Frost’s A-Comin’!
Well, as we are now in late August we have to begin to be aware of old Jack Frost. I’d say the first thing you need to know about frost, is that the temperature, which is measured five feet above the ground, only has to be 36 or so, for frost to have the potential to form.
This is because the temperature, under clear, calm conditions, is several degrees colder at the surface of the grass, where the greatest rate of heat loss is occurring. This explains why you will see or hear frost advisories issued, without an actual forecast low of 32 degrees!
Now if you use your phone (or computer) to get weather alerts, there is something you should know about frost and freeze warnings. Once a killing frost has occurred in a region, the NWS does not issue any further frost and freeze warnings for that region.
This can result in a graphical display that can be confusing, until you understand the reason you are seeing what you’re seeing! If your phone gets an alert, you might see a frost advisory has been issued for, let’s say, central and southern Aroostook, (and the advisory might be shaded light blue on the displayed map), yet there may be no frost advisory for northwestern portions of The County. This would happen if northwest Aroostook had already had a killing frost, but central and southern Aroostook had not. The graphical display described above might cause people to think that there wouldn’t be frost issues for northwest Aroostook (because, again, the color on the map would not be shaded blue for the frost advisory there. Why? Because they would have already had a killing frost.
The upshot? As you nurse your garden along, as deep into the coming autumn as you can, remember to always pay attention to actual forecast low temperatures. And also remember that actual “lawn temps” can be several degrees colder when the sky is clear and the wind is calm than it is at the official temperature-measuring height of five feet.
In terms of a true widespread killing freeze, also known as a hard freeze, you usually have to spend a few hours in the upper 20s.
I’ll close with some frost dates that may be of interest. At Caribou, records go back to 1939, and I’ll be using 36 degrees, since frost can occur at that temperature, as previously described. The earliest 36 degree reading at Caribou was August 13th, 1941, the latest 36 was October 5th, 1970. Finally the average date for “first 36” is September the 10th.
Ted Shapiro holds the Broadcast Seal of Approval from both the American Meteorological Society and the National Weather Association. An Alexandria, Va. native, he has been chief meteorologist at WAGM-TV since 2006. Email him at tshapiro@wagmtv.com.
