Lead (Sea Ice)

A lead is a long, narrow opening in the pack wide enough for a ship to pass, ranging from a fracture to a channel kilometers across, defined in the WMO Sea-Ice Nomenclature (WMO No. 259). Leads open and close as the pack diverges and converges under wind and current, and a flaw lead runs along the boundary between fast ice and drift ice. They are the navigable routes through the pack and the sites of rapid new-ice growth and large ocean-to-atmosphere heat flux when exposed water freezes. Frost flowers and nilas form first in refreezing leads, which show as dark linear features in SAR and thermal imagery.