Boulder, Montana climate: Normal weather by month, frost dates and records
Boulder's normal year runs from 12°F December nights to 83°F July afternoons, with 10.8 inches of precipitation a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 5.9°C, December lows of −11°C, July highs of 28°C, 274 mm of precipitation a year.
- NOAA stationBoulder
- Station IDUSC00241008
- Location46.23°N 112.11°W
- Elevation4,904 ft (1,495 m)
- Population1,305
On October 22, the normal high in Boulder is 54°F and the normal low 26°F, and the chance of measurable rain or snow is 21%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Boulder.
- Jan 35° 12°
- Feb 38° 14°
- Mar 47° 21°
- Apr 55° 27°
- May 64° 35°
- Jun 72° 43°
- Jul 83° 48°
- Aug 82° 46°
- Sep 72° 38°
- Oct 57° 28°
- Nov 42° 19°
- Dec 34° 12°
Each month: normal high (large) and low, °F. Fill: the month's normal mean, colder blue to hotter red.
What is the weather like in Boulder each month?
July is the warmest month, with normal highs of 83°F; December the coldest, with normal lows of 12°F. The most precipitation falls in June (2.30 in), the least in February (0.19 in).
| Month | High | Low | Mean | Precip. | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|
| January | 35°F | 12°F | 23.5°F | 0.28 | 5 | <1 | 30 | 9 h |
| February | 38°F | 14°F | 25.9°F | 0.19 | 5 | <1 | 27 | 10 h 20 min |
| March | 47°F | 21°F | 33.8°F | 0.40 | 6 | <1 | 28 | 11 h 50 min |
| April | 55°F | 27°F | 40.7°F | 0.78 | 7 | <1 | 23 | 13 h 30 min |
| May | 64°F | 35°F | 49.4°F | 1.86 | 12 | <1 | 10 | 15 h |
| June | 72°F | 43°F | 57.2°F | 2.30 | 12 | 1 | 2 | 15 h 50 min |
| July | 83°F | 48°F | 65.4°F | 1.38 | 8 | 6 | <1 | 15 h 30 min |
| August | 82°F | 46°F | 64.1°F | 1.12 | 7 | 4 | 1 | 14 h 10 min |
| September | 72°F | 38°F | 55.0°F | 1.10 | 7 | 1 | 6 | 12 h 40 min |
| October | 57°F | 28°F | 42.6°F | 0.64 | 7 | <1 | 21 | 11 h |
| November | 42°F | 19°F | 30.5°F | 0.48 | 6 | <1 | 27 | 9 h 30 min |
| December | 34°F | 12°F | 22.5°F | 0.27 | 6 | <1 | 30 | 8 h 40 min |
| Year (annual) | 57°F | 29°F | 42.6°F | 10.80 | 89 | 12 | 205 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. |
|---|---|---|---|---|
| January | 2°C | −11°C | −4.7°C | 7 mm |
| February | 3°C | −10°C | −3.4°C | 5 mm |
| March | 8°C | −6°C | 1.0°C | 10 mm |
| April | 13°C | −3°C | 4.8°C | 20 mm |
| May | 18°C | 2°C | 9.7°C | 47 mm |
| June | 22°C | 6°C | 14.0°C | 58 mm |
| July | 28°C | 9°C | 18.6°C | 35 mm |
| August | 28°C | 8°C | 17.8°C | 28 mm |
| September | 22°C | 3°C | 12.8°C | 28 mm |
| October | 14°C | −2°C | 5.9°C | 16 mm |
| November | 6°C | −7°C | −0.8°C | 12 mm |
| December | 1°C | −11°C | −5.3°C | 7 mm |
NOAA publishes no 1991-2020 snowfall normal for this station, and no record within 25 miles keeps one, so snow is left out here rather than estimated.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00241008; annual normals. Wet days: 0.01 inch or more; 90°F days and freezing nights: NOAA's day counts. Daylight: sunrise to sunset on the 15th, computed from the latitude.
When is the best weather to visit Boulder?
Normal highs sit between 65°F and 85°F in June, July, August and September. Of those, September has the fewest days with rain (7).
That is a measurement, not a recommendation: it counts the months whose normal afternoon high sits between 65°F and 85°F, where a light layer is enough by day.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00241008.
When is the last frost in Boulder?
In Boulder, the last spring frost (32°F) comes after June 7 in half of years and after June 25 in one year in ten; the first autumn frost comes before September 8 in half of years and before August 25 in one year in ten.
Frost-free in half of yearsFrost in 1 to 9 years out of 10Frost most years
- Last spring frost, half of years after
- June 71 year in 10 after June 25; 9 in 10 after May 24
- First autumn frost, half of years before
- September 81 year in 10 before August 25; 9 in 10 before September 21
NOAA puts the median frost-free season at 90 days, and the season without a hard freeze (28°F) at 121 days.
A hard freeze (28°F) comes after May 21 in half of springs and before September 21 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Boulder in zone 5a in ZIP code 59632, the nearest ZIP area. By the map's own method, the average of each year's coldest night over 1991-2020, Boulder's NOAA station is in zone 4b: the coldest night of a typical year reaches −24°F (22 complete years); the map interpolates between stations and terrain, so the station and the ZIP area can differ by half a zone.
Frost dates for every NOAA station in Montana · Frost dates by ZIP code
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities), station USC00241008; USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; the station zone: ACIS daily lows, 1991-2020. A 10% date means one year in ten has a frost after it in spring or before it in autumn.
How hot and how cold does it get in Boulder?
In a normal year in Boulder, 12 days a year reach 90°F, 205 nights fall to 32°F or colder and 89 days bring measurable precipitation.
- Days at 90°F or more
- 12a normal year
- Days at 100°F or more
- fewer than 1a normal year
- Nights at 32°F or colder
- 205a normal year
- Days that stay below freezing
- 37high of 32°F or less
- Days with 0.01 inch of precipitation
- 89a normal year
The highest temperature in this station's record (since 1880) is 101°F, on July 14, 2002; the lowest is −42°F, on December 22, 1990.
The wettest calendar year on that record is 1975, with 17.88 in; the snowiest season is 1966–67, with 73.9 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00241008; records: ACIS (NOAA Regional Climate Centers), station USC00241008. Records use complete months only; a month missing more than three days is left out.
How sunny is Boulder?
Over a year 74% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Boulder, 4.0 kWh per square metre a day. July is the sunniest month (86%) and December the cloudiest (61%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 66% | 1.5 |
| February | 70% | 2.5 |
| March | 69% | 3.7 |
| April | 68% | 4.8 |
| May | 69% | 5.7 |
| June | 74% | 6.5 |
| July | 86% | 7.3 |
| August | 84% | 6.0 |
| September | 79% | 4.5 |
| October | 70% | 2.8 |
| November | 64% | 1.6 |
| December | 61% | 1.2 |
Source: NASA POWER solar climatology, 2001-2020 (CERES SYN1deg). Satellite-based, for the 1-degree cell (about 100 km across) around the station, so it describes the area rather than the airport.
How much heating and cooling does Boulder need?
A normal year in Boulder brings 8,312 heating degree days, 55% more than the median U.S. place (5,373), and 157 cooling degree days, 83% fewer than the median (908). Nine months need 300 or more heating degree days, a monthly mean of about 55°F or colder. For gardens and crops, NOAA counts 1,477 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 1,288 | 0 |
| February | 1,095 | 0 |
| March | 967 | 0 |
| April | 728 | 0 |
| May | 483 | 0 |
| June | 245 | 12 |
| July | 68 | 80 |
| August | 85 | 57 |
| September | 307 | 7 |
| October | 696 | 0 |
| November | 1,035 | 0 |
| December | 1,316 | 0 |
| Year (annual) | 8,312 | 157 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00241008. A degree day is one degree of a day's mean temperature below 65°F (heating) or above it (cooling); utilities use them to estimate energy use.
Has Boulder been warmer than normal lately?
Over 55 complete years from 1951 to 2024, the station's annual mean temperature rose at a fitted 3.4°F per century (+0.19°C per decade). A fitted rate summarizes the whole record; single years swing well above and below it.
Source: trend: NOAA GHCN-Monthly v4. A year counts only with all twelve months complete.
How does Boulder's climate compare?
Boulder is warmer than 40 of the other 140 places with their own NOAA record in Montana, and wetter than 7. Across the 7,247 U.S. stations with normals, its annual mean of 42.6°F is warmer than 11%.
In the Köppen classification computed from these normals, Boulder is BSk: cold semi-arid steppe. What the classes mean.
Places with the most similar year elsewhere
Measured month by month on the normal high, low and precipitation, among places of 20,000 or more people in other states:
Source: NOAA NCEI 1991-2020 normals for every station PlainClimate covers. Similarity: the root-mean-square difference of the 36 monthly figures, precipitation weighted so an inch counts like 6°F.
Which places near Boulder share this climate record?
Basin has no NOAA station with 1991-2020 normals of its own, and this station is the nearest full record. A place's own climate can differ from these figures, by several degrees where it sits much higher or lower than the station, nearer the coast, or inside a large city's heat; for the larger places the next nearest records are named with their yearly mean, so the spread around them is visible, and places of 50,000 or more have their own page with NOAA's gridded normals.
- Basin, MT360 people · 9 mi NW
Source: Census Bureau 2024 Gazetteer (place centers) and Vintage 2024 population estimates. Distance and direction from the station to each place's center.
Questions about Boulder's climate
What is the coldest month in Boulder?
December, with a normal high of 34°F and a normal low of 12°F. The warmest is July (83°F by day, 48°F at night).
What is the rainiest month in Boulder?
June, with 2.30 in of precipitation in a normal year; the driest is February with 0.19 in.
What plant hardiness zone is Boulder in?
The USDA's 2023 map puts Boulder in zone 5a in ZIP code 59632, the nearest ZIP area. By the same method at Boulder's NOAA station, the coldest night of a typical year reaches −24°F (22 years, 1991-2020): zone 4b.
How late can frost come in Boulder?
In half of years the last 32°F night comes after June 7; in one year in ten it comes after June 25. Waiting until June 25 leaves about a one-in-ten chance of a later frost.
Nearby and next
- Montana climateThe statewide normals and every place in the state
- Tizer Gardens, MT11 miles from this station, 41.0°F a year
- Butte, MT26 miles from this station, 40.1°F a year
- Helena, MT27 miles from this station, 45.5°F a year
- Elliston, MT27 miles from this station, 40.3°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made