Q: Why is it that snow storms can last a long time and precipitate a deep layer of snow, but hail storms are brief and do not result in a deep layer of fallen hail?
A: Most snow falls from extensive banks of stable nimbus clouds. The genesis of a snowflake requires fairly slow and continuous crystallisation with 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.
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Deep layers of hail do occur but, as the largest cumulonimbus clouds are formed on clear sunny days over hot ground and as the same cloud may also precipitate warm rain, hailstones tend to melt quickly. As hail is macroscopically denser than fresh snow, the same mass of water will produce a much deeper layer of snow over a given area.
A: Very severe hailstorms, such as those that cause crop damage in the summer in North America, can produce a deep layer of fallen hail. I have seen such a storm in Nebraska deposit about 10 centimetres of 5-millimetre diameter hailstones over a wide area in less than 20 minutes. At first glance, the appearance on the ground was of heavy snowfall. Of course, the storm moved on, the sun came out and the hailstones melted. In Britain our thunderstorms are not so severe and we only rarely and briefly see a coating of hail on the ground.
A: It is not necessarily true that hailstorms cannot produce a deep layer of fallen hail. Last summer on a hot, sultry day, we had a thunderstorm that lasted for several hours. In the course of it, there was a fall of enormous hailstones (the largest I have ever seen) and they piled up in impressive drifts. Obviously, it was too warm for them to last for a long time on the ground, and they melted, producing a flash flood. Our garage was flooded to a depth of about 5 centimetres as a result.