East coasts also receive heavy winter swells when low-pressure cells form in the sub-tropics, where slow moving highs inhibit their movement. These lows produce a shorter fetch than polar fronts, however, they can still generate heavy swells since their slower movement increases the duration of a particular wind direction. The variables of fetch and duration both influence how long wind acts over a wave as it travels since a wave reaching the end of a fetch behaves as if the wind died.