Ottawa weather in September

In September, Ottawa sees average daytime highs of 76°F and overnight lows near 56°F, with 3.1 in of rain across 8.4 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 80°F in the first days to 71°F by the end. The sky is clear or mostly clear about 44% of the time in September, and the air most often feels dry.

Daylight stretches from about 13 hours at the start of September to 11 h 36 min by month's end. Set against the rest of the year, September is Ottawa's 1st-clearest and 9th-windiest month. For the whole year in context, see Ottawa's year-round climate.

Daytime high
76°
4th warmest
Overnight low
56°
9th coldest
Rainfall
3.1 in
8.4 wet days
Daylight
12 h 18 min
5:42 AM – 5:58 PM
Cloud cover
49%
1st clearest
Wind
8 mph
9th windiest
Solar energy
4.56 kWh/m²
Travel score
7.1
out of 10

Ottawa is city in Illinois, United States.

Density
553/km²
people per km²
Founded
1853

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 80°F in the first days to 71°F by the end.

A typical September day in Ottawa reaches about 76°F in the afternoon and slips back to 56°F overnight — a 20°F spread between the day's warmth and the night's chill. On most days the high lands between 65°F and 86°F. Set against its neighbours, September runs about 6°F cooler than August and about 13°F warmer than October.

40°60°80°151015202530
Avg high (°F)Avg low (°F)
18061
28061
38061
48060
58060
67960
77959
87959
97858
107858
117858
127757
137757
147756
157656
167656
177655
187555
197554
207454
217454
227453
237353
247352
257352
267251
277251
287151
297150
307150

How September really feels in Ottawa

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.

Ottawa 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
Jan32° / 18°2.28.25.49.40.0
Feb37° / 21°2.07.24.210.30.0
Mar49° / 31°2.89.92.411.71.9
Apr61° / 41°4.311.90.613.14.8
May71° / 52°4.713014.36.1
Jun80° / 62°4.411.7014.96.2
Jul83° / 66°3.811.3014.75.7
Aug82° / 64°3.59.7013.76.6
Sep76° / 56°3.18.4012.37.1
Oct63° / 44°3.190.110.95.7
Nov49° / 33°2.78.61.19.72.1
Dec37° / 23°2.28.13.79.10.0

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 44% of the time in September, the clearest month of the year.

Beyond those clear spells, partly cloudy skies cover about 13% of September, with mostly cloudy or overcast conditions the remaining 43%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
15941
25941
35941
46139
56139
66139
76139
86238
96139
106139
116040
126040
135941
145842
155842
165743
175743
185644
195644
205644
215545
225545
235545
245446
255446
265347
275347
285347
295347
305347

Chance of precipitation in September

Any precipitation

Ottawa gets around 3.1 in of rain over 8.4 wet days in September. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 29% and peaks near 30% on the wettest days.

0%10%20%30%40%151015202530
Rain chance (%)
129
229
329
429
529
629
729
829
930
1030
1130
1230
1330
1430
1530
1630
1730
1829
1929
2029
2128
2228
2328
2428
2528
2627
2727
2827
2927
3027

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 47% of days. Sticky, muggy-or-worse air turns up on roughly 14% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)
1202425228
2222425218
3242525207
4252524196
5272624176
6292624165
7312623155
8322623144
9342622144
10362622133
11382621123
12402521122
13422420122
14442419112
15462318112
16482317111
17512216101
18532115101
1955211491
2057201391
2158191391
2262181280
2364181170
2466171070
2568161060
2668161060
277115950
287115950
297314940
307414840

Wind in September

Winds average 8 mph in September.

Across September, daily wind ranges from about 7 mph on the calmest days to 8 mph at its breeziest. It stays fairly steady from day to day.

051015151015202530
Wind speed (mph)Feels
17calm
27calm
37calm
47calm
57calm
67calm
77calm
87calm
97calm
107calm
117calm
127calm
137calm
147light breeze
157light breeze
168light breeze
178light breeze
188light breeze
198light breeze
208light breeze
218light breeze
228light breeze
238light breeze
248light breeze
258light breeze
268light breeze
278light breeze
288light breeze
298light breeze
308light breeze

Sunrise & sunset in September

DaylightNight

Daylight runs from about 13 hours 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 84 minutes over September. Sunrise moves from 5:26 AM to 5:56 AM, and sunset from 6:24 PM to 5:33 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
113.05:266:24
212.95:276:22
312.95:286:21
412.85:296:19
512.85:306:17
612.75:316:16
712.75:326:14
812.65:336:12
912.65:346:10
1012.65:356:09
1112.55:366:07
1212.55:376:05
1312.45:386:03
1412.45:396:01
1512.35:406:00
1612.35:425:58
1712.25:435:56
1812.25:445:54
1912.15:455:52
2012.15:465:51
2112.05:475:49
2212.05:485:47
2311.95:495:45
2411.95:505:44
2511.85:515:42
2611.85:525:40
2711.85:535:38
2811.75:545:36
2911.75:555:35
3011.65:565:33

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.56 kWh/m².

For context, solar energy here swings from a low near 1.72 kWh/m² to a high of 6.31 kWh/m² in June across the year — September's 4.56 kWh/m² falls in the middle.

0246151015202530
Solar energy (kWh/m²)Level
15.1Bright
25.1Bright
35.0Bright
45.0Bright
55.0Bright
64.9Bright
74.9Bright
84.8Bright
94.8Bright
104.8Bright
114.7Bright
124.7Bright
134.6Bright
144.6Bright
154.5Bright
164.5Bright
174.5Bright
184.4Bright
194.4Bright
204.3Bright
214.3Bright
224.3Bright
234.2Bright
244.2Bright
254.1Bright
264.1Bright
274.0Bright
284.0Bright
294.0Moderate
303.9Moderate
Planning a trip in September

Is September a good time to visit Ottawa?

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

7.1
Sightseeing
1st-best month of the year
6.0
Beach & pool
4th-best month of the year

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

What to pack for Ottawa in September

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

In September, Ottawa sees daytime highs around 76°F and nights near 56°F. Worth packing: light clothes, rain gear and sun protection. About 8 days see rain.

light clothesrain gearsun protection

UV index in Ottawa in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 6 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Ottawa peaks around June 20 at about 9 (very high) — sunscreen, a hat and midday shade are essential. It bottoms out near December 19 at about 2 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 8 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecModerateHighVery highHighModerateLowJun 20UV 9Dec 19UV 2
UV index (midday)Level
Jan2Moderate
Feb3Moderate
Mar5Moderate
Apr6High
May7Very high
Jun9Very high
Jul8Very high
Aug8Very high
Sep6High
Oct4Moderate
Nov3Moderate
Dec2Low

Air quality in Ottawa 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 Ottawa is worst in July — around 17 µg/m³ PM2.5 (moderate), about 1.9× the cleanest month (October, 9 µg/m³).

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

01020JanFebMarAprMayJunJulAugSepOctNovDecModerateGoodJul 1917 µg/m³Oct 229 µg/m³
PM2.5 (µg/m³)Level
Jan12Good
Feb14Moderate
Mar14Moderate
Apr13Moderate
May14Moderate
Jun15Moderate
Jul17Moderate
Aug16Moderate
Sep12Moderate
Oct9Good
Nov9Good
Dec10Good
Location & terrain

Where is Ottawa?

Ottawa sits at 41.3°N, 88.8°W, 482 ft above sea level, in United States. The nearest listed city is Yorkville, 29 mi away.

Topography around Ottawa

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

Within 2 miles, the terrain around Ottawa has only modest variations in elevation — a maximum elevation change of 295 ft around an average of 531 ft above sea level; within 10 miles, only modest variations in elevation — a maximum elevation change of 417 ft; within 50 miles, only modest variations in elevation — a maximum elevation change of 692 ft.

The land around Ottawa is covered by trees (36%) and artificial surfaces (24%) within 2 miles, cropland (69%) within 10 miles, and cropland (78%) within 50 miles.

2 miles
295 ft
10 miles
417 ft
50 miles
692 ft
treesgrasslandcroplandartificial surfacesbare terrainwater
September weather nearby: Yorkville 29 mi · Oswego 34 mi · Shorewood 35 mi · Montgomery 37 mi · Plainfield 39 mi · Aurora 39 mi · DeKalb 40 mi · Crest Hill 41 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 Ottawa (41.3456°, -88.8426°, elevation 482 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).