(Items71-80)

                           Getting the Drift
Most snow falls from extensive banks of stable nimbus clouds. The
genesis of a snowflake requires fairly slow and continuous crystallization
with little turbulence. If the wind continues to carry clouds toward high
ground, or if a warm air mass slides smoothly over a cold one, snow can
5 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, Mosthailstones have a layered structure, showing that they
10 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, localized
burst.
15 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
The first paragraph gives information about _____.
1. what nimbus clouds are
2. when crystallization begins
3. what snowflakes lookl ike
4. how snow forms
5. how many days snow lasts
4. how snow forms