How can it snow when temps are in the 40s?

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Dear Tom,
On Monday afternoon, it was snowing with temperatures well into the 40s. How can it snow when it’s that warm?
— Gwen Biasi, Mount Prospect, and Ken Shilt, LaPorte, Ind.

Dear Gwen and Ken,
Temperatures must be 32 degrees or lower in the clouds where snowflakes form, but because the flakes can fall more than 1,000 feet without melting, they can reach the ground with surface readings well above freezing. As it did Monday, snow has fallen in Chicago with temperatures as high as the middle 40s, though it immediately melted upon impact. In addition to Monday’s cloud-layer temperatures below 20, surface dew points were in the teens, adding cooling from evaporation into the dry air that helped keep the flakes from melting. One of the warmest recorded snowfalls occurred during the 1970s in Jacksonville, Fla., when flakes fell with a temperature of 53.
Temperatures must be 32 degrees or lower in the clouds where snowflakes form, but because the flakes can fall more than 1,000 feet without melting, they can reach the ground with surface readings well above freezing.

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