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Name: Jackie
Status: other
Grade: other
Location: NE
Country: N/A
Date: 11/29/2005

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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