Why does frost sometimes only form on car windows, but not on grass?

Ask Tom Why

Dear Tom,
My car sits outside overnight. Frost sometimes forms on the car windows this time of year, but no place else like on the lawn, garden, etc. Why just my car windows?

James Morton, Morris, Ill.

Dear James,
The ground retains some of summer’s warmth into the early weeks of the cold season and acts as a frost-defeating heat source. Your car windows stand above the warm ground and are therefore free to cool at night. They lack any means of staying warm. The result is no frost on the lawn, even as frost flourishes on your car windows in the colder nighttime hours. This selective frost formation is temporary — it won’t continue indefinitely. Once the ground cools and freezes, nighttime frosts will be easily able to cover both your lawn and your car when it’s parked outdoors.

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