MEMPHIS, Tenn. (WMC) -Many Mid-Southerners have had their first frost which on average its around November 2nd but has been as early as September 29. Some think that the first frost coincides with the first day we hit that magical freezing mark of 32 but you don’t have to have freezing temperatures to get frost. Nights when the air temperature is just above freezing, for example in the middle to upper 30s, the temperature at ground level can actually be a tad colder. On those cold nights, ground temperatures can be below freezing. The colder layer of air sinks to the ground because it is denser compared to the air around it. This can mean temperatures are at or just below freezing at ground level. Different amounts of water vapor can linger in the air, normally in liquid form. This liquid in the air can begin to transform into a frozen state once it sinks to the ground forming frost.
Frost forms on cold, clear, calm mornings when the dew-point temperature (frost point) is at or below freezing. When the air temperature cools to the frost point and you the temperatures cool even more, water vapor can change directly to ice without becoming a liquid first. Frost is related to clear nights and open areas because these are the conditions best suited for the loss of heat. Frost, ice formed when water vapor on a surface surface is at or below 32°F (0°C) Meanwhile the surrounding air temperature could be above 32 degrees. Our temperature readings are usually measured from five to ten feet above the ground since this is the level we live in.
At night when that more dense, colder air sinks towards the ground, that’s when frost can form. While the air temperature just above the surface stays above freezing, the surface temperature itself often falls below freezing which allows frost formation on the ground. Elevated surfaces such as our car windshields and patio furniture will also be susceptible to frost, especially when winds are calm.