Snowfall
Precipitation & snow falling snow
Snowfall occurs when ice crystals or flakes formed from them reach the ground from clouds. The air at ground level does not always need to be below 0 °C.
From water vapour to flakes
In cold clouds, ice crystals grow as water vapour deposits onto them. They may join into larger flakes or collect supercooled cloud droplets. If they reach the ground without melting completely, snow is falling.
Why snow can fall above freezing
Melting takes heat and time. A flake may therefore survive a warm layer that is not too deep. Melting and evaporation also cool the surrounding air. What matters is temperature and humidity along the entire journey, not just the reading on a window thermometer.
What a forecast in centimetres means
A new-snow forecast estimates the depth of freshly fallen snow. Density matters greatly. The rule of thumb that one millimetre of water makes one centimetre of snow is only a rough assumption and can be far from the result.
Falling is not the same as settling
Grass may turn white while a warm road stays wet. Ground warmth, sunshine, traffic and precipitation intensity all affect settling. Snow has not agreed to arrange itself as neatly everywhere as the forecast number suggests.
For icy conditions, current surface observations and warnings matter alongside the expected new-snow amount.