A reader asked us about an observation she made at her home last week. She reports that her thermometer read 36 degrees Fahrenheit, but there was clearly frost on the front lawn.
This set of circumstances does not mean that her thermometer is faulty and in need of replacement. Instead, it reflects a nearly daily reality that goes undetected for most of the year until the cold season. It turns out that the air does not radiate heat away nearly as well as the solid ground beneath it. As a consequence of this difference, given 13 hours of nighttime with clear skies, the ground radiates a lot more energy away (and cools rapidly) while the air above struggles to cool as efficiently. Over those many hours, this difference results in a big difference between the ground temperature and the air temperature even as little as 5 or 6 feet above the ground.
On almost every clear, windless morning throughout the year you could measure such a temperature structure, known as an inversion, in the lowest 6 or so feet of the atmosphere. In the late fall, the colder ground may be below 32 degrees and permit frost to form while the air just above it, at the thermometer level, could be several degrees warmer than that. Hence, it is not at all unusual for a set of seemingly contradictory observations, like those reported by our reader, to occur at this time of year.
Steve Ackerman and Jonathan Martin, professors in the uW-Madison department of atmospheric and oceanic sciences, are guests on WHa radio (970 aM) at 11:45 a.m. the last Monday of each month. send them your questions at email@example.com or firstname.lastname@example.org.