At any point in time the sea condition is a reflection of every storm and disturbance that has occurred within the past few weeks. Distant storms may have churned the ocean 1,000 nautical miles away from your position; a day and a half later the first swells from that storm reach the spot where you sail. For example, sailing northwest in the direction of the oncoming swell, you are likely to encounter higher waves as time passes, until finally the size of the waves once again diminishes as the last waves of that particular storm pass under the hull of the boat.

Now suppose that at the same time, the wind begins to freshen in the northeast, signaling the arrival of a nearby storm or squall. All afternoon the wind from the northeast builds, soon reaching the point at which you reduce sail to just the mainsail with one reef. Soon you are sailing at 7 knots, making good progress to a distant port; meanwhile the squall has pretty much blown itself out. What of the seas at this point?

Looking at the waves, it is difficult to discern a single pattern or uniformity. Instead, superimposed on the swell coming from the northwest are smaller waves from the local storm. Although the wind was out of the northeast, and thus at a right angle to the direction in which you were sailing, the storm waves do not line up downwind. Instead, wave trains arrive at various angles to the direction of the wind—some piling up on top of the swell, some opposing it, and some hitting it at an angle. The combined effect is to give the sea a chaotic appearance—any singular pattern of wave motion is difficult to perceive.

Being close to the local storm, dispersion had little effect and the waves reaching the boat had many different wavelengths. Another way of stating this is that the “sea” has a wide bandwidth spectrum (meaning waves of many different frequencies) compared to swell, which has a narrow bandwidth (because of the filtering effect of dispersion).

You might ask, “What is the significant wave height under these conditions?” Trying to determine how high the waves are in chaotic seas might seem to be nearly impossible, but weather forecasters have devised methods, based on the randomness of chaotic seas, that they use.1



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement