Why Frost Goes Deeper Under a Plowed Driveway Than Under Snow
The surface sets how much cold gets in
Air temperature is not ground temperature. Engineers convert one to the other with an n-factor: the share of the air's freezing index that reaches the surface. The Andersland and Ladanyi table, as reprinted in an Owens Corning frost-protection design guide, gives 1.0 for snow, 0.6 to 1.0 for sand and gravel, 0.29 to 1.0 for asphalt and 0.25 to 0.95 for concrete. A higher n-factor means deeper frost.
Why this site uses snow
Every county page uses 1.0, the top of the table, because it gives the deepest frost and a footing estimate should not err shallow. That choice is listed in each page's assumptions.
The same counties, three surfaces
Same weather, same soil, only the n-factor changed:
| County | Snow cover (what this site uses) (n 1) | Sand and gravel, low end (n 0.6) | Asphalt, low end (n 0.29) |
|---|---|---|---|
| Hennepin County, MN | 48 in | 37 in | 24 in |
| Cook County, IL | 27 in | 20 in | 16 in |
| Denver County, CO | 22 in | 19 in | 14 in |
| New York County, NY | 16 in | 13 in | 10 in |
The two right-hand columns are the low ends of each range. The high end of the gravel and pavement ranges is 1.0, the same as snow, which is what a plowed driveway or a sidewalk kept bare can reach.
What it means on a real lot
Frost under a cleared driveway, a walk or a slab edge kept bare can match or beat the snow figure. Under deep undisturbed snow, real frost is often shallower, because snow insulates. A water line that crosses under a driveway is the classic place pipes freeze: the yard on either side is fine and the line under the plowed strip is not.
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).
How Each County's Frost Depth Is Worked Out, and Where It Is Weakest · Why the Frost Depth Here Is Not the Number Your Inspector Uses