The Hows and Whys of Frost Depth Measurements

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. 

Frost depth is typically deepest during the mid to late winter and early spring, when surface temperatures are coldest.  The duration of cold, sub-freezing temperatures and depth of snow cover have the most influence on frost depth.  Deeper snow cover acts as an insulator, which typically prevents deeper frost depths.
Knowledge of the frost depth is useful in determining the potential for flooding during the spring snow melt.   Frozen ground will not allow the water to penetrate into the ground, resulting in additional water runoff.   This was the case this spring when heavy rainfall affected southern Wisconsin between April 8th and 18th. Many locations received between 4 and 8 inches of rain. Temperatures had remained well below normal prior to the rainfall, while snow cover persisted through most of March. This combination of prolonged cold and snow cover resulted in frost depths around 20 inches, from late January into early April. The frozen soil and periods of heavy rainfall in mid-April resulted in many flooded fields, localized street flooding, and many rivers and streams experiencing significant flooding. 
Approximately seventy National Weather Service frost depth gauges are installed in Wisconsin.   The frost gauge program began in the early 2000’s.   Prior to the installation of the frost depth gauge network, frost depth information was retrieved from grave diggers and utility companies. These observations tended to be inconsistent and less reliable.

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.

Frost gauge illustration

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)

MKX Frost Depth

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.

Frost tube

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:

Frost depth readings from around the upper Midwest are also available from the North Central River Forecast Center at the following location:
National Weather Service - Milwaukee/Sullivan

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