Why Frost Forms at Temperatures Above Freezing

Have you ever wondered why frost can form on nights when the temperatures only fall to 36 or 37 degrees? The Kentucky Mesonet recently added some extra temperature sensors at varying heights that show just why this could occur. The plot below shows a 24-hour trace of the temperature measured at 0.5 meters above the ground (in blue), at 2 meters above the ground (in red), and at 3 meters above ground (in orange). An official temperature reading is taken in a special instrument shelter that shades from the direct sunlight as well as provides ventilation AND is situated at 2 meters above the ground level. So the readings you see at Standiford Field (aka, the Louisville Airport) as well as at Bluegrass Airport in Lexington are taken at 2 meters above the ground.

Notice on the right side of the image above, the readings taken at night are cooler closer to the ground. The difference between the 0.5- and 2-meter temperatures in this plot are on the order of 3-5 degrees at night! On a clear and calm night, the temperature close to the ground easily can be 5 or more degrees cooler than at the standard height. Thus you sometimes will see a forecast of frost even with lows in the upper 30s. This property of warmer air above the surface is night is called an "inversion", as it is typical to see temperatures drop as you go aloft during the day (see the daytime readings in the plot above).