buoyant debris get caught in the convergent currents, but don’t follow them down. The downwelling currents become visible as lines of foam and flotsam streaming out parallel to the wind.
Diagram of Langmuir Circulation by Andrés E. Tejada-Martínez from the Journal of Applied Mechanics, 2012.
The internet says that Irving Langmuir was the first to observe this pattern in 1927. That’s a bunch of hooey. Pirates and paddlers, Vikings and voyageurs would certainly have observed this pattern for millennia. It is common, and occurs on oceans, seas, lakes, estuaries, and rivers. Langmuir, however, was the first to publish a paper describing the phenomenon in the journal Science.
An American-born chemist and physicist, Langmuir eventually was recognized with a Nobel Prize in 1932 for his work on surface chemistry. Back in 1927, he simply noticed that a type of free-floating seaweed called sargassum was converging into long windrows roughly parallel to the wind direction, and asked himself “Why?”
After returning home from his sailing trip to the Sargasso Sea in the North Atlantic, Langmuir found his pattern also visible in Lake George, New York. There he described what would become known as “Langmuir circulation” in greater detail, and measured that the downwelling currents move much faster than the upwelling currents.
On the open ocean, the windrows concentrate little critters who can’t overcome the currents; attract predators to eat those prey; and offer a calmer place for birds to rest. The currents also may play an important role in mixing water and nutrients.
While Langmuir currents can form at a wide variety of wind speeds, the sweet spot for visibility is between 4.5 and 7 miles per hour. This provides enough turbulence to create the foam by mixing air into surface scum, but not so much wind that the bubbles get broken or dissipated. Gentle breezes may not initiate the currents. Stronger winds will form the currents, but also whip up whitecaps that mask the pattern.
The lakes up here are currently painted solid white instead of striped. That’s just fine. I’ll tuck this information away until next spring. Hopefully when the next visitor asks about those lines of foam, the term Langmuir circulation will upwell easily from my memory and float merrily along the surface in an organized fashion.
Emily’s second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.
For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new Curiosity Center kids’ exhibit and Pollinator Power annual exhibit are now open! Call us at 715-798-3890 or email email@example.com.