The National Weather Service measures frost depth in Wisconsin from the beginning of November until the end of the spring thaw. Since the National Weather Service started measuring frost depth, the deepest measure was 24 inches in February 2009.
Frost depth is normally deepest at the end of winter and the beginning of spring before surface temperatures have warmed the underground soil. Typically, underground temperatures are above freezing by May due to elevated surface temperatures, meaning there is no longer any frost depth to measure.
Frost depth measures how far down into the soil frost has penetrated or how deep in the ground the temperature has fallen below freezing.