Nilas

Nilas is a thin, uniform crust of sea ice that forms on the surface of seawater in polar regions when the water begins to freeze. It is typically 2 to 10 centimeters thick and appears smooth, pale blue to gray, and translucent. Nilas is flexible and can bend with slight wave action without breaking, a property that distinguishes it from thicker, more rigid ice.

Nilas forms as the upper surface freezes under cold air conditions, often following the development of frazil ice or grease ice. It requires relatively calm sea conditions to allow the ice crystals to fuse into a coherent crust. The surface is generally smooth, with few ridges or hummocks.

In the sea-ice growth sequence, nilas represents the initial solid ice stage. When the crust thickens to about 10 centimeters, it is commonly classified as young ice or first-year ice rather than nilas. Despite its thinness, nilas contributes to albedo changes and plays a role in the heat exchange between ocean and atmosphere during the early stages of ice formation.

Geographically, nilas is observed in Arctic and sub-Arctic seas during winter and in some Antarctic marginal seas where surface temperatures fall below freezing. It serves as an indicator of freezing conditions used in oceanography and remote sensing and is a transient phase in the progression toward thicker sea ice.