Denver ice dams and freeze-thaw: what 20 winters of weather data show
Denver winters swing above and below freezing constantly. At Denver International Airport, winters from 2006–07 to 2025–26 had a median of 122.5 freeze-thaw days and 40 days with an inch or more of snow on the ground. Sunny daytime melt followed by an overnight refreeze is what fills gutters with ice and builds ice at roof edges.
122.5
Freeze-thaw days
High above 32°F and low below 32°F on the same day.
median per season, Denver International Airport, 2006–07 to 2025–26
40
Snow-cover days
At least 1 inch of snow on the ground at the daily observation.
median per season, Denver International Airport, 2006–07 to 2025–26
22.5
Snow-cover days with a thaw and refreeze
Snow cover of 1 inch or more on a freeze-thaw day.
median per season, Denver International Airport, 2006–07 to 2025–26
13
Snow-cover days that stayed below freezing all day
Snow cover of 1 inch or more and a high at or below 32°F.
median per season, Denver International Airport, 2006–07 to 2025–26
How ice dams form
Heat escaping from the house warms the roof deck and melts the underside of snow on the roof. The meltwater runs down to the colder eave overhang and refreezes into a ridge of ice. Water then pools behind that ridge and can back up under roofing, causing leaks, wet insulation, and ceiling stains.
Denver also has a sun-driven pattern. Strong sun melts snow on sunny slopes, and the water refreezes overnight in shaded gutters and eaves, which gives icicles and ice-filled gutters. The two snow-cover measures in this data map onto the two patterns:
- Snow cover plus an all-day freeze is classic ice-dam weather, where the melt comes mainly from heat escaping through the roof. The airport median is 13 days per season.
- Snow cover plus a thaw and refreeze is melt-refreeze at the gutters and eaves. The airport median is 22.5 days per season.
Snow depth in this data is measured on the ground at each station, not on roofs. Real roofs vary with sun exposure, pitch, insulation, and ventilation, so these figures describe the weather, not the condition of any one roof.
Month by month at Denver International Airport
Monthly figures are averages per month across the winters in which that month was counted (2006–07 to 2025–26).
| Month | Freeze-thaw days | Snowfall (in) | Snow-cover days | Snow cover + all-day freeze | Snow cover + thaw/refreeze |
|---|---|---|---|---|---|
| October | 6.1 | 3.9 | 1.2 | 0.5 | 0.7 |
| November | 18.5 | 5.3 | 3.5 | 1.3 | 2.1 |
| December | 21.9 | 9.0 | 9.2 | 4.3 | 4.8 |
| January | 23.2 | 7.1 | 9.5 | 3.9 | 5.4 |
| February | 19.6 | 9.3 | 9.6 | 3.9 | 5.5 |
| March | 18.3 | 9.4 | 4.8 | 1.1 | 3.5 |
| April | 10.7 | 4.9 | 1.8 | 0.1 | 1.3 |
| May | 1.7 | 1.4 | 0.4 | 0.0 | 0.3 |
March has the most snowfall at the airport, averaging 9.4 inches. February and January hold snow cover longest, averaging 9.6 and 9.5 days with at least an inch on the ground. February has the most snow-cover days with a thaw and refreeze, averaging 5.5.
The median first fall freeze at the airport is October 11 and the median last spring freeze is May 7 (18 seasons with a recorded freeze).
Across the metro
Station figures are medians per season. Freeze-thaw days, snowfall, and snow-cover days are totals for a July 1 to June 30 season.
| Area | Station | Elevation (ft) | Freeze-thaw days | Snowfall (in) | Snow-cover days | Snow cover + thaw/refreeze | Seasons counted (freeze-thaw) | Seasons counted (snow cover) |
|---|---|---|---|---|---|---|---|---|
| Denver's official airport record | Denver International Airport | 5,404 | 122.5 | 48.9 | 40 | 22.5 | 18 | 18 |
| Central Denver | Denver Water Department | 5,225 | 119 | 40.0 | 30 | 24 | 13 | 9 |
| West metro: Wheat Ridge, Lakewood, Arvada | Wheat Ridge 2 | 5,395 | 144 | 73.0 | 58 | 37.5 | 18 | 18 |
| North metro: Northglenn, Thornton, Westminster | Northglenn | 5,370 | 126.5 | 50.3 | 32.5 | 16 | 16 | 15 |
| Southwest Denver and Littleton | Marston Filter Plant | 5,520 | 120.5 | 44.9 | 37 | 23 | 16 | 16 |
| Southwest edge near Roxborough and Waterton Canyon | Kassler | 5,498 | 125 | 60.1 | 41 | 22.5 | 16 | 16 |
| South metro: Centennial and Englewood | Centennial Airport | 5,870 | 123.5 | Not recorded | Not recorded | Not recorded | 20 | Not recorded |
Wheat Ridge 2 (West metro: Wheat Ridge, Lakewood, Arvada) has the highest median snowfall, 73.0 inches per season versus 48.9 at the airport, and also the most snow-cover days with a thaw and refreeze, 37.5 versus 22.5 (about 1.7 times the airport figure). The lowest median among stations with snow records is 16 days at Northglenn.
Each station describes conditions at its own site, not a whole city. Stations also differ in which seasons met the completeness rule, so their medians are not computed over identical winters. Compare them as a general pattern, not as a ranking.
What winter does to gutters
- Ice-filled gutters add weight that can loosen hangers and open seams.
- Frozen downspouts back water up into the gutter.
- Downspout discharge can refreeze on walks and driveways, so check where the water lands.
- Some gutter-guard designs can ice over on top. Guards do not stop ice dams, because ice dams start with heat from the house reaching the roof.
- Heat cables can keep a drainage path open, but they do not fix heat loss, and they must follow the product’s installation and electrical requirements.
Related pages: seamless gutters in Denver and gutter guards in Denver.
What prevents ice dams
- Reduce the heat reaching the roof deck: attic air sealing, insulation, and balanced intake and exhaust ventilation.
- At roof replacement, ask whether an ice-barrier membrane at the eaves is part of the proposed scope.
- Make sure gutters and downspouts drain before winter starts.
Best Roof And Gutter evaluates roof-edge ice conditions, drainage, ventilation, and insulation factors. See the ice dam prevention service page.
What to watch for safely this winter
Look from the ground or from inside the house.
- Icicles forming along the same stretch of roof edge, season after season
- Ice building up at the eaves
- Ceiling or wall stains near exterior walls
- Frost or damp insulation in an attic you can safely reach
- Frozen downspouts, or gutters that overflow during a thaw
- Ice on walks and driveways where downspouts discharge
Stay off the roof
- Do not climb onto a snowy or icy roof.
- Do not chip or pry at ice on the roof or gutters.
- Do not put salt or chemicals on the roof.
- Keep people and pets away from large icicles and from under snow-loaded eaves.
Ice dam and freeze-thaw questions
Methodology and download
Source: NOAA NCEI Global Historical Climatology Network Daily (GHCN-Daily). Coverage is 2006-07-01 through 2026-06-30 (20 winter seasons, 2006–07 to 2025–26). Summaries generated 2026-10-21.
Seasons run July 1 to June 30. A season counts for a measure only when the needed values are present on at least 97% of days from September 1 through May 31; missing days inside a counted season are not filled in. Station figures are medians across counted seasons, so stations can differ in which seasons they include. Cooperative stations report the 24 hours ending at the observer's reading; airports report the calendar day. Snow depth is measured on the ground, not on roofs.
Definitions
- Freeze-thaw days:
- High above 32°F and low below 32°F on the same day.
- Freezing nights:
- Low at or below 32°F.
- All-day freeze days:
- High at or below 32°F.
- Snowfall (in):
- Total new snowfall, July 1 to June 30.
- Snow-cover days:
- At least 1 inch of snow on the ground at the daily observation.
- Snow cover + all-day freeze:
- Snow cover of 1 inch or more and a high at or below 32°F.
- Snow cover + thaw/refreeze:
- Snow cover of 1 inch or more on a freeze-thaw day.
Stations
- Denver International Airport (NOAA ID USW00003017), daily observation: calendar day; primary station for the airport figures
- Denver Water Department (NOAA ID USC00052223), daily observation: 24 hours ending at the observer's daily reading
- Wheat Ridge 2 (NOAA ID USC00058995), daily observation: 24 hours ending at the observer's daily reading
- Northglenn (NOAA ID USC00055984), daily observation: 24 hours ending at the observer's daily reading
- Marston Filter Plant (NOAA ID USC00055402), daily observation: 24 hours ending at the observer's daily reading
- Kassler (NOAA ID USC00054452), daily observation: 24 hours ending at the observer's daily reading
- Centennial Airport (NOAA ID USW00093067), daily observation: calendar day
One row per station per season.