feature that widens and spreads. When islands are close together, as in the Siberian Sea, fast ice blankets the sea surface, and bridges the waters between all land areas. On the average in the Northern Hemisphere, sea ice is at a minimum in September, and at a maximum in March. In the Southern Hemisphere, these times are nearly opposite; minimum in March and maximum in September. The first indication of ice formation is the presence of  fine  ice  crystals  on  the  surface  of  the  water, producing a "slushy" water appearance. Sea Ice Classification As  sea  ice  forms  and  grows,  it  is  generally categorized into one of four groups: newly formed ice, young ice, first-year ice, and old ice. NEWLY  FORMED  ICE.—In the open sea, the first sign that the sea surface is freezing is an oily appearance  of  the  water.  This  is  caused  by  the formation of spicules (minute ice needles) and frazil crystals (thin plates of ice). As formation continues, the surface attains a thick, soupy consistency termed grease ice. Eventually, slush and/or shuga (spongy white ice clumps) will begin to appear. Next, depending on the wind, waves, and salinity, an elastic or brittle crust forms. The elastic crust (nilas) has a matte appearance, while the brittle crust (ice rind) is shiny. As the crust thickens, the wind and sea cause the ice to break up into rounded  masses  known  as  pancake  ice.  With continued freezing, the pancake ice forms into a continuous sheet. YOUNG ICE.—This ice sheet forms in 1 year or less, and its thickness ranges from 10 to 30 centimeters (4 to 12 inches). It is further classified as gray ice and gray-white  ice. FIRST-YEAR  ICE.—This ice is a more or less unbroken sheet of ice of not more than one winter’s growth that starts as young ice. Its thickness is from 30 centimeters to 2 meters (1 foot to 6 1/2 feet). First-year ice may be subdivided into thin first-year ice, medium first-year ice, and thick first-year ice. The latter is more than 4 feet thick. OLD ICE.—Old ice is extremely heavy sea ice that has survived at least one summer’s melt. It occurs primarily in the Arctic and Antarctic polar packs as a mass of converging and dividing ice floes of various ages, sizes and shapes. Old ice may be subdivided into second-year ice and multi-year ice. 1-51 Sizes of Sea Ice Sea ice generally forms in vast sheets frozen solidly to the shores of islands and land masses; such areas of ice are called fast ice. The effects of winds and currents may break up fast ice sheets into smaller free-floating pieces of sea ice. Sea ice is categorized into seven different sizes, ranging from "small ice cakes" to "giant ice floes." Refer to figure 1-35 for relative sizes and a comparison  to  more  common  features.  When  the majority of the water area is covered in ice floes, the ice is generally called pack ice. A similar term, the ice puck,  refers to any very large area that is predominately covered in ice. In the Arctic and Antarctic, some areas of fast ice persist for many years. The frozen layers of seawater collect layers of snow that build up and are compressed into ice. These areas may develop ice sheets many hundreds of feet in thickness. Vast areas of seawater may be covered by these permanent areas of ice, which is called shelf ice or an ice shelf.  As pieces of shelf ice along the edges of the ice sheet break free, they become icebergs. Open Water Around Sea Ice The same forces that separate ice floes from the fast-ice sheet also create various openings of unfrozen water between the ice floes. Naval operations in and around  fields  of  sea  ice  can  be  hazardous.  The movement of massive floes of ice can cut off ships from open water; worse yet, the ice may close in around a ship, leaving it stranded in a sea of ice. Therefore, changes in the size of open water areas in ice-covered seas becomes very important. Many water features are associated with sea ice. Some of the more common features are as follows: Fracture—Any break or crack through the ice sheet Lead—A  long,  narrow  break  or  passage  through the sea ice sheet or between floes; a navigable fracture. A lead may be open or refrozen Puddle—A depression in sea ice usually filled with melted water caused by warm weather Thaw hole—A hole in the ice that is caused by the  melting  associated  with  warm  weather Polynya—Any  sizable  area  of  seawater  enclosed by sea ice. Put simply, a large hole in the ice