Why was Sunday’s snowfall almost entirely composed of needle-shaped snow?

Dear Tom,

Sunday’s snowfall was almost entirely composed of needle-shaped snow, not the usual snowflakes. Why was that?

Thanks,
Carol Huntowski,
Schaumburg

Jean SmilingCoyote,
Chicago

Cindy Lochmann,
Maple Park

Dear Carol, Jean and Cindy,

The shape of the snowflake is determined by the temperature and the humidity at which the crystal forms. The most ideal temperature for dendritic growth (snowflake formation) is typically centered around 10-degrees F and produces the usual six-sided flakes. On Sunday, the atmosphere in the Chicago area was considerably warmer, with an extensive area of uniform temperature hovering around 23-degrees F, that extended from near the ground to cloud level where the snowflakes were being produced. While those temperatures were low enough for snow, it was not cold enough for typical dendritic growth and the result was the needle snowflakes.

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