Sneaker wave

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Sneaker wave is a term used to describe disproportionately large coastal waves that can often appear in a wave train without warning. Sneaker waves form when the energy of a number of smaller waves becomes "focused," meaning that several smaller waves have run into each other and created constructive interference. Constructive interference means that the individual wave peaks coincided and created a new wave that is the sum of those that were superimposed. This is similar to the way deep ocean rogue waves are thought to form, albeit on a much smaller scale.

Because they are much larger than preceding waves, sneaker waves can catch unwary swimmers, washing them out to sea. Sneaker waves are a universal coastal phenomenon, although they are known to be more common in some areas than others; in the United States, Northern California, Oregon and Washington are particularly affected.

[edit] Seventh wave

In many parts of the world local lore predicts that out of a certain number of waves, one will be much larger than the rest ("every seventh wave" is one common belief that has wide circulation and has entered popular culture through music and literature). While these ideas have no scientific basis, because sneaker waves do not follow a predictable pattern, they do demonstrate a general familiarity with the phenomenon itself, along with a fascination with the number 7. The legend also serves to educate shore dwellers about the necessity of remaining vigilant when near the sea.