The ground was covered with frost this morning but I believe the low was reported as 35 degrees. It is my understanding that in order for frost to form, the temperature where frosts forms must be at least freezing. Does that mean, that although the air temperature was 35 degrees, the ground temperature was 32 or below?
Yes, if there is frost, then the temperature where the frost occurs (frost on the ground) must be at or below 32°F.
Okay . . . here are two possible explanations:
#1 - The "simplest" explanation is that at your location the temperature was actually at or below freezing that morning. While Metro Airport's official low was 35°F, many WAFB neighborhoods, especially those to the east and north of Baton Rouge, reported a light freeze that morning. The potential variability in temperatures across the WAFB viewing area is exactly why we show our Weather Watcher numbers almost every day. The airport is a "general reference" observation site, but when we get into the low to mid 30°s, a degree or two here or there can make the difference between a light freeze or not. (Fact is, the temperature difference from one side of your backyard to another can sometimes be a degree or more!)
#2 - Yes, it is still very possible -- and not all that uncommon -- to have frost on the ground even though the "official" air temperature only dipped into the mid 30°s. In fact, from an agricultural perspective, we typically use 35°F or 36°F as a guideline for the potential for frost. Frost with "official" temps in the mid 30°s is most likely to occur prior to dawn, under fair-to-clear skies and with winds running light-to-calm.
How? Because the "official" temperature is recorded roughly 4 feet above the ground level. But under clear skies and little wind, the temperature right at the ground level can be 2° to 4° (or more!) cooler than the 4-foot above-ground reading. Often the ground will cool more quickly than the air, especially if the soil is relatively dry. So, while the air temp at 4 feet might be 35°, the temperature at the soil surface may be closer to 30°-32°F -- enough to freeze any condensation (dew).
Bottom line -- if there is frost/ice, it MUST be 32° or below.
Hope this helps!
WAFB Storm Team