| A snow storm is usually the result
of the warm air associated with an extratropical cyclone
(as in the diagram above) flowing up and over the cold air surrounding part of the cyclone.
The air being lifted, combined with the abundant water vapor available in the warm air mass,
causes cloud water to keep forming. This cloud water is collected by falling ice particles,
which grow as the super-cooled cloud water freezes. The ice particles in the
clouds grow
and combine to become so large that upward flowing air (updrafts) in the clouds can no longer
support them, and they fall to the ground. The more water vapor there is available to the cloud,
and the stronger the updrafts that cause this water vapor to
condense into cloud water or ice particles, the more
likely it is that snow will form within the cloud. A cold, cloudy day with no snow indicates
that there is either not enough water vapor available to the cloud, or that the rising motion
creating the cloud is not enough to cause snow (or both). (Even at temperatures as low as -40 degrees
F, tiny cloud water droplets remain liquid, until they become attached to an ice particle, and
then they freeze.) Snow can also form from very cold air flowing over a large ice-free lake, a
situation called lake effect snow.
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