Boulder weather in September

In September, Boulder sees average daytime highs of 71°F and overnight lows near 44°F, with 2.1 in of precipitation across 9.7 wet days and about 12.3 hours of daylight. It is the 4th-warmest and 6th-wettest month of the year here.

Highs ease over the month, from about 75°F in the first days to 66°F by the end. The sky is clear or mostly clear about 48% of the time in September, and the air most often feels dry.

Daylight stretches from about 12 h 54 min at the start of September to 11 h 36 min by month's end. Set against the rest of the year, September is Boulder's 4th-clearest and 10th-windiest month. For the whole year in context, see Boulder's year-round climate.

Daytime high
71°
4th warmest
Overnight low
44°
9th coldest
Precipitation
2.1 in
9.7 wet days
Daylight
12 h 18 min
5:48 AM – 6:03 PM
Cloud cover
43%
4th clearest
Wind
4 mph
10th windiest
Solar energy
5.48 kWh/m²
Travel score
6.8
out of 10

Boulder is city in and county seat of Boulder County, Colorado, United States.

Density
1,606/km²
people per km²
Founded
1858

Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Highs ease over the month, from about 75°F in the first days to 66°F by the end.

A typical September day in Boulder reaches about 71°F in the afternoon and slips back to 44°F overnight — a 26°F spread between the day's warmth and the night's chill. On most days the high lands between 58°F and 81°F. Set against its neighbours, September runs about 7°F cooler than August and about 12°F warmer than October.

20°40°60°80°151015202530
Avg high (°F)Avg low (°F)
17649
27549
37549
47548
57448
67448
77447
87347
97347
107346
117246
127246
137245
147145
157144
167144
177044
187043
196943
206942
216942
226942
236841
246841
256741
266740
276740
286640
296639
306539

How September really feels in Boulder

Once humidity and wind are folded in, September averages 0 days that feel above 90°F and 0 that feel at or below 32°F — the comfort signal a plain temperature reading hides.

Days feeling ≥ 90°F
0
heat index over the day's high
Days feeling ≤ 32°F
0
wind chill below the day's low

September sits comfortably in between — no days that feel dangerously hot or drop to a hard freeze once humidity and wind are folded in.

Boulder weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 6th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan37° / 14°0.95.75.59.50.0
Feb39° / 16°1.06.7710.40.0
Mar48° / 22°2.08.87.211.71.7
Apr54° / 29°3.111.17.9133.1
May63° / 38°4.014.93.514.24.8
Jun74° / 47°3.014.30.314.86.2
Jul80° / 53°3.014.8014.56.0
Aug78° / 52°2.814.9013.66.2
Sep71° / 44°2.19.70.912.36.8
Oct58° / 33°1.97.43.910.94.9
Nov46° / 22°1.36.45.79.81.4
Dec37° / 15°0.95.35.49.20.0

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 48% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 24% of September, with mostly cloudy or overcast conditions the remaining 29%. Skies tend to open up as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
16931
26931
37030
47030
57030
67129
77030
87030
97129
107129
117129
127228
137228
147129
157228
167228
177228
187327
197327
207327
217426
227327
237327
247327
257228
267129
277129
287129
297030
307030

Chance of precipitation in September

Any precipitationSnow

Boulder gets around 2.1 in of rain and snow over 9.7 wet days in September. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable precipitation averages around 33% and peaks near 39% on the wettest days, and about 1 day brings snow. The odds ease toward the end of the month.

0%10%20%30%40%50%151015202530
Rain chance (%)Snow chance (%)
1390
2391
3391
4381
5381
6381
7381
8371
9371
10372
11362
12362
13362
14353
15353
16343
17334
18324
19314
20304
21304
22295
23285
24285
25285
26285
27276
28276
29276
30276

Humidity in September

DryComfortableHumidMuggyOppressiveMiserable

Most of September feels dry — the stacked bands show each day's mix of comfort levels, read off the dew point.

By dew point, September lands in the dry band about 99% of days. It rarely turns muggy — the humid and sticky bands barely register.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)
1991
2991
3991
4991
5991
6991
7991
8991
9991
10991
11991
12991
13991
14991
15991
16991
17991
181000
191000
201000
211000
221000
231000
241000
251000
261000
271000
281000
291000
301000

Wind in September

Winds average 4 mph in September.

Across September, daily wind ranges from about 4 mph on the calmest days to 5 mph at its breeziest. It stays fairly steady from day to day.

0246810151015202530
Wind speed (mph)Feels
14calm
24calm
34calm
44calm
54calm
64calm
74calm
84calm
94calm
104calm
114calm
124calm
134calm
144calm
154calm
164calm
174calm
184calm
194calm
204calm
214calm
225calm
235calm
245calm
255calm
265calm
275calm
285calm
295calm
305calm

Sunrise & sunset in September

DaylightNight

Daylight runs from about 12 h 54 min at the start of September to 11 h 36 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 78 minutes over September. Sunrise moves from 5:33 AM to 6:02 AM, and sunset from 6:29 PM to 5:39 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.95:336:29
212.95:346:27
312.85:356:25
412.85:366:24
512.85:376:22
612.75:386:20
712.75:396:19
812.65:406:17
912.65:416:15
1012.55:426:13
1112.55:436:12
1212.45:446:10
1312.45:456:08
1412.35:466:07
1512.35:476:05
1612.35:486:03
1712.25:496:01
1812.25:506:00
1912.15:515:58
2012.15:525:56
2112.05:535:55
2212.05:545:53
2311.95:555:51
2411.95:565:49
2511.95:575:48
2611.85:585:46
2711.85:595:44
2811.76:005:43
2911.76:015:41
3011.66:025:39

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 5.48 kWh/m².

For context, solar energy here swings from a low near 2.56 kWh/m² to a high of 7.49 kWh/m² in June across the year — September's 5.48 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
16.0Intense
26.0Bright
36.0Bright
45.9Bright
55.9Bright
65.8Bright
75.8Bright
85.8Bright
95.7Bright
105.7Bright
115.6Bright
125.6Bright
135.5Bright
145.5Bright
155.5Bright
165.4Bright
175.4Bright
185.4Bright
195.3Bright
205.3Bright
215.2Bright
225.2Bright
235.2Bright
245.1Bright
255.1Bright
265.0Bright
275.0Bright
284.9Bright
294.9Bright
304.8Bright
Planning a trip in September

Hazard calendar for Boulder

In seasonPeak

In September, Boulder sits inside its rainy season season. The full year is below, with each hazard's peak months called out.

Boulder's rainy season runs March–October, when most of the year's rain falls.

Rainy season
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

Is September a good time to visit Boulder?

Our two travel scores rate September for general sightseeing and for beach or pool weather — 0 (poor) to 10 (ideal).

6.8
Sightseeing
1st-best month of the year
4.4
Beach & pool
4th-best month of the year

September is a reasonable time to visit Boulder for sightseeing.

What to pack for Boulder in September

From September's temperature range, rain chance, sun and wind — what's worth putting in the bag.

In September, Boulder sees daytime highs around 71°F and nights near 44°F. Worth packing: a warm layer, rain gear and sun protection. About 10 days see rain.

a warm layerrain gearsun protection

UV index in Boulder in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 7 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Boulder peaks around July 13 at about 11 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 23 at about 2 (moderate).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 10 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highExtremeVery highHighModerateJul 13UV 11Dec 23UV 2
UV index (midday)Level
Jan2Moderate
Feb4Moderate
Mar7High
Apr6High
May7Very high
Jun11Extreme
Jul11Extreme
Aug10Very high
Sep7Very high
Oct5High
Nov3Moderate
Dec2Low

Air quality in Boulder in September

GoodModeratePoorUnhealthyVery unhealthy

Average fine-particle pollution (PM2.5) through the year — so you can see where September falls, not just the annual average.

Air quality in Boulder is worst in August — around 21 µg/m³ PM2.5 (moderate), about 1.6× the cleanest month (May, 13 µg/m³).

Modeled monthly averages (CAMS reanalysis, 2015–2024).

0102030JanFebMarAprMayJunJulAugSepOctNovDecModerateAug 1620 µg/m³May 1113 µg/m³
PM2.5 (µg/m³)Level
Jan17Moderate
Feb15Moderate
Mar16Moderate
Apr13Moderate
May13Moderate
Jun14Moderate
Jul14Moderate
Aug21Moderate
Sep13Moderate
Oct15Moderate
Nov15Moderate
Dec16Moderate
Location & terrain

Where is Boulder?

Boulder sits at 40.0°N, 105.3°W, 5,328 ft above sea level, in United States. The nearest listed city is Louisville, 8.1 mi away.

Topography around Boulder

How much the land rises and falls, and what covers it, within 2, 10 and 50 miles of Boulder.

Within 2 miles, the terrain around Boulder has significant variations in elevation — a maximum elevation change of 1,703 ft around an average of 5,495 ft above sea level; within 10 miles, very significant variations in elevation — a maximum elevation change of 4,147 ft; within 50 miles, extreme variations in elevation — a maximum elevation change of 9,623 ft.

The land around Boulder is covered by trees (45%) and artificial surfaces (39%) within 2 miles, grassland (44%) and trees (41%) within 10 miles, and trees (41%) and grassland (34%) within 50 miles.

2 miles
1,703 ft
10 miles
4,147 ft
50 miles
9,623 ft
treesgrasslandcroplandartificial surfacesbare terrainwatermoss and lichen
September weather nearby: Louisville 8.1 mi · Lafayette 9.9 mi · Broomfield 12 mi · Erie 12 mi · Longmont 14 mi · Westminster 17 mi · Northglenn 17 mi · Arvada 17 mi

United States travel facts

Power plugs
Type A / B
120 V · 60 Hz
Prices
≈ 100% of US
price level, 2025
GDP per capita
$90,027
2025
Happiness
6.8 / 10
World Happiness, 2025
Safe drinking water
98%
of population, 2024
Homicide rate
5.8
per 100k, 2023
Road deaths
12.7
per 100k/yr, 2019
Income group
High
World Bank
Religion
Protestant 47%unaffiliated 23%Roman Catholic 21%Jewish 2%

Sources: World Bank, US State Department, World Happiness Report, WHO/UNICEF, EF EPI, CIA World Factbook.

Data sources

Temperature, precipitation, cloud, humidity, wind and solar figures are a 1991–2020 climatology for Boulder (40.015°, -105.2706°, elevation 5,328 ft), computed from hourly ERA5 reanalysis — contains modified Copernicus Climate Change Service information. Sun figures are astronomical calculations, not observations. City data © GeoNames, CC BY 4.0. Map outlines from Natural Earth (public domain); interactive map © Google. UV index is derived from CAMS (Copernicus Atmosphere Monitoring Service) clear-sky UV data — contains modified Copernicus Atmosphere Monitoring Service information. Monthly air-quality (PM2.5) seasonality is from the CAMS EAC4 reanalysis (Copernicus Atmosphere Monitoring Service), scaled to the WHO annual mean where available. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).