Ice in lakes, rivers, and seas forms and grows in distinctly different conditions and ways. 

For lakes, the process begins under calm conditions near the shore when the water temperature reaches 32°F (0°C). There, a clear layer of “primary ice” made up of hexagonal plates, needles, or sheath-like structures with large crystals oriented in up-and-down forms. If cold conditions persist, this ice sheet will grow out from the shorelines until it covers the entire lake, making it “fast ice” because it’s holding fast to the land. This ice usually melts away in spring and summer.

Ice in turbulent rivers and the sea form very differently. Under rougher or salty conditions, water temperature needs to approach 28°F (-2°C), when fine ice crystals called “frazil” form a sort of slush in the water. The slush can gather into thicker icy 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 recently formed ice that is less than 10 centimeters thick. "Young" ice is 10 to 30 cm, and first-year ice is greater than 30 cm. Multiyear ice has survived a summer melt season and is typically 2 to 4 meters thick.

Ice melts when air and water temperatures warm as seasonal sunlight intensifies. 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.