Cross sea

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.

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

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]

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