United States · 1991–2020

Best time to visit
Boulder.

September leads for comfortable weather in Boulder: typically 25.9°C by day, 9.2°C at night, with 53 mm of rain. Based on measured 1991–2020 NOAA station history.

Best comfortSeptember

Typical high25.9°C

Typical low9.2°C

Rain in September53 mm

Comfort

September

September · June · August

79climate match

Beach

August

August · July · June

71climate match

Outdoors

October

October · September · May

81climate match

The weakest month is December, scoring 9 against September’s 79 — days reach only 7.9°C, 16° below the 24°C the score treats as ideal, and nights drop to -5.7°C.

Month by month

The whole year,
without the guesswork.

The bars show the comfort score. Day and night temperatures come from daily observed highs and lows; rain is the historical monthly estimate.

Jan

118.3° / -5.5°

Feb

128.9° / -5.1°

Mar

3213.7° / -1.3°

Apr

4317° / 1.7°

May

6521.7° / 6.2°

Jun

7527.9° / 10.9°

Jul

7031.1° / 14.2°

Aug

7430° / 13.4°

Sep

7925.9° / 9.2°

Oct

5918.8° / 3.2°

Nov

3012.4° / -1.9°

Dec

97.9° / -5.7°

Monthly climate evidence for Boulder — daylight is calculated from latitude, everything else is measured
Month	Day	Night	Rain	Wet days	Daylight	Comfort	Coverage
January	8.3°C	-5.5°C	20 mm	3.9	9 h 37	11	Strong coverage
February	8.9°C	-5.1°C	25 mm	4.9	10 h 37	12	Strong coverage
March	13.7°C	-1.3°C	51 mm	6.1	11 h 49	32	Strong coverage
April	17°C	1.7°C	77 mm	7.3	13 h 12	43	Strong coverage
May	21.7°C	6.2°C	81 mm	9	14 h 22	65	Strong coverage
June	27.9°C	10.9°C	49 mm	6.4	15 h	75	Strong coverage
July	31.1°C	14.2°C	47 mm	6.7	14 h 45	70	Strong coverage
August	30°C	13.4°C	43 mm	5.9	13 h 45	74	Strong coverage
September	25.9°C	9.2°C	53 mm	5.2	12 h 24	79	Strong coverage
October	18.8°C	3.2°C	41 mm	5.2	11 h 04	59	Strong coverage
November	12.4°C	-1.9°C	30 mm	4.4	9 h 55	30	Strong coverage
December	7.9°C	-5.7°C	21 mm	3.7	9 h 20	9	Strong coverage

What has changed

Boulder runs +0.9°C
on the 1961–1990 baseline.

Each bar compares the 2011–2025 daytime average for that month against the same month in 1961–1990, measured at the same station. Months where the older record is too thin to compare are left blank.

Jan

+1.0

Feb

-0.7

Mar

+1.8

Apr

+0.4

May

-1.1

Jun

+1.5

Jul

+0.6

Aug

+1.3

Sep

+2.5

Oct

+0.1

Nov

+1.8

Dec

+1.4

September daytime highs, 2011–2025. Last recorded: 26.6°C.

Records at BOULDER, 1893–2026
Month	Record high	Record low	Wettest day	Wettest / driest month	Years
January	22.8°C20 Jan 2005	-36.1°C17 Jan 1930	26 mm6 Jan 1998	64 mm1948 · driest 0 mm in 1919	127
February	26.1°C8 Feb 1954	-33.3°C8 Feb 1936	40 mm3 Feb 2024	94 mm2015 · driest 0 mm in 1925	129
March	31.7°C21 Mar 2026	-25.0°C9 Mar 1932	68 mm24 Mar 1909	138 mm2003 · driest 0 mm in 2012	130
April	31.1°C21 Apr 1989	-19.4°C2 Apr 1975	90 mm3 Apr 1986	233 mm1900 · driest 3 mm in 2022	131
May	35.0°C29 May 2000	-8.3°C2 May 2013	89 mm17 May 1995	244 mm1995 · driest 0 mm in 1974	129
June	40.0°C23 Jun 1954	-6.7°C20 Jun 1935	86 mm4 Jun 1949	188 mm1949 · driest 0 mm in 1924	130
July	40.0°C11 Jul 1954	4.4°C4 Jul 1921	122 mm31 Jul 1919	190 mm1919 · driest 2 mm in 2002	130
August	38.9°C24 Aug 1936	4.4°C10 Aug 1912	78 mm3 Aug 1951	190 mm1951 · driest 1 mm in 1985	127
September	37.8°C2 Sep 1983	-9.4°C30 Sep 1985	231 mm12 Sep 2013	461 mm2013 · driest 0 mm in 1944	127
October	32.2°C5 Oct 1947	-18.9°C29 Oct 1917	64 mm15 Oct 1916	153 mm1942 · driest 0 mm in 1934	129
November	27.2°C2 Nov 2025	-24.4°C27 Nov 1919	34 mm26 Nov 2019	94 mm1946 · driest 0 mm in 1913	127
December	24.4°C17 Dec 1980	-31.1°C22 Dec 1990	37 mm5 Dec 1913	107 mm1913 · driest 0 mm in 1905	128

Typical days per month at BOULDER, 1991–2020
Month	Days 25°C+	Days 30°C+	Frost nights	Heavy rain (10 mm+)	Snow days
January	0	0	27	<1	6
February	0	0	24	<1	7
March	<1	0	20	1	6
April	2	<1	11	3	5
May	10	2	2	3	<1
June	23	12	0	2	0
July	29	21	0	1	0
August	28	18	0	<1	0
September	20	7	<1	2	<1
October	6	<1	8	1	2
November	<1	0	20	1	5
December	0	0	27	<1	5

The best month has moved. On the 1961–1990 numbers the strongest month here was September (scoring 84); on 2011–2025 it is June (76). Both are scored the same way, from the same station — only the climate underneath changed.

How often, not how much — every day in BOULDER’s record sorted by what it was actually like, rather than averaged
Month	Warm & dry days	Wet days	Washouts	Week with 3+ wet days	Daytime mean has ranged
January	1%	11%	1%	7%	4.0–10.4°C
February	3%	16%	2%	11%	4.8–12.3°C
March	13%	20%	4%	18%	8.2–15.9°C
April	35%	23%	8%	24%	13.6–19.7°C
May	56%	28%	8%	32%	19.0–23.6°C
June	77%	21%	5%	19%	24.4–30.1°C
July	77%	23%	4%	24%	27.7–32.5°C
August	79%	21%	3%	18%	27.4–31.3°C
September	77%	17%	4%	15%	22.9–28.2°C
October	50%	15%	4%	11%	16.1–22.0°C
November	13%	13%	3%	7%	9.2–15.3°C
December	2%	11%	2%	6%	5.1–10.9°C

A warm and dry day reaches 20°C with under 1 mm of rain; a wet day gets 1 mm or more; a washout gets 10 mm or more. The week column is the share of seven-day stretches in that month, across the station’s whole daily record, that contained three or more wet days — the odds a week-long trip is rained on repeatedly, counted rather than modelled. The last column is the 10th to 90th percentile of that month’s daytime average across individual years: a narrow band means the average is what you will get, a wide one means the average is a midpoint between years that felt quite different.

1898Each stripe is one year’s mean temperature against the 115-year average. Warmest: 1954. Coolest: 1923.2025

The evidence behind this page

Nearby, measured,
and named.

Place identity: GeoNames 5574991
NOAA station: USC00050848 · BOULDER
Station distance: 2.5 km from the place centre
Climate period: 1991–2020
Minimum month coverage: 29 usable years
Daylight: Calculated from latitude 40.01°, mid-month, sunrise to sunset including refraction. Not a station measurement.
Important limit: Historical climate is not a forecast. Nearby stations can miss mountains, coasts, and local microclimates.

These places read the same station, so their numbers are identical — distance from BOULDER is what differs

Louisville→Superior→Gunbarrel→Pine Brook Hill→Paragon Estates→Lazy Acres→

Or put two places side by side: Boulder vs Louisville

Best time to visitBoulder.