Frost dates in Thornton, Colorado
NOAA 1991–2020 normals for a 32 °F freeze. Low spots and the edge of town run colder than downtown.
Short answer
The average last spring frost in Thornton is May 2, and the average first fall frost is October 11, from NOAA's Northglenn station, 4 miles from the middle of Thornton. 1 year in 10 the last frost comes after May 14, and the first one by September 26. That leaves about 162 frost-free days. Thornton is in USDA planting zone 6a.
Weather stations around Thornton
The first row is the date most people in Thornton go by. The others show how far the dates move across the area.
| Station | Last spring frost | First fall frost | Frost-free |
|---|---|---|---|
| Northglenn 4 mi away, 5,407 ft | May 2 1 in 10 after May 14 | October 11 1 in 10 by September 26 | 162 days |
| Brighton 3 SE 6 mi away, 5,016 ft | May 3 1 in 10 after May 16 | October 7 1 in 10 by September 26 | 157 days |
| Denver-Stapleton 12 mi away, 5,286 ft | May 4 1 in 10 after May 16 | October 8 1 in 10 by September 25 | 157 days |
| Denver Water Dept 14 mi away, 5,228 ft | April 30 1 in 10 after May 13 | October 12 1 in 10 by September 28 | 165 days |
| Wheat Ridge 2 14 mi away, 5,398 ft | May 5 1 in 10 after May 18 | October 5 1 in 10 by September 20 | 153 days |
Distances are from the middle of Thornton as the Census Bureau maps it. The average date is NOAA's 50% date: half of years have a later last frost, or an earlier first one. Plan by the 1-in-10 date if a freeze would cost you plants. The planting zone is the USDA 2023 zone for ZIP code 80241, at the middle of the city.
Your exact ZIP code
Thornton covers a lot of ground. For the station nearest your own street, and its planting zone, type your ZIP code.
How these dates work
Frost dates come from NOAA's 1991 to 2020 Climate Normals, which work out from each weather station's daily lows how likely the last spring freeze and the first fall freeze (32 °F) are by each date. We use the station nearest the middle of your ZIP code and show its height and distance, since a station on a hill or by a lake can differ from your yard.
The average date is the 50% date: half of years have a freeze after the spring date or before the fall one. The 1-in-10 dates are NOAA's 10% dates, the ones to plan by if a late freeze would cost you plants. Within one yard, cold air pools in low spots while walls and paving hold heat, so some corners freeze before others.
More frost dates
All of Colorado, station by station.
Other Colorado cities: Denver · Colorado Springs · Aurora · Fort Collins · Lakewood
Where the numbers come from
- USDA Plant Hardiness Zone Map, 2023. Agricultural Research Service, U.S. Department of Agriculture
- PRISM Climate Group, Oregon State University (zone data)
- NOAA NCEI, U.S. Climate Normals 1991–2020, annual/seasonal
- NOAA, Normals calculation methodology (frost-freeze probability dates)
- US Census Bureau, 2025 ZCTA gazetteer (ZIP code centers)
- US Census Bureau, 2025 city population estimates and place gazetteer