ST. LOUIS – Frost can form when the reported air temperature is still several degrees above freezing. The reason “why” has everything to do with where we measure the official air temperature. The “where” is about six feet above ground level. That’s where thermometers are placed at weather stations around the world.
We measure temperatures at that level to reduce the influence that radiant heat from the ground can have on the air temperature. The thermometer is placed inside what is called a Stevenson Shelter, which protects the thermometer from excessive heat from direct sunlight and the cooling effects of precipitation. It provides the purest temperature possible.
On a clear, calm night like we are experiencing early this week, temperatures fall quickly very near the ground because, as the air cools, the density of the cool air forces it to collect near the ground. But only a few feet up, where the thermometer is located, temperatures can be several degrees warmer.
So, it is possible to have a reported air temperature in the mid-30s and above freezing, while the temperature of air directly in contact with the ground is below freezing, which allows frost to form.
Using 36 degrees as a proxy for frost, the earliest frost date in St. Louis’s 150 years of weather records was Sept. 23, 1974. The average first frost date over the past 30 years is Oct. 23.
When you look strictly at freezing temperatures, the earliest freeze in St. Louis recorded history was September 28, 1942. But the average first freeze date over the past 30 years is Nov. 2.
So, we are running about a week or so ahead of normal for our first frost and freeze in the St. Louis area.
