Any non-linear shaped opening enclosed in ice. Polynyas may contain brash ice and/or be covered with new ice, nilas or young ice; submariners refer to these as skylights. Sometimes the polynya is limited on one side by the coast and is called a shore polynya or by fast ice and is called a flaw polynya. If it recurs in the same position every year, it is called a recurring polynya. Polynyas range in size from relatively small to enormous. The largest polynya observed in the Antarctic was the Weddell Polynya of 1975-77, covering an area of 2x105km2. The two main categories of polynya are sensible heat and latent heat, depending on the mechanism responsible for maintaining their presence. Latent heat polynyas are maintained by persistent katabatic winds that drain off the continent. Newly formed ice is advected away by the wind, leaving the surface ice-free and open to more ice formation. In this manner latent heat polynyas can be major sources of new ice production. Coastal polynyas are primarily of this type. Sensible heat polynyas are maintained by upwelling warm water that supplies a sufficiently large oceanic heat flux to the base of the ice to reduce its thickness, or melt it completely. These polynyas are not responsible for large quantities of new ice production. A polynya may also form by a combination of the sensible and latent heat processes.