Why is it snowing?
Snow happens when the air is below freezing from the cloud almost all the way to the ground. Ice crystals grow in the cloud, clump into flakes and fall. It can even snow at 34 to 36°F if the air is dry, because melting and evaporating snow cools the air around it.
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How a snowflake grows
In a cloud below freezing, water vapor deposits directly onto tiny ice crystals, which grow into six-sided flakes. The exact shape depends on the temperature and moisture the crystal passes through, which is why no two look alike. Flakes stick together into clumps on the way down.
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Rain, snow, or something in between
What reaches you depends on the temperature of every layer the flake falls through. All cold: snow. A warm layer that melts it, then a deep cold layer near the ground: it refreezes into sleet. A warm layer, then only a thin cold layer at the surface: it lands as rain and freezes on contact, which is freezing rain.
Is it ever too cold to snow?
Not really, but very cold air holds very little water, so the heaviest snows usually fall when it's just below freezing. The snowiest places get both cold and a steady moisture supply: high mountains and the downwind shores of the Great Lakes.
Questions people ask
- Why does snow make everything quiet?
- Fresh snow is mostly air trapped between crystals, which absorbs sound.
- Why is wet snow heavier?
- When it's near freezing, flakes are partly melted and clump together, holding more water. Very cold snow is light and powdery.