Frost at 35 Degrees Fahrenheit on my Thermometer?
Is that Possible?
Since the weather has been more than a little flaky lately, I thought we would talk a little about frost. Why are we talking about frost in mid-April? Well, because a friend of mine (Call him Bob) from Southern Indiana mentioned the other day that us Northern flatlanders in the state were looking at a hard frost and he was looking at 35 degrees and would be good to go. When he woke up “Good to go” still had frost. The low was 35 degrees according the TV and his thermometer, just as advertised by the weather folks on TV. So this morning when he arrived back up state and we had some coffee the following conversation took place…
Bob: Frost at 35 degrees? WTF? They even said on TV the low was 35…my fancy digital thermometer hooked to my furnace controls said 35 was the low outside…why the frost? It is my understanding that for frost to form, the temperature where frosts forms must be at least freezing. That is still 32 right? The feds didn’t get that changed as well did they? Or did I miss something in science class? Me: Bob, you missed a lot in science class, but that is beside the point. But in this case you are right…Yes, if there is frost on the ground, then the temperature must be at or below 32 F. Bob: Okay…then what the … look the wife in angry because she told me to cover her flowers and I said yea, yea, and now they look like they are done being flowers and I have to go shopping with her Saturday to get new ones and I just do not get it. Me: Well, first there is more than one answer. The first answer is that at your location, at the exact spot of your wife’s flower garden the temperature was actually at or below freezing. Although, the official low was 35 degrees, there is a potential for temperatures across an area to vary a few degrees. The weather observation site may have been 35 degrees, but when we get into the low to mid 30s, a degree or two here or there can make the difference between a light freeze or no freeze at all. I have read that the temperature difference from one side of your backyard to another can sometimes be a degree or more!. Also, my partner Art will tell you that he has often experienced a five degree temperature difference between the house and his barns.
Bob: Yea but my thermometer said 35 degrees and my ground is flat around the house. So I don’t buy that answer. I think it should be pretty uniform across my yard.
Me: Well Bob that is why I have another answer. (Bob rolls his eyes…mumbles something about geeks, to much reading and I think called my cows ugly…anyway) Believe it or not it is not at all uncommon to have frost on the ground when the air temperature is 35 degrees according to your thermometer, which is likely at least four feet above-the-ground. In fact, for us farmer types, 35 or 36 degrees is used as a guideline that frost is possible. Frost at or around these mid 30 degree temperatures is most likely to occur prior to dawn, under fair-to-clear skies and with light winds or none at all.
Bob: OK, Mr, Wish I Dream I was Weatherman…how is that?
Me: Because the temperatures you hear in a weather report or likely even your thermometer records temperatures at approximately four feet above the ground. And because of this, under he right weather conditions as we discussed, the temperature at the ground level can be 2 to 4 degrees cooler than the 4-foot above-ground sampling. So, while the air temp at four feet high might be 35 degrees, the temperature at ground level may be closer to 30-32 degrees, which is enough to freeze any dew (condensation). So buddy, science has not changed and the conditions were right, so it got 32 degrees Fahrenheit or below at your place. Next time listen to your wife, cover the flowers, because I recall she listened in class, you just sit there and stared at her for an hour.
Good Luck this growing season, be safe in all you do and don’t forget to take a moment and enjoy your time connected to the land and agriculture.