What Is a Swell in the Ocean?
Ocean swells are characteristically smooth, without the peaks and breakers of a wave. They might appear to be gently rolling across the seascape like watery creatures lumbering peacefully toward an unknown destination, possibly imparting a gentle rocking motion to the vessels they encounter on the journey. They may even possibly leave a boat's occupants in a temporary state of zero gravity, if their period is short enough and if they are occur in concert with waves.
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Wind Sea
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A "wind sea" is a set of waves or a wave train generated by local transfer of wind energy to the surface of the ocean. Typically, such a wave train has peaks that either retain their shape or have a "breaking top" resulting from a horizontal wind acting on the wave at its top.
Rules of Thumb for Determining Wave Heights
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A general formula for calculating significant wave heights--that is, the height, from trough to crest of the largest third of waves occurring at a particular time and place--requires knowing the fetch of the wind (the unencumbered distance a wind blows at sea is called its "fetch"), the wind speed at the observer's location and the depth of the water at the observer's location. A general rule of thumb used by sailors for centuries is that a 20-knot wind, blowing unencumberedacross a distance of 20 nautical miles in a bottomless ocean (a depth greater than 600 meters) can produce a 20-foot sea one-third of the time. Another ancient formula for calculating the average wave height for the same conditions calls for simply dividing the wind speed by four, meaning that a 20-knot wind with a fetch of 20 miles in the same depth of water produces waves with an average height of five feet.
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The Wave is a Local Response to Wind
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Waves and swells do not occur independently of each other, because both are the result of friction at the interface of two fluids. The difference between a wave and a swell is that a wave is a local response to that friction: The wind "whips up the water" in much the same way that an electric kitchen mixer whips "peaks" into egg whites and powdered sugar to form a meringue.
Swells
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A swell is generated by wind as well; however, a swell is the remains of a wind sea. It may have been generated by a storm miles or even hundreds of miles away from the point at which it is observed. It no longer has the characteristic wind-generated wave peak, it is not apparently the result of local winds and it represents the remaining vertical component of the horizontal transfer of wind energy to the ocean at some other point. If you leave a meringue sitting for too long after mixing, the peaks collapse, much like waves, and it becomes more compact as the air that was whipped into it by the mixer compacts. It begins to resemble a set of gentle rises--in a series of lines rather than a set of choppy peaks--in much the same way that, as the ocean settles down after a stiff wind, swells become the norm.
Swells Represent Traveling Energy
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Swells can travel for many, many miles. The gentle swell rocking boats in the marina, making their rigging ring off of their masts, may have begun as waves generated by a storm far away. Water, because it is essentially incompressible, tends to retain the energy imparted by the wind blowing across its surface. The individual particles of water do not move; they transfer the energy from the wind to the next particle, converting only the smallest bit of that potential energy to kinetic energy. A 20-foot swell in the Pacific is not uncommon, but there may be 10 miles between the crests of two swells because of the depth of the ocean, making that 20-foot swell completely unnoticeable.
Tsunamis
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Tsunamis result from undersea geological events like volcanic eruptions or earthquakes. Aboard a boat, even a short distance from the geological event that triggers a tsunami, the wave may be easily confused with a simple swell. The only way to tell the difference is when that "swell" encounters a shallow bottom: The swell breaks down, returning its load of energy to the atmosphere as it collapses against the bottom. The waters of the tsunami, however, are so energized that they continue ashore, leaving a path of destruction before their energy--much greater than that of wind--is dissipated.
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