Leads tend to open parallel to the direction of water flow
A lead is a fracture or large crack in the sea ice that results in a passageway of open water. Leads are less important to sea life than the open areas of polynyas, but they are still important to many organisms. Leads provide important arteries for the navigation of marine mammals and birds.
Leads can occur almost anywhere that sea ice is packed relatively loosely. They may appear suddenly, and tend to open and close over a period of just a few hours following changes in winds or tides. There are places, however, where wide, predictable leads form between the landfast ice and the free-floating pack ice. These long-lasting leads are created by winds blowing offshore that push ice away from the coast, causing the pack ice to aggregate. These distinct leads provide open water for new ice formation.
Leads can often be detected from a great distance because they make the clouds above them appear dark. This occurs because the open water in leads absorbs sunlight, which is normally reflected off ice or snow. This causes a dark streak to appear on the underside of the clouds above. This phenomenon is called ‘watersky’.