The National Weather Service serving south central and southeast Wisconsin keeps track of frost depth each year from November 1st through the following spring thaw. Frost depth is the depth into the soil that frost has penetrated, or the depth that the ground temperature has fallen below freezing.
The National Weather Service frost depth gauge consists of a vinyl tube inserted 4 to 6 feet into the ground. The vinyl tube is protected by an outer 1 inch PVC tube. An additional larger PVC tube inserted into the ground at the surface protects the inner PVC and vinyl tubes from damage during the winter.
The inner vinyl tube is marked with a one inch scale, and is filled with a combination of fluorescein dye and distilled water. The fluorescein dye causes the water to take on a green color, however when the water and dye mixture freeze, the mixture turns clear. This allows for a more straightforward interpretation of the depth of the freezing temperatures, or frost depth.
Frost depth has been measured at the National Weather Service office near Sullivan since 2004. Check out the below graph which shows the frost depth and compares it to the snow cover each day, through the winter season. (Click for larger view)
The deepest frost depth measured since record keeping began, was 24 inches in mid February, 2009. The second deepest frost depth was this year, when it reached 22 inches in late February, and lasted through mid-March.
Yearly maintenance of the frost depth gauge is required. Moisture may have collected in the outer tube, which needs to be removed. Also, the fluorescein dye/distilled water solution may need to be replaced.
The latest frost depth readings from the National Weather Service office near Sullivan, as well as across the state are available from November through the following spring thaw at the following link: