A cross swell at the Phares des Baleines (Lighthouse of the Whales) on Île de Ré, France, February 2011. Photo by Michel Griffon.

In surface navigation, a cross sea is a sea state with two wave systems traveling at oblique angles. 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, creating a shifting, dangerous pattern. Until the older waves have dissipated, they create a sea hazard among the most perilous.