Published: 2007-10-07 12:55:29
Updated: 2007-10-07 12:55:29
Posted October 7, 2007 12:55 p.m. EDT
By Tommy Hoots
MIKE MOSS SAYS: Tommy, The trick in answering your question is that there can be a large difference, on cold nights with fair skies and light or calm winds, between the reported or forecast air temperature (which applies about 4 to 5 feet above the ground) and the temperature of exposed surfaces such as plant leaves, mailboxes, roofs, car windshields and so on. It is not unusual under the right conditions to have pockets of frost on mornings when the air temperature only drops into the mid or upper 30s. Of course, in addition to this effect, one also has to consider that localized variations in terrain, elevation, cloud cover, wind speeds, and land use pattern can lead to significant temperature variations from one nearby location to another.
As implied abovge, the strongest surface layer temperature inversions (colder surfaces beneath warmer air a few feet above) and the largest horizontal variations from place to place typically occur on almost clear nights in the absence of much wind. Conversely, nights with persistent wind or significant cloud cover tend to reduce the difference in temperature between one place and another, and also tend to prevent or mix out surface layer inversions. In those cases, plants usually only freeze if the air temperature falls below freezing as well.