Leads are narrow, linear cracks in the ice that form when ice floes diverge or shear as they move parallel to each other. The formation of leads is similar to mid-ocean ridges or shear zones that form from Earth's moving tectonic plates. The width of leads varies from a couple of meters to over a kilometer. Leads can often branch or intersect, creating a complex network of linear features in the ice. In the winter, leads begin to freeze almost immediately from the cold air.

Aerial view of a lead with frazil ice.
—Photo courtesy of Todd Arbetter, National Snow and Ice Data Center.

Leads are important for several reasons. First, seasonal changes influence local and regional climate. Leads are much darker than surrounding ice, which during the summer, results in relatively lower albedo, or the ability to reflect light. Because of lower albedo, leads absorb more solar energy than surrounding ocean, which heats the water in the leads and speeds up the melting of surrounding ice. At the beginning of winter, as sea ice begins to refreeze in leads, brine adds salt to the open ocean layer. In leads that persist throughout the winter, relatively warm ocean water is exposed to the cold atmosphere, releasing heat and moisture into the atmosphere. Thus, leads are often accompanied by low-level clouds downwind.

Leads are also important for wildlife. Seals, whales, penguins, and other animals rely on leads for access to oxygen. Polar bears in the Arctic often hunt near leads, because they know that their prey is likely come to the surface to breathe in such areas.

Finally, leads are important for navigation. Even when they freeze, leads tend to contain thinner and weaker ice that allows submarines to more easily surface through the ice and icebreakers to more easily traverse the ice.