Polystyrene Foam-like Snow
A few years ago we had a thunder snow storm. The snow
that fell was like polystyrene foam when we walked on it. What causes that kind
of snow? I could walk on it and I barely made my footprints in it. It was
really cool though and that was the first and only time I have seen that.
You are describing could have been either graupel or soft hail.
Graupel occurs when snow falls from a cloud and then through
warm air below the cloud. The points of the ice crystal melt
forming a little ball of snow. More wetted snow can accumulate
on the little ball and it can become a much larger ball of snow.
Since there were thunderstorms at the time, what you saw may
have been soft hail. Soft hail forms in somewhat warmer
conditions and in shorter thunderstorms than occurs in the
summer. Since the temperatures in the thunderstorm are not
extremely cold, the hail can partially melt and then accumulate
more water (or even ice crystals) as it travels up and down
in the cloud, eventually becoming too heavy for the cloud
updrafts to support and falling out as soft hail.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012