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Rogue waves have been part of marine folklore for centuries. They are generally considered to be unexpectedly high waves which in some instances come from a direction different from the predominant waves in the local area. A single rogue wave has certainly been known to spell disaster for the mariner. They have, over the past twenty or thirty years, come to be recognized as a unique phenomena albeit with several possible causes.
1) Constructive interference. Several different wave trains of differing speeds and directions meet at the same time. The heights of the crests are additive so that an extreme wave may result when very high waves are included in the wave trains. The effect is normally short lived since the wave trains continue to separate and move on.
2) Focusing of wave energy. When storm forced waves are developed in a water current counter to the wave direction an interaction can take place which results in a shortening of the wave frequency. The result is the superimposing of the wave trains and the generation of extreme waves. Examples of currents where these are sometimes seen are the Gulf Stream and Agulhas current. Extreme wave developed in this regime tend to be longer lived.
3) Normal part of the wave spectrum. The generation of waves on water
results not in a single wave height but in a spectrum of waves distributed
from the smallest capillary waves to large waves indeed. Within this spectrum
there is a finite possibility of each of the wave heights to occur with the
largest waves being the least likely. The wave height most commonly observed
and forecast is the significant wave height. This is defined as the average
of the one third highest waves. The random nature of waves implies that
individual waves can be substantially higher than the significant wave height.
In fact, observations and theory show that the highest individual waves in a
typical storm with typical duration to be approximately two times the
significant wave height. Some reported rogue waves are well within this factor
of two envelope. Waves higher than roughly twice the significant wave height
fall into the category of extreme or rogue waves.