Longmont weather in September

In September, Longmont sees average daytime highs of 76°F and overnight lows near 49°F, with 1.5 in of precipitation across 5.9 wet days and about 12.3 hours of daylight. It is the 4th-warmest and 8th-wettest month of the year here.

Highs ease over the month, from about 81°F in the first days to 72°F by the end. The sky is clear or mostly clear about 51% 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 Longmont's 1st-clearest and 7th-windiest month. For the whole year in context, see Longmont's year-round climate.

Daytime high
76°
4th warmest
Overnight low
49°
9th coldest
Precipitation
1.5 in
5.9 wet days
Daylight
12 h 18 min
5:47 AM – 6:03 PM
Cloud cover
41%
1st clearest
Wind
4 mph
7th windiest
Solar energy
5.39 kWh/m²
Travel score
7.6
out of 10

Longmont is city in Boulder County, Colorado, United States.

Density
1,228/km²
people per km²
Founded
1871

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 81°F in the first days to 72°F by the end.

A typical September day in Longmont reaches about 76°F in the afternoon and slips back to 49°F overnight — a 27°F spread between the day's warmth and the night's chill. On most days the high lands between 63°F and 87°F. Set against its neighbours, September runs about 8°F cooler than August and about 13°F warmer than October.

40°60°80°151015202530
Avg high (°F)Avg low (°F)
18255
28254
38154
48154
58153
68053
78053
87952
97952
107951
117851
127851
137750
147750
157649
167649
177649
187548
197548
207547
217447
227447
237446
247346
257346
267245
277245
287244
297144
307144

How September really feels in Longmont

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.

Longmont weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 8th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°F)Rainfall (inches)Wet daysSnow daysDaylight (hours)Travel score
Jan41° / 18°0.84.74.59.50.1
Feb44° / 20°0.85.3510.41.0
Mar54° / 27°1.67.34.411.73.6
Apr61° / 35°2.69.54135.0
May69° / 44°3.411.70.614.26.2
Jun80° / 53°2.18.5014.86.6
Jul86° / 59°1.77.4014.65.3
Aug84° / 57°1.66.8013.66.1
Sep76° / 49°1.55.90.412.37.6
Oct63° / 37°1.66.51.910.96.2
Nov50° / 27°1.15.84.49.82.7
Dec41° / 19°0.74.54.49.20.2

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 51% of the time in September, the clearest month of the year.

Beyond those clear spells, partly cloudy skies cover about 21% of September, with mostly cloudy or overcast conditions the remaining 27%. Skies tend to open up as the month goes on.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
17228
27228
37228
47327
57327
67327
77327
87327
97426
107426
117426
127327
137327
147327
157426
167327
177327
187327
197327
207426
217426
227327
237327
247228
257228
267129
277129
287129
297129
307030

Chance of precipitation in September

Any precipitationSnow

Longmont gets around 1.5 in of rain and snow over 5.9 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 21% and peaks near 23% on the wettest days, and about 0 day brings snow.

0%10%20%30%151015202530
Rain chance (%)Snow chance (%)
1170
2170
3180
4181
5191
6191
7191
8201
9201
10211
11211
12221
13221
14221
15231
16231
17231
18221
19221
20221
21212
22212
23212
24212
25212
26212
27212
28212
29222
30222

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 97% of days. It rarely turns muggy — the humid and sticky bands barely register.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)
19280
29280
39370
49460
59460
69451
79451
89541
99541
109541
119631
129631
139820
149820
159820
169910
179910
189910
199910
209910
2110000
2210000
2310000
2410000
2510000
2610000
2710000
2810000
2910000
3010000

Wind in September

Winds average 4 mph in September.

Across September, daily wind ranges from about 4 mph on the calmest days to 4 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
224calm
234calm
244calm
254calm
264calm
274calm
284calm
294calm
304calm

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 5:32 AM to 6:01 AM, and sunset from 6:28 PM to 5:38 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
112.95:326:28
212.95:336:26
312.85:346:25
412.85:356:23
512.85:366:21
612.75:376:20
712.75:386:18
812.65:396:16
912.65:406:15
1012.55:416:13
1112.55:426:11
1212.45:436:09
1312.45:446:08
1412.45:456:06
1512.35:466:04
1612.35:476:03
1712.25:486:01
1812.25:495:59
1912.15:505:57
2012.15:515:56
2112.05:525:54
2212.05:535:52
2311.95:545:50
2411.95:555:49
2511.95:565:47
2611.85:575:45
2711.85:585:44
2811.75:595:42
2911.76:005:40
3011.66:015:38

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 5.39 kWh/m².

For context, solar energy here swings from a low near 2.42 kWh/m² to a high of 7.36 kWh/m² in June across the year — September's 5.39 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
16.0Intense
26.0Bright
35.9Bright
45.9Bright
55.8Bright
65.8Bright
75.8Bright
85.7Bright
95.7Bright
105.6Bright
115.6Bright
125.5Bright
135.5Bright
145.4Bright
155.4Bright
165.3Bright
175.3Bright
185.3Bright
195.2Bright
205.2Bright
215.1Bright
225.1Bright
235.0Bright
245.0Bright
254.9Bright
264.9Bright
274.8Bright
284.8Bright
294.7Bright
304.7Bright
Planning a trip in September

Is September a good time to visit Longmont?

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

7.6
Sightseeing
1st-best month of the year
6.6
Beach & pool
4th-best month of the year

September is one of the better times to visit Longmont for general sightseeing. The water and warmth also suit a beach or pool day.

What to pack for Longmont in September

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

In September, Longmont sees daytime highs around 76°F and nights near 49°F. Worth packing: light clothes and sun protection. About 6 days see rain.

light clothessun protection

UV index in Longmont 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 Longmont 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 Longmont 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 Longmont is worst in October — around 30 µg/m³ PM2.5 (moderate), about 3.1× the cleanest month (November, 10 µg/m³).

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

010203040JanFebMarAprMayJunJulAugSepOctNovDecGoodModerateGoodModerateGoodOct 1528 µg/m³Nov 2310 µg/m³
PM2.5 (µg/m³)Level
Jan12Good
Feb12Moderate
Mar14Moderate
Apr11Good
May11Good
Jun12Good
Jul14Moderate
Aug19Moderate
Sep17Moderate
Oct30Moderate
Nov10Good
Dec11Good
Location & terrain

Where is Longmont?

Longmont sits at 40.2°N, 105.1°W, 4,984 ft above sea level, in United States. The nearest listed city is Erie, 8.7 mi away.

Topography around Longmont

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

Within 2 miles, the terrain around Longmont has only modest variations in elevation — a maximum elevation change of 197 ft around an average of 4,993 ft above sea level; within 10 miles, significant variations in elevation — a maximum elevation change of 1,765 ft; within 50 miles, extreme variations in elevation — a maximum elevation change of 9,721 ft.

The land around Longmont is covered by artificial surfaces (50%) and trees (22%) within 2 miles, grassland (54%) and cropland (25%) within 10 miles, and grassland (42%) and trees (31%) within 50 miles.

2 miles
197 ft
10 miles
1,765 ft
50 miles
9,721 ft
treesgrasslandcroplandartificial surfacesbare terrainwatermoss and lichen
September weather nearby: Erie 8.7 mi · Lafayette 12 mi · Louisville 13 mi · Boulder 14 mi · Loveland 16 mi · Broomfield 17 mi · Brighton 19 mi · Northglenn 21 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 Longmont (40.1672°, -105.1019°, elevation 4,984 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).