How Each County's Frost Depth Is Worked Out, and Where It Is Weakest

Step 1: the nearest weather station

Each county is matched to the nearest NOAA station with 1991-2020 temperature normals. Half are within about 6 miles. 158 counties are more than 17.6 mi from theirs, and their pages say so. Alaska has the most (21), then Georgia and Texas; San Bernardino County, CA, Kern County, CA, San Francisco County, CA are the most populous. A station at a different elevation from where you are building can be several degrees off.

Step 2: a freezing index from daily normals

The freezing index adds up how far below 32°F each day is, over the winter. Averaged daily normals hide freezing days: a day that averages 34°F over thirty years includes plenty of years when it was 20. So the index here uses each day's normal mean and its standard deviation, and adds up the expected degrees below freezing. That runs high where winters thaw and refreeze, which keeps the estimate on the deep side. 107 stations publish no daily spread; their counties use the plain averages and are flagged as likely too shallow.

Step 3: the Modified Berggren equation

Frost depth = λ × √(48 × k × nFI / L), where k is the soil's thermal conductivity, L its latent heat from water content and density, and λ a correction read off a chart from the length and severity of the season. The soil is fixed to the sandy silt in Pavement Interactive's worked example (dry density 100 lb/ft³, 15% water), and this site's code reproduces that example's 2.7 ft. Wetter silts and clays freeze shallower per degree-day; dry sands and gravels freeze deeper.

Where the chart runs out

The λ chart covers a limited range of climates. In 1,784 counties, more than half, the climate falls off its edge and the nearest edge value is used. Those pages are marked. It is the largest source of uncertainty in the model. 877 of them come out at 12 in or less, where the code minimum sets the dig depth whatever λ is.

Where to go next

Compare across the range: Cass County, ND (64 in), Hennepin County, MN (48 in), Cook County, IL (28 in), New York County, NY (16 in), Miami-Dade County, FL (0 in).

Why the Frost Depth Here Is Not the Number Your Inspector Uses · When the 12 Inch Footing Minimum Is Deeper Than the Frost