waves -- 6/29/16

Today's selection from -- Barbarian Days by William Finnegan. How waves are formed.

"Here is how waves form. A storm out at sea churns the surface, creating chop -- smaller and then larger disorganized wavelets, which amalgamate, with enough wind, into heavy seas. What we are wait­ing for on distant coasts is the energy that escapes from the storm, radiat­ing outward into calmer waters in the form of wave trains -- groups of waves, increasingly organized, that travel together. Each wave is a column of orbiting energy, most of it below the surface. All the wave trains pro­duced by a storm constitute what surfers call a swell. The swell can travel thousands of miles. The more powerful the storm, the farther the swell may travel. As it travels, it becomes more organized -- the distance be­tween each wave in a train, known as the interval, increases. In a long­ interval train, the orbiting energy in each wave may extend more than a thousand feet beneath the ocean surface. Such a train can pass easily through surface resistance like chop or other smaller, shallower swells that it crosses or overtakes.

Water particle motion of a deep water wave.

"As waves from a swell approach a shoreline, their lower ends begin to feel the sea bottom. Wave trains become sets -- groups of waves that are larger and longer-interval than their more locally generated cousins. The approaching waves refract (bend) in response to the shape of the sea bot­tom. The visible part of the wave grows, its orbiting energy pushed higher above the surface. The resistance offered by the sea bottom increases as the water gets shallower, slowing the progress of the lowest part of the wave. The wave above the surface steepens. Finally, it becomes unstable and prepares to topple forward -- to break. The rule of thumb is that it will break when the wave height reaches 80 percent of the water's depth­ an eight-foot wave will break in ten feet of water. But many factors, some of them endlessly subtle -- wind, bottom contour, swell angle, currents determine exactly where and how each wave breaks. [S]urfers ... just hope that it has 3, catchable moment (a takeoff point), and a ridable face, and that it doesn't break all at once (close out) but instead breaks gradu­ally, successively (peels), in one direction or the other (left or right), allow­ing us to travel roughly parallel to the shore, riding the face, for a while, in that spot, in that moment, just before it breaks."


William Finnegan


Barbarian Days: A Surfing Life


Penguin Books


Copyright 2015 by William Finnegan


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