cross sea
{{short description|Sea state with two wave systems traveling at oblique angles}}
{{distinguish|Square wave (waveform)}}
File:Ile de ré.JPG, France. The waves themselves are examples of cnoidal waves.]]
File:Cross sea near Lisbon, Portugal - April 2019.jpg, Portugal.]]
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.{{cite web |url=http://glossary.ametsoc.org/wiki/Cross_sea |website=Glossary of Meteorology |publisher=American Meteorological Society |title=Cross Sea |access-date=4 Feb 2019 |language=en}} 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.{{Cite web|title=Is the Cross Sea Dangerous?|url=http://earth.esa.int/cgi-bin/confsea100415.html?abstract=349|access-date=2020-09-07|website=earth.esa.int|archive-date=2017-08-24|archive-url=https://web.archive.org/web/20170824180828/http://earth.esa.int/cgi-bin/confsea100415.html?abstract=349|url-status=dead}}
This sea state is fairly common and a large percentage of ship accidents have been found to occur in this state.{{cite journal |last1=Toffoli |first1=A. |last2=Lefevre |first2=J. M. |last3=Bitner-Gregersen |first3=E. |last4=Monbaliu |first4=J. |date=2005 |title=Towards the identification of warning criteria: Analysis of a ship accident database |journal=Applied Ocean Research |doi=10.1016/j.apor.2006.03.003 |volume=27 |issue=6 |pages=281–291}} 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.{{cite web |author=Xiaoming Li |title=Is the Cross Sea Dangerous? |website=Wayback machine.|url=http://earth.eo.esa.int/cgi-bin/confsea10.pl?abstract=349 |archive-url=https://web.archive.org/web/20140503193016/http://earth.eo.esa.int/cgi-bin/confsea10.pl?abstract=349 |access-date=10 February 2019|archive-date=2014-05-03 }}
Cross seas present a particular hazard for small boats and swimmers, since they can generate rip currents and waves up to {{convert|10|feet|meters|0}}.{{cite web |title=Square Waves in Ocean Settings Are Rare But Dangerous |url=https://science.howstuffworks.com/environmental/earth/oceanography/cross-seas.htm |website=HowStuffWorks |access-date=6 March 2025 |language=en-us |date=1 January 1970}}
A cross swell is generated when the wave systems are longer-period swells, rather than short-period wind-generated waves.{{cite book |last=Bowditch |first=Nathaniel |author-link=Nathaniel Bowditch |year=1995 |title=The American Practical Navigator |title-link=The American Practical Navigator |chapter=Glossary C |chapter-url=http://www.irbs.com/bowditch/pdf/glossary/gloss-c.pdf |publisher=National Imagery and Mapping Agency |location=Bethesda, MD |isbn=0-403-09895-5 |page=758 |access-date=2007-11-29 |archive-date=2011-07-16 |archive-url=https://web.archive.org/web/20110716165325/http://www.irbs.com/bowditch/pdf/glossary/gloss-c.pdf |url-status=dead }}