Frost - Knowledgebase Question

Deer Park, NY
Avatar for fiskj
Question by fiskj
October 27, 1998
I realize that my question may fit more into the realm of general science than gardening however I did not know who else to ask. I did not concern myself with the possibility of frost the other night because the forcast temperature was above freezing and indeed the temperature did not fall below freezing. the low was reported at 37 degrees however when I went out to go to work I had to scrape frost off of my car. There was frost all over the place. How does frost form when the temperature does not drop to or below freezing?

Answer from NGA
October 27, 1998
It does seem puzzling, doesn't it. My guess would be that you are located in a particularly cold microclimate--a frost pocket. Heat rises, so cold air flows downhill. You may be in a particularly cold spot if you are located at the base of a hill, in a depression surrounded by higher ground, or you are on a hillside and there is something below you to stop the cold air from flowing--another building, woods, etc. There can be a difference of several degrees between the bottom of a hill and the top--so perhaps that 37 degree reading was from higher ground, or near a lake that tends to moderate temperatures. Weather reports can only report the temperature at a given location--so it sounds like you may need to subtract a few degrees from the temperature given on that report to accurately represent your conditions.

Frost can be a localized phenomenon. For example, steady breeze can prevent frost from forming even when the temperature hits 32. On a cool, clear night, the earth (and other objects like your car) radiate heat they accumulated during the day. They can't radiate so much heat that they fall below the air temperature, so the air temperature around the frosted areas must have reached 32.

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