So, I put a thermometer out in the middle of an open field to see the temperature difference tonight. I expect it to be impressively different than my more sheltered thermometer closer to the house.
To understand frost you need to know a little about temperature. During the day, the sun heats the land and the land then heats the air above it. At night, when the sun is gone the land loses heat quickly. In fact, the land loses heat faster than the air above it does. This creates colder air nearest the surface. So, if you’re thermometer is several feet above the ground it will be several degrees warmer than the air at the ground.
Air at night tends to be damp and moisture holds on to heat. It’s like warm, humid nights in the summer. That moisture really holds on to the heat. In the desert, the dry air loses heat quickly, causing hot afternoons and cold nights. No moisture, no heat retention in the air.
Frost forms at 32 degrees. Period. Yes, your thermometer says 35 degrees but if your thermometer were at ground level it would be 32 degrees (if you have frost).
That colder air also hugs the ground and eases toward lower places. Cold air is heavy and likes to gather in the lower-lying areas of fields. That’s where you see frost on nights like tonight. For that reason, we see frost quite often around our pond out in the field (lower area).
Keep in mind that the winds must be calm for all this to take place. Wind stirs up the air and causes temperatures to be more evenly dispersed. Skies also have to be clear. Clouds hold in the day’s warmth and cause us to be warmer.
So, my thermometer out in the field is unsheltered and the heat is radiating nicely from the ground. That thermometer currently reads 38 degrees at 8:25! My thermometer under the trees reads 46 degrees. And that’s how you end up with scattered frost across the fields by morning.
Thanks for letting me be a total nerd for this post (ha!).