A sneaker wave, sleeper wave, or in Australia a king wave is a disproportionately large coastal wave that can sometimes appear in a wave train without warning. The terminology is popular rather than scientific: there is no scientific coverage (or evidence) of the phenomenon as a distinct sort of wave with respect to height or predictability—as there is on other extreme wave events such as rogue waves. One Australian authority treats "king wave" as a synonym for "rogue wave". One American oceanographer distinguishes "rogue waves" as occurring on the ocean and sneaker waves as occurring at the shore.
Because they are much larger than preceding waves, sneaker waves can catch unwary swimmers, washing them out to sea. It is not uncommon for people walking or standing on beaches and ocean jetties to also be washed into the sea. Sneaker waves are mainly referred to in warnings and reports of incidents for the coasts of Northern California, Oregon and Washington in the United States . These sneaker waves also occur on the west coast of Canada, they are commonly seen in Tofino and Ucluelet. King waves occur especially in Western Australia and Tasmania.
In many parts of the world, local folklore 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. These ideas have some scientific basis, due to the occurrence of wave groups at sea, along with a culture-inflected fascination with the number 7. This saying may also serve to educate shore dwellers about the necessity of remaining vigilant when near the sea. There is no scientific evidence that these wave groups are related to sneaker waves.
- See the external links
- Kinsman, Blair (1984). Wind waves : their generation and propagation on the ocean surface. Dover Publications. ISBN 0-486-49511-6. p. 10.
- For example: Massel, Stanislaw R. (1996). Ocean surface waves: their physics and prediction. Singapore: World Scientific. ISBN 981-02-2109-6. §4.6, pp. 192–200.
- National Park Service US Department of the Interior warning page
- Oregon Parks and Recreation Department warning page
- Washington State Department of Ecology warning page
-  article about a nearly fatal incident