Cornwall weather in September

In September, Cornwall sees average daytime highs of 21°C and overnight lows near 11°C, with 93 mm of rain across 10.5 wet days and about 12.3 hours of daylight. It is the 4th-warmest and 7th-wettest month of the year here.

Highs ease over the month, from about 23°C in the first days to 18°C by the end. The sky is clear or mostly clear about 30% of the time in September, and the air most often feels dry.

Daylight stretches from about 13 h 06 min at the start of September to 11 h 36 min by month's end. Set against the rest of the year, September is Cornwall's 1st-clearest and 9th-windiest month. For the whole year in context, see Cornwall's year-round climate.

Daytime high
21°
4th warmest
Overnight low
11°
9th coldest
Rainfall
93 mm
10.5 wet days
Daylight
12 h 18 min
5:44 AM – 6:03 PM
Cloud cover
58%
1st clearest
Wind
11 km/h
9th windiest
Solar energy
3.91 kWh/m²
Travel score
6.4
out of 10

Cornwall is city in Ontario, Canada.

Air quality
Good
PM2.5 6 µg/m³ · 2019
Density
793/km²
people per km²
Founded
1784

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 23°C in the first days to 18°C by the end.

A typical September day in Cornwall reaches about 21°C in the afternoon and slips back to 11°C overnight — a 10°C spread between the day's warmth and the night's chill. On most days the high lands between 15°C and 27°C. Set against its neighbours, September runs about 4°C cooler than August and about 7°C warmer than October.

10°20°30°151015202530
Avg high (°C)Avg low (°C)
12413
22313
32313
42313
52313
62313
72312
82212
92212
102212
112212
122212
132211
142111
152111
162111
172111
182110
192010
202010
212010
222010
23199
24199
25199
26199
27189
28188
29188
30178

How September really feels in Cornwall

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

Days feeling ≥ 32°C
0
heat index over the day's high
Days feeling ≤ C
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.

Cornwall weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 7th-wettest month here — the current month is highlighted below.

MonthHigh / Low (°C)Rainfall (mm)Wet daysSnow daysDaylight (hours)Travel score
Jan-4° / -12°7311.410.590.0
Feb-2° / -12°61108.710.10.0
Mar / -6°7611.2711.60.2
Apr12° / 10112.12.813.22.8
May19° / 9812.20.114.65.6
Jun24° / 13°10512.4015.46.3
Jul26° / 16°10012.9015.16.2
Aug25° / 15°9411.6013.96.6
Sep21° / 11°9310.5012.36.4
Oct13° / 105120.610.73.6
Nov / -1°8412.43.79.30.6
Dec-0° / -7°8212.49.38.60.0

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 30% of the time in September, the clearest month of the year.

Beyond those clear spells, partly cloudy skies cover about 18% of September, with mostly cloudy or overcast conditions the remaining 52%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
15149
25149
35149
45149
55050
65149
75050
85050
95149
105050
115050
124951
134951
145050
155050
164951
174951
184951
194951
204951
214852
224753
234753
244753
254555
264456
274357
284357
294258
304159

Chance of precipitation in September

Any precipitation

Cornwall gets around 93 mm of rain over 10.5 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 35% and peaks near 38% on the wettest days.

0%10%20%30%40%50%151015202530
Rain chance (%)
133
233
333
434
534
634
734
835
935
1035
1134
1234
1334
1434
1534
1634
1734
1834
1934
2035
2135
2235
2336
2436
2537
2637
2738
2838
2938
3038

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 60% of days. Sticky, muggy-or-worse air turns up on roughly 5% of them.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)Oppressive (%)
1402821111
2412820101
3422819101
444271991
545271891
647271781
748261781
850261671
951251671
1052251661
1154241561
1257231550
1357231550
1459221450
1560221440
1661211440
1762211340
1864201330
1965201230
2066191230
2167191130
2269181030
2370181020
247217920
257416820
267416820
277715710
287814710
298013610
308212510

Wind in September

Winds average 11 km/h in September.

Across September, daily wind ranges from about 10 km/h on the calmest days to 11 km/h at its breeziest. It stays fairly steady from day to day.

051015151015202530
Wind speed (km/h)Feels
110calm
210calm
310calm
410calm
510calm
610calm
710calm
810calm
910calm
1010calm
1110calm
1210calm
1311calm
1411calm
1511calm
1611calm
1711calm
1811calm
1911calm
2011calm
2111calm
2211calm
2311calm
2411calm
2511calm
2611calm
2711calm
2811calm
2911calm
3011calm

Sunrise & sunset in September

DaylightNight

Daylight runs from about 13 h 06 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 90 minutes over September. Sunrise moves from 5:25 AM to 6:02 AM, and sunset from 6:32 PM to 5:35 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
113.15:256:32
213.15:276:30
313.05:286:28
412.95:296:26
512.95:306:24
612.85:326:22
712.85:336:20
812.75:346:18
912.75:356:16
1012.65:376:14
1112.65:386:12
1212.55:396:10
1312.55:406:08
1412.45:416:06
1512.45:436:04
1612.35:446:03
1712.35:456:01
1812.25:465:59
1912.15:485:57
2012.15:495:55
2112.05:505:53
2212.05:515:51
2311.95:535:49
2411.95:545:47
2511.85:555:45
2611.85:565:43
2711.75:585:41
2811.75:595:39
2911.66:005:37
3011.66:025:35

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 3.91 kWh/m².

For context, solar energy here swings from a low near 1.2 kWh/m² to a high of 5.91 kWh/m² in June across the year — September's 3.91 kWh/m² falls in the middle.

024151015202530
Solar energy (kWh/m²)Level
14.6Bright
24.5Bright
34.5Bright
44.4Bright
54.4Bright
64.3Bright
74.3Bright
84.3Bright
94.2Bright
104.2Bright
114.1Bright
124.1Bright
134.0Bright
144.0Moderate
154.0Moderate
163.9Moderate
173.9Moderate
183.8Moderate
193.8Moderate
203.7Moderate
213.7Moderate
223.6Moderate
233.5Moderate
243.5Moderate
253.4Moderate
263.4Moderate
273.3Moderate
283.3Moderate
293.2Moderate
303.2Moderate
Planning a trip in September

Is September a good time to visit Cornwall?

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

6.4
Sightseeing
2nd-best month of the year
3.8
Beach & pool
4th-best month of the year

September is a reasonable time to visit Cornwall for sightseeing.

What to pack for Cornwall in September

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

In September, Cornwall sees daytime highs around 21°C and nights near 11°C. Worth packing: rain gear. About 11 days see rain.

rain gear

UV index in Cornwall in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 5 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Cornwall peaks around July 14 at about 8 (very high) — sunscreen and a hat are worth it in the middle of the day. It bottoms out near January 10 at about 1 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 7 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecLowModerateHighVery highModerateModerateLowJul 14UV 8Jan 10UV 1
UV index (midday)Level
Jan1Low
Feb2Low
Mar3Moderate
Apr4Moderate
May6High
Jun6High
Jul8Very high
Aug4Moderate
Sep5High
Oct3Moderate
Nov2Low
Dec1Low
Location & terrain

Where is Cornwall?

Cornwall sits at 45.0°N, 74.7°W, 59 m above sea level, in Canada. The nearest listed city is Les Coteaux, 49 km away.

Topography around Cornwall

How much the land rises and falls, and what covers it, within 3, 16 and 80 km of Cornwall.

Within 3 km, the terrain around Cornwall has only modest variations in elevation — a maximum elevation change of 40 m around an average of 58 m above sea level; within 16 km, only modest variations in elevation — a maximum elevation change of 100 m; within 80 km, large variations in elevation — a maximum elevation change of 1,164 m.

The land around Cornwall is covered by trees (35%) and artificial surfaces (26%) within 3 km, trees (51%) within 16 km, and trees (58%) and cropland (28%) within 80 km.

3 km
40 m
16 km
100 m
80 km
1,164 m
treesgrasslandcroplandartificial surfaceswaterwetland
September weather nearby: Les Coteaux 49 km · Salaberry-de-Valleyfield 53 km · Saint-Lazare 63 km · Vaudreuil-Dorion 69 km · Clarence-Rockland 74 km · Orléans 78 km · Fallingbrook 78 km · Gloucester 80 km

Canada travel facts

Travel advisory
Level 1
Exercise Normal Precautions
Power plugs
Type A / B
120 V · 60 Hz
Prices
≈ 83% of US
price level, 2025
GDP per capita
$55,698
2025
Happiness
6.7 / 10
World Happiness, 2025
Safe drinking water
97%
of population, 2024
Homicide rate
2
per 100k, 2023
Road deaths
5.3
per 100k/yr, 2019
Income group
High
World Bank
Religion
Christian 53%none 35%Muslim 5%Hindu 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 Cornwall (45.0181°, -74.7281°, elevation 59 m), 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. Topography is sampled from the Copernicus DEM GLO-90 elevation model (© ESA/Airbus) and ESA WorldCover 2021 land cover (CC BY 4.0).