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

A cross sea (also referred to as a squared sea or square waves[a]) 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]


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


  1. ^ "Cross Sea". Glossary of Meteorology. American Meteorological Society. Retrieved 4 Feb 2019.
  2. ^ "Is the Cross Sea Dangerous?". Retrieved 2020-09-07.
  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?". Wayback machine. Archived from the original on 2014-05-03. Retrieved 10 February 2019.
  5. ^ Bowditch, Nathaniel (1995). "Glossary C" (PDF). The American Practical Navigator. Bethesda, MD: National Imagery and Mapping Agency. p. 758. ISBN 0-403-09895-5. Archived from the original (PDF) on 2011-07-16. Retrieved 2007-11-29.