Ice in lake, river, and sea water form and grow in distinctly different conditions and ways. For lakes, the process begins under calm conditions near the shore as the water temperature reaches 32F. There, a clear layer of “primary ice” made up of hexagonal plates, needles, or sheath-like structures with large crystals oriented up-and-down forms. If cold conditions persist, the ice sheet will grow until it covers the entire lake, making it “fast ice” because it’s holding fast to the shoreline. This ice usually melts away in summertime.

Ice in turbulent rivers and the sea form very differently. Under rougher or salty conditions, water temperature needs to approach 28F. Then, fine ice crystals called “frazil” form a sort of slush in the water. The slush can gather into thicker sludge. While frazil can form in open water, it normally grows from the shore outward. As the sludge accumulates it solidifies as fast ice.

New ice in the ocean is classified as recently formed ice less than 10 centimeters thick. Young ice is 10-30 centimeters. First year ice is greater than 30 centimeters. And multiyear ice has survived a summer melt season and is typically two to four meters thick.

Ice, of course, melts when temperatures warm and sunlight intensifies in summer. Thick ice covered with snow reflects 90% of solar radiation, making it harder to melt. As the snow melts, melt ponds may form on the ice, reducing its reflective properties and increasing its rate of melting. Warm surface waters can also increase melting and produce holes in ice called polynyas.

NASA offers dozens of datasets suitable for tracking the growth and loss of ice from day to day or year to year across the planet.