Even though snow only falls in certain parts of the world, it plays an important role in Earth's hydrologic cycle, forms part of many northern ecosystems, and provides irrigation for crops and drinking water for communities that may be far downstream.
Scientists at NSIDC study ways to map global snow cover from satellite and ways to determine the contribution of melting snow to regional water supplies. NSIDC's All About Snow provides general information about snow, as well as links to resources for learning more about all aspects of snow.
What is snow?
Snow cover is a part of the cryosphere, which traces its origins to the Greek word kryos for frost. Snow is precipitation in the form of ice crystals. It originates in clouds when temperatures are below the freezing point (0 degrees Celsius, or 32 degrees Fahrenheit), when water vapor in the atmosphere condenses directly into ice without going through the liquid stage. Once an ice crystal has formed, it absorbs and freezes additional water vapor from the surrounding air, growing into a snow crystal or snow pellet, which then falls to Earth.
Snow falls in several forms:
- Snowflakes are clusters of ice crystals that fall from a cloud.
- Snow pellets, or graupel, are opaque ice particles in the atmosphere. They form as ice crystals fall through supercooled cloud droplets, which are below freezing but remain a liquid. The cloud droplets then freeze to the crystals, forming a lumpy mass. Graupel tends to be soft and crumbly.
- Sleet is composed of drops of rain or drizzle that freeze into ice as they fall, and is sometimes called a wintery mix of rain and snow. These small, translucent balls of ice are usually smaller than 0.76 centimeters (0.30 inches) in diameter. Official weather observations may list sleet as ice pellets. In some parts of the United States, the term sleet can refer to a mixture of ice pellets and freezing rain.
Graupel is composed of small pellets of snow. Unlike the hard balls of ice that form hail, graupel tends to be smaller, with a soft and crumbly texture.
—Credit: James Halliday, flickr
One form of precipitation, hail, while frozen, is not considered snow. Hail tends to be larger than sleet, and is usually generated during thunderstorms, which happen more often in spring and summer than in winter. Hailstones form when upward moving air, or updrafts, in a thunderstorm prevent pieces of graupel from falling. Drops of supercooled water hit the graupel and freeze to it, causing the graupel to grow. When the balls of ice become too heavy for the updrafts to continue supporting them, they fall as hailstones.
Snow as mineral
Because snow is composed of frozen water, or ice, it can also be classified as a mineral. A mineral is a naturally occurring homogeneous solid, inorganically formed, with a definite chemical composition and an ordered atomic arrangement. Ice is naturally occurring, given a temperature below 0 degrees Celsius (32 degrees Fahrenheit). It is homogenous (of one material), formed inorganically, and has an ordered atomic structure. Ice has a definite chemical composition (H20), with hydrogen and oxygen atoms bonding in a specific manner.