In my travels throughout the region, one person after another talks about how they heard that the soil had frozen more than 5 feet this winter. To find out how accurate these rumors were, I began speaking to people that may have more than rumors: DATA.
As part of the Illinois State Water Survey's Illinois Climate Network, soil temperature measurements are collected by instruments buried 4 or 8 inches below sod or 4 inches below bare soil at the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC) outside of Monmouth. This is good information to have, but is not what I was looking for.
I then contacted meteorologists at the National Weather Service's Davenport Field Office, 60 miles North of Monmouth near Eldridge, Iowa. In addition to modeling current weather variables and satellite data to forecast the regional weather, these scientists measure how deep the frost reaches below the soil surface under sod.
Just how deep did the soil freeze and how do we know? To measure frost depth, the meteorologists read what is called a frost depth gauge. This gauge is a flexible, clear tube that is filled with a liquid that changes temperature when frozen. The gauge fits tightly into a larger, covered PVC sleeve that is buried 60 inches deep. Each morning at 7 a.m., a meteorologist pulls the gauge out from the sleeve and collects two measurements: how deep the soil is frozen (frost depth) and how deep the soil surface has thawed.
Early each spring, the top several inches of soil thaws while soil deeper in the profile remains frozen. The top 8 inches of the soil profile tend to reflect more day to day temperature variability, while soil below 8 inches tends to reflect more seasonal variability.
Despite the record low temperatures during February, and the many rumors of the frost line being more than 5 feet deep throughout Western Illinois, the deepest that the frost line reached during the entire 2013-2014 winter at the Davenport NWS office was 27 inches. Despite the sporadically warm high over the past several weeks, the soil is still frozen 24 inches deep (Figure). The top of the soil profile has thawed between 3 and 5 inches between March 10 and 16, although the cold weather on March 16 re-froze the entire soil profile down to 24 inches on March 17.
The National Weather Service has cooperators throughout the region who tend to be average citizens with an interest in weather. Cooperators collect and then share weather data with the National Weather Service. One cooperator is located in Altona, Illinois in North-central Knox County. At this location, the frost depth on Friday March 14 and Monday March 17 were 15 and 14 inches deep, respectively.