The Bird in the Waterfall by Jerry Dennis and Glenn Wolff



W A V E S
How Waves Begin
Jerry Dennis & Glenn Wolff

How waves begin is no mystery. Children in bathtubs learn quickly that when they push water with their hands it gives way, absorbing the push and sending it rolling across the surface. Drop a pebble and the water yields to it, the surface opening a hole for a moment then springing back elastically, lifting a column of splash that rises, pauses, and plummets, sending waves outward like rings from a bull's-eye. In the complex world of wave dynamics, waves begin simply as a disturbance of the surface and a transfer of energy. And energy of one kind or another is forever disturbing water.

Watch a pond or lake and you can see waves being born. On mornings of mirror calm, the water is kept smooth by surface tension -- the clinging nature of water molecules that makes it possible to float a needle on a glass of tap water. Even after the first breeze springs up the lake remains unruffled. But if the breeze continues it etches the surface with tiny waves known as ripples or capillary waves. These smallest of all water waves are also the most abundant, present not only on the ruffled surface of nearly calm water but across the slopes of large waves. They are produced when wind or raindrops or some other minor disturbing force stretches the surface film, causing it to wrinkle into waves measuring less than 1.5 centimeters from crest to crest. Like ants that can fall from ten-story buildings without harm, these half-inch waves are too small to be bothered much by gravity. Gravity can subdue larger waves to calmness, but ripples are flattened only by the cohesive force of the water molecules at the surface, the same capillary action that drives water up narrow glass tubing.

As a breeze touches a calm lake, areas of ripples appear, often in uneven, roundish shapes mariners have for centuries called "cat's- paws."

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