Snow, ice crystals that fall from the sky. Snow, like rain, sleet, and hail, is a form of precipitation —moisture reaching the earth from the sky. Snow appears white because of the many reflecting surfaces of the ice crystals. Occasionally, snow may be red, pink, yellow, or brown; and more rarely green or blue. The coloring is caused by soot, pollen, or other particles collected by the crystals as they fall through the air. Snowflakes can be individual crystals, or clusters of crystals and crystal fragments.

Snow is the principal source of water in many regions. Winter snows in the mountains and plains melt in spring, and thus provide water to feed lakes, rivers, and streams. Melted snow seeps into the soil and increases groundwater supplies. Because of its insulating properties, snow often protects vegetation from freezing and provides shelter for wild animals. Snow provides recreational opportunities for children and adults. Snow sports include skiing, snowshoeing, sledding, and snowmobiling.

Snow also is a potential danger. The sudden melting of a deep snow cover may produce floods that endanger lives, damage crops and buildings, and erode valuable topsoil. Snow and ice on streets make driving hazardous. The weight of snow can break tree branches and power and telephone lines. Severe snowstorms can paralyze transportation and shut down businesses. In mountain areas, dwellings and even whole communities may be destroyed by avalanches.

Distribution and Measurement

Snowstorms occur in cold months in temperate and polar climates. In the tropics, snow falls only on high mountain peaks. The snow line is the lowest altitude above which an area is covered with snow the year round. The line is higher at the Equator than in a temperate climate, and is lowest in the polar regions. Glaciers develop where the annual accumulation of snow exceeds the amount that can melt and evaporate.

Snowfall is difficult to measure because even moderate winds cause the drifting of snow on the ground. Meteorologists use various devices and gauges to determine snowfall indirectly—by measuring the water equivalent of snow. For example, snow is collected and melted in a rain gauge, and the resulting water is measured. Approximately 10 inches (25.4 cm) of snow equal 1 inch (2.54 cm) of water, but this figure can vary greatly. Snow depth also is measured with a ruler. The recorded depth is the average of several measurements taken at different points in the same area where drifting is minimal.

How Snow Forms

Snow forms in clouds where moisture in the air freezes into ice crystals. In general, the crystals form around dust or other very small particles of solid matter. These particles, called ice nuclei, are present in the atmosphere in large numbers. Some of the crystals may splinter, increasing the number of crystals in the cloud; other crystals may grow as moisture in the cloud freezes onto them. As crystals grow or become joined with other crystals, their increased weight causes them to fall to the ground. If the air near the ground is cold enough, the crystals reach the ground as snow; otherwise, the crystals melt and become rain.

Snow crystals are classified according to form. The most common types are hexagonal plates, hexagonal columns, needles, six-pointed stars, dendrite (fernlike) crystals, and irregular crystals. The variations in these forms are so numerous that no two crystals are identical. The star crystals and dendrite crystals often have beautiful and intricate patterns.

Snow crystals range in diameter from 11000 inch (0.025 mm) to 12 inch (13 mm). Size and shape depend largely on the temperature and moisture conditions during the growth of the crystal. In general, larger crystals with intricate patterns form at relatively low altitudes where the air is moist and slightly below freezing. Small crystals with simple patterns form at higher altitudes in drier and colder air.

Snowmaking

Artificial snow is made by spraying a fine mist of water into the air when the temperature is below freezing. In another type, which looks somewhat like a large cannon, the water is injected into a strong stream of air produced by a turbine-like fan. On some ski slopes, a commercially grown quantity of a naturally occurring bacterium, Pseudomonas syringae, is mixed with the water. The bacteria serve as ice nuclei and are used to increase the amount of snow produced from a given amount of water.

Snow also is produced by dispersing dry ice, silver iodide, or other substances into clouds. This method, called cloud seeding, is successful only under ideal conditions; it is used primarily for experimental purposes.