How can frost form on outdoor surfaces even when the temperature is several degrees above freezing?

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Dear Tom,

How can frost form on outdoor surfaces even when the temperature is several degrees above freezing?

Todd Truby, Zion


Dear Todd,
National Weather Service air temperatures are determined from thermometers located in a wooden instrument shelter about 4.5 feet above the ground. The shelter is placed in an open area, away from any obstructions. On clear, calm nights, the earth’s surface radiates heat out to space more efficiently allowing the ground to be colder (perhaps by several degrees) than the air above it. Because the thermometers are raised off the ground, reported shelter-level temperatures may be above freezing while the ground surface below is sub-freezing. If those surfaces become lower to the dew point temperature, frost will form.

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