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Name: William
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Question:
We live in the Appalachian Mountains. After a bad storm, we found hail in the shape of hemispheres: round on one side and flat on the other. Why this shape?


Replies:
William,

The round side is the one that was facing the ground as the hail fell downwards in a fairly non-turbulent area of the cloud. The hailstone develops a preferential fall orientation as more ice accumulates on it, making the round side heavier and more aerodynamic than the other side. Therefore, that side continues to face downwards each time that the hailstone drops through the cloud, throughout its lifetime of being repeatedly carried aloft and dropped. Once the hailstone becomes too heavy to be lifted or the updrafts in the storm weaken, the hail falls to the ground with the rounded shape on the bottom and flat (or sometimes a scooped out bowl) shape on top.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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