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.
A cross swell is generated when the wave systems are longer period swell, rather than short period wind generated waves.
This phenomenon may be dangerous (boats are meant to hit waves head on, not from all sides), but at the same time it’s also one of the most beautiful naturally occurring shapes.