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a recent storm in the North Pacific will finally reach shore in Newport Beach, California; Hawaii’s North Shore; Mexico; or Tahiti as the long, undulating waves known as swell.
Just what is swell? As I explain in Chapter 4, storm winds typically follow a circular path, stirring up the sea as the storm propagates. Storm waves mix with preexisting sea waves, and in a short time the sea roils with a mixture of waves large and small, with long and short wavelengths, coming from various directions. This scenario is known as a confused sea. (See Chapter 7 for more on this subject.) Waves radiate from the storm center, in much the same manner as they emanate from a rock tossed into a quiet pond. Some of the waves will head off in the direction in which the storm is moving, in which case, if the storm continues to blow, they will grow larger. Others, headed in the opposite direction, will not increase in size. In either case, storm-generated waves propagate from the center of the storm, the faster waves (those with longer periods and wavelengths) gradually outstripping their slower counterparts. In this process, known as dispersion, the sea acts as a filter so that at locations distant from the source, the waves that traverse a particular point eventually will exhibit similar characteristics. These distant, dispersed waves are known as swell. (Later, when the slower waves reach this same point, their characteristics will be different.) Swell travels long distances, only slowly giving up its energy. Swell from a given North Pacific storm can be tracked clear across the Pacific until it finally dissipates on the coast of North America.
Often the yachtsman will note that swell does not consist of nice uniform waves, but instead has a set of waves that seems to come at one angle to his course and another set that comes at a different angle. When swells from two different sources interact, the two (or more) wave fronts can produce various patterns. For example, two swell wave fronts intersecting at 45 degrees would produce a diamond-shaped pattern of crests and troughs. There can be more than two swells interacting—for example, near islands where swell impinges on the shore and reflects or is refracted back into the sea. If two sets of swell meet at