Duluth Freeze-Thaw, Snow and Street Data for Pavement Owners

Last reviewed 2026-07-25. Journalists, researchers, and community groups are welcome to cite any figure on this page with attribution and a link.

The climate figures on this page are NOAA's 1991-2020 U.S. Climate Normals for three stations: Duluth International Airport (USW00014913) on the ridge, the Superior, Wisconsin co-op station across the bay, and the Cloquet co-op station up the St. Louis River. A freeze-thaw count is not one of NOAA's published fields, so this page derives it: take the average number of days with a minimum of 32 degrees or lower, subtract the days whose maximum also stays at 32 or lower, and what remains are the days the pavement froze and then thawed.

The street, salt and rule figures come from the City of Duluth, the DuluthStreams program, the Minnesota Revisor of Statutes and MnDOT. Each value below names its source, and the source list at the end links to the pages used, so anyone can check the arithmetic.

Freeze-thaw days by month at the airport

The monthly split is what sets Duluth apart. In the three coldest months the pavement is mostly frozen solid: December, January and February average 72.3 days with a high at or below freezing and only 16.4 freeze-thaw days. Spring is the damage season, with 36.2 freeze-thaw days from March through May, 32 of them in March and April. Fall adds 25.7, mostly in October and November. The practical lesson is to have cracks closed before October and to expect new damage to show up in April and May.

NOAA 1991-2020 monthly and annual normals, Duluth International Airport (USW00014913). The monthly freeze-thaw values sum to 78.3 because of rounding. For August's freezing nights NOAA publishes the special code -7777, a nonzero value that rounds to zero, shown here as a trace.

Hill, bay and river valley compared

Three stations in the same metro area describe three different pavement climates. The hilltop airport gets the most snow. Superior, at lake level across the bay, gets about 30 inches less and has the longest frost-free season. Cloquet, inland on the St. Louis River, has the warmest summer afternoons and the most freezing nights, which gives it the highest freeze-thaw count of the three by about 20 days.

NOAA NCEI 1991-2020 annual and seasonal normals for USW00014913 (Duluth International Airport), USC00478349 (Superior, WI) and USC00211630 (Cloquet, MN). Freeze-thaw count = freezing nights minus the days that never rose above 32 F.

The sealing window in monthly normals

The working rule for sealer is a reading of 50 degrees and climbing, for both air and pavement, plus a dry day after application. At the airport the normal daily high first clears 60 degrees in May, but May nights still average 40.6 degrees, and with a median last freeze of May 17, half of all years see a freezing night after that date. June through August carry nearly all of the dependable warm days, and by October the normal high is back to 51.5.

NOAA 1991-2020 monthly normals, Duluth International Airport (USW00014913).

City streets, salt and stormwater

The city's plow priorities and salt rates explain how road sand and brine reach private driveways, and its stormwater figures explain why the city asks residents to sweep that sand up each spring. By DuluthStreams' figures, chloride from road salt harms fish above about 230 milligrams per liter over long exposure, with a short-term maximum of 860.

• More than 530 miles of roadway cleared by the City of Duluth after snowfalls
• Plow targets: main arterials within 36 hours, residential streets to one lane within 48, alleys within 56
• Salt: 200 to 400 lb per two-lane mile above 30 F, rising to 1,200 to 1,500 lb for dry snow below 15 F
• A half-salt, half-sand blend, plus plain sand in designated sand hazard areas
• Stormwater system: 415 miles of pipe, over 100 miles of ditches, about 3,000 culverts and 11,204 catch basins
• 44 named streams within city limits, 16 of them designated trout streams
• Street Improvement Program funded by a local half-percent sales tax since 2020

Frost depth, thaw and the paving calendar

These rules were written for buildings and state highways, not driveways, but they describe the same ground. Frost deep enough to justify five-foot footings reaches well into any driveway base, and the spring weeks when MnDOT limits truck weights are the same weeks when a driveway's base is at its wettest and weakest.

• Minnesota Rule 1303.1600 sets frost footings at 5 feet in Zone I, which includes St. Louis and Carlton counties, against 3.5 feet in Zone II
• MnDOT's 2026 spring load restrictions ran March 20 to May 15 in its North zone and March 3 to April 20 in its Metro zone
• MnDOT specification 2360.3.A.4 bars the final wearing course after October 15 north of a line from Browns Valley to Holyoke
• Holyoke, the line's eastern end, sits at about 46.47 degrees north in Carlton County, south of Duluth, Proctor, Hermantown and Cloquet

Sources & Methodology

Every figure above was taken from the primary sources below (accessed 2026-07-25). Where a number is derived (arithmetic on published figures) or covers a broader period or area, the text says so.

1. NOAA NCEI, 1991-2020 annual and seasonal normals, Duluth International Airport (USW00014913) — Values in tenths: ANN-TMIN-AVGNDS-LSTH032 1767 (176.7 days), ANN-TMAX-AVGNDS-LSTH032 982 (98.2), ANN-SNOW-NORMAL 861 (86.1 in); DJF, MAM and SON counts for the seasonal split; frost dates from ANN-TMIN-PRBLST-T32FP50 (05/17) and ANN-TMIN-PRBFST-T32FP50 (09/27).
2. NOAA NCEI, 1991-2020 monthly normals, Duluth International Airport — Monthly normal highs, lows and snowfall, and days with highs at or above 60 F.
3. NOAA NCEI, monthly freezing-day counts, Duluth International Airport — Month-by-month fields MLY-TMIN-AVGNDS-LSTH032 (freezing nights) and MLY-TMAX-AVGNDS-LSTH032 (frozen days), used for the freeze-thaw table; August's -7777 code marks a trace.
4. NOAA NCEI, 1991-2020 normals, Superior, WI (USC00478349) — Snowfall 562 (56.2 in), nights at or below 32 F 1587, days at or below 32 F 813, frost dates 05/07 and 10/05, season 148 days.
5. NOAA NCEI, 1991-2020 normals, Cloquet, MN (USC00211630) — Snowfall 662 (66.2 in), nights at or below 32 F 1862, days at or below 32 F 879, frost dates 05/28 and 09/22, season 116 days.
6. City of Duluth, How the City Plows Streets — Over 530 miles of roadway; priority targets of 36, 48 and 56 hours.
7. DuluthStreams, road salt — Salt application table in pounds per two-lane mile by pavement temperature, the 50/50 salt and sand blend, sand hazard areas, and chloride thresholds of 230 and 860 mg/L.
8. City of Duluth, Stormwater — 44 named streams, 16 designated trout streams, 415 miles of pipe, 11,204 catch basins, and the impervious-surface basis of the storm utility fee.
9. Minnesota Rules 1303.1600, footing depth for frost protection — Five feet in Zone I and 3.5 feet in Zone II; St. Louis and Carlton counties are not in the Zone II list.