Cross sea

Short description: Sea state with two wave systems traveling at oblique angles

Cross swell at Île de Ré, France. The waves themselves are examples of cnoidal waves.

Cross sea near Lisbon, Portugal.

A cross sea (also referred to as a squared sea or square waves[lower-alpha 1]) is a sea state of wind-generated ocean waves that form nonparallel wave systems. Cross seas have a large amount of directional spreading.[1] This may occur when water waves from one weather system continue despite a shift in wind. Waves generated by the new wind run at an angle to the old.

Two weather systems that are far from each other may create a cross sea when the waves from the systems meet at a place far from either weather system. Until the older waves have dissipated, they can present a perilous sea hazard.[2]

This sea state is fairly common and a large percentage of ship accidents have been found to occur in this state.[3] Vessels fare better against large waves when sailing directly perpendicular to oncoming surf. In a cross sea scenario, that becomes impossible as sailing into one set of waves necessitates sailing parallel to the other.[4]

A cross swell is generated when the wave systems are longer-period swells, rather than short-period wind-generated waves.[5]

## Notes

1. ↑ Not to be confused with square wave, a waveform.

## References

1. ↑ "Cross Sea" (in en). American Meteorological Society. http://glossary.ametsoc.org/wiki/Cross_sea.
2. ↑ "Is the Cross Sea Dangerous?". http://earth.esa.int/cgi-bin/confsea100415.html?abstract=349.
3. ↑ Toffoli, A.; Lefevre, J. M.; Bitner-Gregersen, E.; Monbaliu, J. (2005). "Towards the identification of warning criteria: Analysis of a ship accident database". Applied Ocean Research 27 (6): 281–291. doi:10.1016/j.apor.2006.03.003.
4. ↑ Xiaoming Li. "Is the Cross Sea Dangerous?". http://earth.eo.esa.int/cgi-bin/confsea10.pl?abstract=349.
5. ↑ Bowditch, Nathaniel (1995). "Glossary C". The American Practical Navigator. Bethesda, MD: National Imagery and Mapping Agency. p. 758. ISBN 0-403-09895-5. http://www.irbs.com/bowditch/pdf/glossary/gloss-c.pdf. Retrieved 2007-11-29.

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