Hemispheric Hail Shape
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?
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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Update: June 2012