Snow is solid precipitation in the form of ice crystals that grow inside clouds when water vapour deposits directly onto ice nuclei at temperatures below −12 °C. The resulting crystals exhibit the famous hexagonal symmetry, though their exact shape — plates, columns, needles, dendrites, or stellar — depends on temperature and humidity during growth. The classic six-armed dendritic snowflake forms at around −15 °C, the temperature of maximum crystal branching.
For snow to reach the ground without melting, the entire air column must be near or below 0 °C. The critical factor is the wet-bulb temperature (accounting for evaporative cooling as flakes fall through unsaturated air), which is why snow can occur with surface temperatures up to 2–3 °C. The snow level (the altitude where snow turns to rain) drops roughly 150 m for each 1 °C of cooling. In Spain, significant snowfall affects mountainous areas regularly and occasionally reaches lowland cities — Madrid recorded 50 cm during Storm Filomena (January 2021).
Snow has profound meteorological and climatic effects. Fresh snow reflects 80–90 % of incoming solar radiation (albedo), creating a powerful cooling feedback: snow cover leads to colder temperatures, which preserves the snow — the snow-albedo feedback, a critical factor in climate change. Accumulated snowpack in mountains is a vital water reservoir, feeding rivers during spring melt. Measurement is by depth (cm) and water equivalent (SWE) — typically 10 cm of fresh snow yields 1 cm of water, though this ratio varies from 5:1 (wet snow) to 30:1 (powder).