Feel free to straighten me out on my errors or add comments! 
From hilltop to bottom/valley I would see a 1-3 degree change in the thermometer. It was very interesting to see the difference. And this was just a variation of probably 200-250 feet at the most.
)
Intheswamp said: With some cold air in the 30Fs coming in tonight it got me to wondering about frost and what conditions are required. I, through, looking back through my local data have settled on November 14 as my "first frost date"...it certainly did not frost this year on that date, though! Anyhow, here are some thoughts and things I came up with...
It has been noted that there is an area next to the ground that reaches roughly up about 5' feet from ground level that can be "super cooled" to five or six degrees colder than the average air temperature.
Low-lying areas are more prone to frosts than adjoining higher elevation areas. For instance...a valley is more apt to have a frost than the hills surrounding the valley. That makes sense being as cold air sinks and warm air rises. Also, calm or near-calm winds can enhance the chance of frost...I've woken up after a clear, cold, calm night to find frost looking like a solid light snow had fallen.
I've found on several places these temperatures noted for frost:
From 38 to 42 F can lead to patchy frost.
From 33 to 37 areas of frost.
From 32 and below widespread frost/freeze.
So, it looks like there can be a chance of frost anywhere from the low-40s on down...with the chance of frost increasing with every few degree-drops in temperature.
There also has to be moisture in the air for frost to form...pretty easy for that to happen in the humid deep south where I'm located. We just got 1.84" of rain yesterday so we have plenty of moisture in the environment to check that box for frost creation.
Anyhow, just some points of interest regarding frost that I thought I'd share...right or wrong.
Weedwhacker said: I would have been happy to see frost this morning...
Weedwhacker said: I would have been happy to see frost this morning...
RpR said: Some of those numbers are wrong for reason of physics.
Water is at equilium at 32 degrees Standard Temperature and Pressure.
Pure water can change to ice at that point but other factors do not mean it absolutely will.
I.E. at glass of Pure water is still liquid at 32 degrees STP, but can change back to liquid as at the temp. it is at equilibrium.
The water in a glass of ice water is 32 degrees or equilibrium.
Non-Pure water can have a even lower freezing poing, ditto for higher boiling point.
Frost is frozen water, so, even if the water on the plants is pure, it will not freeze above 32 degrees F.
Cold air is heavier and settles, so if you have a low point on your lawn, it may frost while the rest of your yard, and flowers are untouched due to temp. inversion.
It is a bit more complex than that, but if the air temp is not at least, and really below 32, you will not get a frost.
I could swear that we've had frost when the temperature was not below 32F. 
We only hit 39F this morning and I didn't see any frost. The forecast is for 34F and 5mph winds to calm in the morning...plus a "Frost Advisory". I've got some graphing thermometers that, *if* I an remember to 
, I'll set outside...one at ground level and one at 5' and see what they record. Not NASA grade equipment, but it might be interesting to compare them. It only seems natural that ice would form at 32F or lower, but....
TomatoNut95 said: I hate the winter. I hate frost. I hate temps below fifty. (I sound like Grumpy Smurf, don't I?
Here it can frost around the thirty's and usually upsets my poor plants. I have a lot of insulation board I use to cover my plants with to protect from frost. Works wonders, that stuff.
