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Dear Tom,
A friend in West Virginia mentions they get a lot of freezing rain, and I know Chicago sometimes gets it too. If it is cold enough for freezing rain, why wouldn't it just snow instead?
—Donna Smith, Chicago
Dear Donna,
When a wintertime cold spell gives way to warmer weather, milder air first arrives aloft — several thousand feet up —because the warmer air is lighter and slides over, rather than displaces, the colder air. This wintertime situation yields rain if temperatures are above freezing in the precipitation-formation layer, typically 2,000-6,000 feet aloft, even when surface readings are still below 32 degrees. Rain, falling from the milder air aloft into subfreezing surface air, will freeze on impact to form a coating of glaze on exposed objects — freezing rain. Subfreezing temperatures aloft will yield snow.