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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. Two weather systems that are far from each other may create a cross sea when the waves from the systems meet, usually at a place far from either weather system. Until the older waves have dissipated, they create a sea hazard among the most perilous. This sea state is fairly common and a larger percentage of ship accidents were found to have occurred in this state.
A cross swell is generated when the wave systems are longer period swell, rather than short period wind generated waves.