Answer to "frost" question

FROST -- George Irwin of Gambrills, MD wanted to know. "If it were possible to have a frost when nighttime low temperatures were expected in the mid to upper 30's." -- 10/11/96

(1) Official temperature recording equipment (often located in an instrument shelter) is typically placed 4 to 5 feet above the ground. In some urban locations, it might even have to be placed on a rooftop. Some of the automatic reporting weather stations at schools are located on school roofs, as well.

(2) At night, the Earth cools faster than the air above the Earth. This means that the temperature at the instrument shelter level is often many degrees warmer than the temperature right near the ground. During the day, especially with sunny skies and no snow cover, the reverse occurs.

(3) One clear, calm morning, the type of morning on which frost is most likely to form, I placed several calibrated thermometers around my house. One was placed on the hood of my car (no trees overhead); another was placed on the ground; a third on an azalea bush (3 feet above the ground); and a fourth under my carport on a box (a foot or so above the concrete. The thermometer on the car was 9 degrees Fahrenheit colder than the azalea thermometer; the ground thermometer was 5 degrees Fahrenheit colder than the azalea thermometer.

Hence, it is possible for the air temperature to be in the mid to upper 30's and still have a frost.

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