On the coast of California wave recorders have detected swell from as great a distance, for some of the surf that breaks on that coast in summer is born in the west-wind belt of the Southern Hemisphere. The Cornwall recorders and those in California, as well as a few on the east coast of America, have been in use since the end of the Second World War. These experiments have several objects, among them the development of a new kind of weather forecasting. In the countries bordering the North Atlantic there is no practical need to turn to the waves for weather information because meteorological stations are numerous and strategically placed. The areas in which the wave recorders are presently used have served rather as a testing laboratory to develop the method. It will soon be ready for use in other parts of the world, for which there are no meteorological data except those the waves bring. Especially in the Southern Hemisphere, many coasts are washed by waves that have come from lonely, unvisited parts of the ocean, seldom crossed by vessels, off the normal routes of the air lines. Storms may develop in these remote places, unobserved, and sweep down suddenly on mid-ocean islands or exposed coasts. Over the millions of years the waves, running ahead of the storms, have been crying a warning, but only now are we learning to read their language. Or only now, at least, are we learning to do so scientifically. There is a basis in folklore for these modern achievements in wave research. To generations of Pacific Island natives, a certain kind of swell has signaled the approach of a typhoon. And centuries ago, when peasants on the lonely shores of Ireland saw the long swells that herald a storm rolling in upon their coasts, they shuddered and talked of death waves.
Now our study of waves has come of age, and on all sides we can find evidence that modern man is turning to the waves of the sea for practical purposes. Off the Fishing Pier at Long Branch, New Jersey, at the end of a quarter-mile pipeline on the bed of the ocean, a wave-recording instrument silently and continuously takes note of the arrival of waves from the open Atlantic. By electric impulses transmitted through the pipeline, the height of each wave and the interval between succeeding crests are transmitted to a shore station and automatically recorded as a graph. These records are carefully studied by the Beach Erosion Board of the Army Corps of Engineers, which is concerned about the rate of erosion along the New Jersey coast.
Off the coast of Africa, high-flying planes recently took a series of overlapping photographs of the surf and the areas immediately offshore. From these photographs, trained men determined the speed of the waves moving in toward the shore. Then they applied a mathematical formula that relates the behavior of waves advancing into shallow water to the depths beneath them. All this information provided the British government with usable surveys of the depths off the coast of an almost inaccessible part of its empire, which could have been sounded in the ordinary way only at great expense and with endless difficulty. Like much of our new knowledge of waves, this practical method was born of wartime necessity.
Forecasts of the state of the sea and particularly the height of the surf became regular preliminaries to invasion in the Second World War, especially on the exposed beaches of Europe and Africa. But application of theory to practical conditions was at first difficult; so was the interpretation of the actual effect of any predicted height of surf or roughness of sea surface on the transfer of men and supplies between boats or from boats to beaches. This first attempt at practical military oceanography was, as one naval officer put it, a ‘most frightening lesson’ concerning the ‘almost desperate lack of basic information on the fundamentals of the nature of the sea.’
As long as there has been an earth, the moving masses of air that we call winds have swept back and forth across its surface. And as long as there has been an ocean, its waters have stirred to the passage of the winds. Most waves are the result of the action of wind on water. There are exceptions, such as the tidal waves sometimes produced by earthquakes under the sea. But the waves most of us know best are wind waves.
It is a confused pattern that the waves make in the open sea—a mixture of countless different wave trains, intermingling, overtaking, passing, or sometimes engulfing one another; each group differing from the others in the place and manner of its origin, in its speed, its direction of movement; some doomed never to reach any shore, others destined to roll across half an ocean before they dissolve in thunder on a distant beach.
Out of such seemingly hopeless confusion the patient study of many men over many years has brought a surprising amount of order. While there is still much to be learned about waves, and much to be done to apply what is known to man’s advantage, there is a solid basis of fact on which to reconstruct the life history of a wave, predict its behavior under all the changing circumstances of its life, and foretell its effect on human affairs.
Before constructing an imaginary life history of a typical wave, we need to become familiar with some of its physical characteristics. A wave has height, from trough to crest. It has length, the distance from its crest to that of the following wave. The period of the wave refers to the time required for succeeding crests to pass a fixed point. None of these dimensions is static; all change, but bear definite relations to the wind, the depth of the water, and many other matters. Furthermore, the water that composes a wave does not advance with it across the sea; each water particle describes a circular or elliptical orbit with the passage of the wave form, but returns very nearly to its original position. And it is fortunate that this is so, for if the huge masses of water that comprise a wave actually moved across the sea, navigation would be impossible. Those who deal professionally in the lore of waves make frequent use of a picturesque expression—the ‘length of fetch.’ The ‘fetch’ is the distance that the waves have run, under the drive of a wind blowing in a constant direction, without obstruction. The greater the fetch, the higher the waves. Really large waves cannot be generated within the confined space of a bay or a small area. A fetch of perhaps 600 to 800 miles, with winds of gale velocity, is required to get up the largest ocean waves.
Now let us suppose that, after a period of calm, a storm develops far out in the Atlantic, perhaps a thousand miles from the New Jersey coast where we are spending a summer holiday. Its winds blow irregularly, with sudden gusts, shifting direction but in general blowing shoreward. The sheet of water under the wind responds to the changing pressures. It is no longer a level surface; it becomes furrowed with alternating troughs and ridges. The waves move toward the coast, and the wind that created them controls their destiny. As the storm continues and the waves move shoreward, they receive energy from the wind and increase in height. Up to a point they will continue to take to themselves the fierce energy of the wind, growing in height as the strength of the gale is absorbed, but when a wave becomes about a seventh as high from trough to crest as the distance to the next crest it will begin to topple in foaming whitecaps. Winds of hurricane force often blow the tops off the waves by their sheer violence; in such a storm the highest waves may develop after the wind has begun to subside.
But to return to our typical wave, born of wind and water far out in the Atlantic, grown to its full height on the energy of the winds, with its fellow waves forming a confused, irregular pattern known as a ‘sea.’ As the waves gradually pass out of the storm area their height diminishes, the distance between successive crests increases, and the ‘sea’ becomes a ‘swell,’ moving at an average speed of about 15 miles an hour. Near the coast a pattern of long, regular swells is substituted for the turbulence of open ocean. But as the swell enters shallow water a startling transformation takes place. For the first time in its existence, the wave feels the drag of shoaling bottom. Its speed slackens, crests of following waves crowd in toward it, abruptly its height increases and the wave form steepens. Then with a spilling, tumbling rush of water falling down into its trough, it dissolves in a seething confusion of foam.