Photo: Billy Hathorn · CC BY-SA 3.0Thornton’s winters are colder and snowier than most of the U.S.
A typical July afternoon reaches 90°, a January night drops to 18°, and about 74 inches of snow falls in a normal winter.
A year in Thornton at a glance
Each month’s typical high and low, and what it brings.
What stands out about Thornton’s climate
Compared with towns across the U.S., with Thornton’s place in Colorado beside it.
What a year in Thornton feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
About 155 frost-free days a year
In half of years the last 32° night comes by May 3 and the first by October 7. Frost and day counts come from NOAA’s station at Brighton 3 Se, CO, 6 miles from the city’s center point, 180 ft lower.
The 2023 USDA map puts Thornton’s center in zone 6a: the coldest night of a typical winter, averaged over 1991–2020, reaches about −9°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 74 inches of snow a winter
Snow, measured at Broomfield 1.1 Ne, CO, falls mostly in March. Precipitation (rain plus melted snow) adds up to 14 inches a year; May is the wettest month and January the driest. Rain or snow falls on about 87 days a year.
Rain + melted snow by month
Inches of water
Snowfall by month
Inches of snow
Heating and cooling a home in Thornton
89% of the work is heating, so the furnace matters far more than the air conditioner. Months averaging under 55° run from October through April.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Brighton 3 Se, CO.
Living with Thornton’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
155 frost-free days in zone 6a. The last frost comes by May 3 in half of years, so many gardeners wait a week or two past that to set out tomatoes. The first fall frost comes by Oct 7.
Months with highs of 62–84° and lows of 40° or more: May, June and September. 38 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in Thornton from natural hazards, hail is the largest share (31%); heat waves, cold waves, winter storms and ice storms together are 22%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Thornton and its neighbors
Todd Creek has the coldest January nights next door (17°, 2° colder). Differences under a degree are within what the 5 km grid can tell apart. Tap a town to see its climate.
Climates most like Thornton’s, farther away
TownRatings’ own comparison of NOAA 1991–2020 normals, not a NOAA product: places of 5,000+ at least 150 miles away, one per state, closest on January low, July high, precipitation and snow.
| Town | Jan. low | July high | Precip. | Snow |
|---|---|---|---|---|
| Thornton, CO | 18° | 90° | 14" | 74" |
| Herriman, UT | 22° | 90° | 15" | 74" |
| Casper, WY | 15° | 89° | 12" | 82" |
| Pocatello, ID | 19° | 87° | 15" | 64" |
| Billings, MT | 15° | 89° | 14" | 48" |
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value. Snow and frost-free days come from the nearest weather station, so neighbors often share them.
Warmer than it used to be
The last 30 years in Thornton averaged 1.6° warmer than the 1900s. The biggest change is in March (+3.5°).
A bigger shift than in 64% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods. In a city of 148,000 people, part of the change is the city itself: pavement and buildings hold heat (the urban heat island).
How Thornton compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Thornton’s place among Colorado’s.
Thornton weather by month
Highs, lows and precipitation from NOAA’s grid at Thornton’s center; snow, wet days and snow on the ground from Broomfield 1.1 Ne, CO. 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | 30-yr range |
|---|---|---|---|---|---|---|
| January | 46° | 18° | 0.5" | 9.0" | 5 | 22° – 38° |
| February | 47° | 21° | 0.5" | 12.4" | 6 | 29° – 42° |
| March | 56° | 28° | 1.0" | 16.3" | 7 | 36° – 49° |
| April | 63° | 34° | 1.8" | 8.1" | 9 | 42° – 54° |
| May | 72° | 44° | 2.4" | 2.6" | 11 | 52° – 62° |
| June | 84° | 53° | 1.5" | 0.0" | 8 | 64° – 73° |
| July | 90° | 59° | 1.7" | 0.0" | 9 | 70° – 77° |
| August | 87° | 56° | 1.5" | 0.0" | 9 | 68° – 75° |
| September | 79° | 48° | 1.2" | 0.0" | 7 | 59° – 68° |
| October | 66° | 36° | 1.0" | 5.2" | 6 | 43° – 57° |
| November | 54° | 26° | 0.7" | 8.4" | 5 | 32° – 47° |
| December | 45° | 18° | 0.5" | 11.9" | 5 | 25° – 36° |
| Year | 66° | 37° | 14.2" | 73.9" | 87 |
How much a month can swing
A normal is an average. January swings the most: from 1991 to 2020 its average ranged from 22° to 38°. July is the most reliable, staying within 7°.
Where Thornton’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at Thornton’s center.
- Snow: Broomfield 1.1 Ne, CO.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Brighton 3 Se, CO, 6 miles from the city’s center point, 180 ft lower; the nearest station that passes our checks for distance, height and agreement with the grid.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about Thornton’s climate
What is the weather like in Thornton?
A typical July afternoon reaches 90°, a January night drops to 18°, and about 74 inches of snow falls in a normal winter. Colder January nights than 68% of U.S. towns; hotter July afternoons than 75% of U.S. towns.
How cold does it get in Thornton in winter?
December nights average 18°. About 163 nights a year reach 32° or below and 8 reach 0° or below. In the coldest December of the last 30 years the month averaged 25°.
How much snow does Thornton get?
About 74 inches in a normal winter at Broomfield 1.1 Ne, CO, most of it in March.
How hot does it get in Thornton?
July, with a typical high of 90° and a low of 59°. About 49 days a year reach 90° and 3 reach 100° (Brighton 3 Se, CO).
When is the first and last frost in Thornton?
In half of years the last spring frost (32°) comes by May 3 and the first fall frost by October 7, about 155 frost-free days, at Brighton 3 Se, CO.
What growing zone is Thornton in?
Zone 6a on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about −9°.
What is the best time of year to visit Thornton?
By one simple rule (highs of 62–84° and lows of 40° or more), May, June and September. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Thornton getting warmer?
Yes: 1991–2020 averaged 1.6° warmer than 1901–2000 on NOAA’s grid. A bigger shift than 64% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Thornton.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.