Cross sea

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Cross swell at Île de Ré.
Aircraft view of cross sea near Lisbon.

A cross sea (also referred to as a squared sea or square waves) 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 create a perilous sea hazard.

This sea state is fairly common and a large percentage of ship accidents have been found to occur in this state.[2] Vessels fare better against large waves when perpendicular to the waves. In a cross sea, vessels are more likely to be struck in a dangerous way.[3]

A cross swell is generated when the wave systems are longer-period swell, rather than short-period wind-generated waves.[4]

See also[edit]


  1. ^ "Cross Sea". Glossary of Meteorology. American Meteorological Society. Retrieved 4 Feb 2019.
  2. ^ A. Toffolia & 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.CS1 maint: Uses authors parameter (link)
  3. ^ Li, Xiaoming. "Is the Cross Sea Dangerous?". Wayback machine. Retrieved 10 February 2019.
  4. ^ Bowditch, Nathaniel (1995). "Glossary C" (PDF). The American Practical Navigator. Bethesda, MD: National Imagery and Mapping Agency. p. 758. ISBN 0-403-09895-5.