Lead

Linear crack or fracture in continuous sea ice, opened by tidal stress, wind shear, or differential current, and large enough to allow navigation by ice-class vessels. The World Meteorological Organization Sea-Ice Nomenclature distinguishes leads from polynyas by their elongated shape and by their transient nature — leads can close and refreeze within hours. Arctic shipping uses leads to reduce icebreaking power demand, but voyage planning under Polar Code Part I-A requires contingency for lead closure and re-routing through heavier ice.