snowfall

Snowfall is a form of precipitation consisting of snow, produced when water vapor in clouds deposits as ice crystals that grow into snowflakes and descend to the surface. This requires air temperatures near the surface at or below freezing, sufficient atmospheric moisture, and some upward motion to keep the cloud particles from melting before reaching the ground.

Snow that reaches the ground accumulates as a layer of snowpack whose depth depends on surface temperature, solar radiation, wind, melting and sublimation. Ground accumulation is measured in centimeters or inches and is often reported as snow depth or as overall snow amount during a storm. Snow density varies with temperature and moisture, giving snow-to-liquid ratios that commonly range from about 5:1 to 20:1, with wetter snow being closer to 5:1 and dry powder closer to 20:1.

Different atmospheric processes produce snowfall, including lake-effect snow, orographic uplift, frontal systems and convective squalls. Geographic and seasonal patterns reflect latitude and elevation: temperate regions have most snowfall in winter; high latitudes and mountain areas may accumulate year-round; coastal regions can see lighter, more frequent events while inland, dry regions see less frequent snow.

Snow affects ecosystems and human activity. A layer of snow provides insulation for soil and winter organisms and stores freshwater as part of the snowpack; its high albedo reflects sunlight, influencing local climate. Snowfall can disrupt transport, damage roofs, and strain energy systems, especially during rapid thaws or heavy storms.

Forecasting and measurement rely on weather stations, radar, satellites and numerical models. Snow forecasts consider temperature profiles, humidity, wind, and storm dynamics, and are essential for aviation, transportation, and hazard planning.