Most snow falls from extensive banks of stable nimbus clouds.
The genesis of a snowflake requires fairly slow, continuous crystallisation and little turbulence. If the wind continues to carry clouds towards high ground, or if a warm air mass slides smoothly over a cold one, snow can persist for as long, and over as large a continuous area, as ordinary rainfall. Hailstones are formed in conditions of violent convection inside tall, isolated, unstable cumulonimbus clouds, where there is very strong internal circulation. Most hailstones have a layered structure, showing that they have been cycled several times between freezing and melting levels. Eventually, the cloud grows to the point where its shadow prevents the sun from heating the ground which normally supplies the rising thermals necessary to keep the stones airborne, and so they fall in a brief, localised burst.