earth, sun, and moon; and even more distant planets have a small, albeit negligible, effect.
These attractive forces are strong enough to move mountains and exert a measurable effect on oceans, tugging the water one way when the sun and moon are close and the force is greater, tugging it another way as the force is relaxed or moves to another part of the sea. Since water is a fluid, if it rises at one point, it must lower elsewhere, causing a current to flow from one point to the other. If left undisturbed, any body of water will eventually settle into a stable configuration with a flat surface. The surface tension of the fluid acts to preserve that stability. It is only in the presence of some disturbing force that the surface begins to change shape.
Wind is the usual disturbing force, producing wind-generated waves, the waves most commonly experienced. Oceanographers distinguish six major types of waves, typically categorized in terms of increasing period (Figure 1). These are capillary waves, which have periods of a fraction of a second; ultragravity waves, which have periods ranging from 0.1 to 1 second; gravity waves, 1 to 30 seconds; infragravity waves, 30 seconds to 5 minutes; long-period waves, 5 minutes to 24 hours; and transtidal waves, 24 hours or longer. In terms of the topic of this book, wind-generated gravity waves are the most likely source of extreme waves, but the next category, long-period waves, can also produce extremely dangerous and damaging waves.
Gravity waves—the name arises from the fact that wind piles water up, but gravity pulls it back down—are further subdivided into seas and swell. Seas—wind-driven waves—usually have shorter periods and are of greater significance to the mariner. Swell—a succession of waves that have moved beyond the immediate influence of the wind that caused them—have longer periods (10 to 30 seconds), but there are overlaps and no distinct dividing line between periods. Because the wavelength of swell is usually long compared to the length of a vessel, it generally does not concern the mariner in the open ocean. When the swell is large and its wavelength close to the length of a vessel, it is a different matter.