Thunder Bay weather in September

In September, Thunder Bay sees average daytime highs of 17°C and overnight lows near 9°C, with 84 mm of rain across 10.5 wet days and about 12.4 hours of daylight. It is the 4th-warmest and 1st-wettest month of the year here.

Highs ease over the month, from about 20°C in the first days to 14°C by the end. The sky is clear or mostly clear about 23% of the time in September, and the air most often feels dry.

Daylight stretches from about 13 h 12 min at the start of September to 11 h 30 min by month's end. Set against the rest of the year, September is Thunder Bay's 4th-clearest and 4th-windiest month. For the whole year in context, see Thunder Bay's year-round climate.

Daytime high
17°
4th warmest
Overnight low
9th coldest
Rainfall
84 mm
10.5 wet days
Daylight
12 h 18 min
6:41 AM – 7:02 PM
Cloud cover
64%
4th clearest
Wind
13 km/h
4th windiest
Solar energy
3.65 kWh/m²
Travel score
5.2
out of 10

Thunder Bay is city in Thunder Bay District, Ontario, Canada.

Air quality
Excellent
PM2.5 5 µg/m³ · 2019
Density
243/km²
people per km²
Founded
1683

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 20°C in the first days to 14°C by the end.

A typical September day in Thunder Bay reaches about 17°C in the afternoon and slips back to 9°C overnight — a 8°C spread between the day's warmth and the night's chill. On most days the high lands between 12°C and 22°C. Set against its neighbours, September runs about 5°C cooler than August and about 8°C warmer than October.

10°15°20°25°151015202530
Avg high (°C)Avg low (°C)
12012
22012
32011
41911
51911
61911
71911
81911
91810
101810
111810
121810
131810
14179
15179
16179
17179
18168
19168
20168
21168
22158
23157
24157
25157
26147
27146
28146
29136
30136

How September really feels in Thunder Bay

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.

Thunder Bay weather by month

How September compares with the rest of the year at a glance. September is the 4th-warmest and 1st-wettest month here — the current month is highlighted below.

MonthHigh / Low (°C)Rainfall (mm)Wet daysSnow daysDaylight (hours)Travel score
Jan-10° / -18°296.57.58.60.0
Feb-7° / -16°224.95.49.90.0
Mar-1° / -10°316.85.611.60.0
Apr / -3°548.3513.40.7
May13° / 6610.10.8153.5
Jun19° / 10°6510.5015.85.8
Jul22° / 14°7610.6015.56.6
Aug22° / 13°679.1014.16.7
Sep17° / 8410.5012.45.2
Oct / 7210.41.310.61.7
Nov / -5°497.85.790.0
Dec-6° / -13°387.37.88.20.0

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 23% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 18% of September, with mostly cloudy or overcast conditions the remaining 60%.

0%25%50%75%100%151015202530
Clearer (%)Cloudier (%)
14753
24753
34753
44753
54654
64654
74456
84456
94258
104258
114060
124060
133961
143961
153862
163763
173763
183763
193763
203763
213763
223862
233862
243862
253862
263862
273862
283862
293862
303763

Chance of precipitation in September

Any precipitation

Thunder Bay gets around 84 mm of rain over 10.5 wet days in September, the wettest month. The line is the day-by-day chance of measurable precipitation.

Day to day, the chance of measurable rain averages around 34% and peaks near 35% on the wettest days.

0%10%20%30%40%50%151015202530
Rain chance (%)
133
233
333
433
533
634
734
834
934
1034
1134
1234
1334
1433
1533
1633
1733
1833
1934
2034
2134
2234
2335
2435
2535
2635
2735
2835
2935
3035

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 77% of days. It rarely turns muggy — the humid and sticky bands barely register.

0%25%50%75%100%151015202530
Dry (%)Comfortable (%)Humid (%)Muggy (%)
15725153
25924143
36023143
46222133
56222133
66521122
76620122
86720112
96819112
106919102
11721891
12731791
13741781
14751681
15771571
16781570
17801460
18801460
19821350
20831250
21851140
22851140
23871030
2488930
2590820
2690820
2791720
2893610
2993610
3094510

Wind in September

Winds average 13 km/h in September.

Across September, daily wind ranges from about 12 km/h on the calmest days to 14 km/h at its breeziest. It stays fairly steady from day to day.

05101520151015202530
Wind speed (km/h)Feels
112light breeze
213light breeze
313light breeze
413light breeze
513light breeze
613light breeze
713light breeze
813light breeze
913light breeze
1013light breeze
1113light breeze
1213light breeze
1313light breeze
1413light breeze
1513light breeze
1613light breeze
1713light breeze
1813light breeze
1913light breeze
2013light breeze
2113light breeze
2213light breeze
2314light breeze
2414light breeze
2514light breeze
2614light breeze
2714light breeze
2814light breeze
2914light breeze
3014light breeze

Sunrise & sunset in September

DaylightNight

Daylight runs from about 13 h 12 min at the start of September to 11 h 30 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 102 minutes over September. Sunrise moves from 6:19 AM to 7:01 AM, and sunset from 7:34 PM to 6:31 PM.

12 AM4 AM8 AM12 PM4 PM8 PM12 AM151015202530
Daylight (h)Sunrise (AM)Sunset (PM)
113.26:197:34
213.26:217:32
313.16:227:30
413.16:247:28
513.06:257:26
612.96:277:23
712.96:287:21
812.86:297:19
912.86:317:17
1012.76:327:15
1112.66:347:13
1212.66:357:10
1312.56:377:08
1412.56:387:06
1512.46:397:04
1612.36:417:02
1712.36:427:00
1812.26:446:57
1912.26:456:55
2012.16:476:53
2112.06:486:51
2212.06:506:49
2311.96:516:46
2411.96:526:44
2511.86:546:42
2611.76:556:40
2711.76:576:38
2811.66:586:36
2911.67:006:33
3011.57:016:31

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 3.65 kWh/m².

For context, solar energy here swings from a low near 1.12 kWh/m² to a high of 6.21 kWh/m² in June across the year — September's 3.65 kWh/m² falls in the middle.

012345151015202530
Solar energy (kWh/m²)Level
14.4Bright
24.3Bright
34.3Bright
44.2Bright
54.2Bright
64.1Bright
74.1Bright
84.0Bright
94.0Moderate
103.9Moderate
113.9Moderate
123.8Moderate
133.8Moderate
143.7Moderate
153.7Moderate
163.6Moderate
173.6Moderate
183.5Moderate
193.5Moderate
203.4Moderate
213.4Moderate
223.3Moderate
233.3Moderate
243.2Moderate
253.2Moderate
263.1Moderate
273.1Moderate
283.0Moderate
293.0Moderate
303.0Moderate
Planning a trip in September

Is September a good time to visit Thunder Bay?

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

5.2
Sightseeing
4th-best month of the year
1.4
Beach & pool
4th-best month of the year

September is a reasonable time to visit Thunder Bay for sightseeing.

What to pack for Thunder Bay in September

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

In September, Thunder Bay sees daytime highs around 17°C and nights near 9°C. Worth packing: a warm layer and rain gear. About 11 days see rain.

a warm layerrain gear

Sea temperature near Thunder Bay in September

The nearest coast averages about 14°C in September — the other half of the beach question. The full-year curve and swim-comfort zones are below.

The sea offshore is warmest in August at about 15°C and coldest in March near 1°C.

The water stays below 20°C all year — cool enough that most swimmers want a wetsuit.

0°5°10°15°JanFebMarAprMayJunJulAugSepOctNovDecbracingcool
Sea temperature (°C)Comfort
Jan2Bracing
Feb1Bracing
Mar1Bracing
Apr2Bracing
May3Bracing
Jun4Bracing
Jul9Bracing
Aug15Cool
Sep14Bracing
Oct10Bracing
Nov6Bracing
Dec4Bracing

UV index in Thunder Bay 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 Thunder Bay peaks around June 17 at about 8 (very high) — sunscreen and a hat are worth it in the middle of the day. It bottoms out near December 21 at about 1 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 6 months of the year.

025710JanFebMarAprMayJunJulAugSepOctNovDecLowModerateHighVery highHighModerateLowJun 17UV 8Dec 21UV 1
UV index (midday)Level
Jan1Low
Feb2Low
Mar3Moderate
Apr5Moderate
May6High
Jun8Very high
Jul6High
Aug6High
Sep5Moderate
Oct3Moderate
Nov1Low
Dec1Low
Location & terrain

Where is Thunder Bay?

Thunder Bay sits at 48.4°N, 89.3°W, 188 m above sea level, in Canada. The nearest listed city is Marquette, 247 km away.

WausauWestonKenoraMarshfield

Topography around Thunder Bay

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

Within 3 km, the terrain around Thunder Bay has only modest variations in elevation — a maximum elevation change of 20 m around an average of 188 m above sea level; within 16 km, significant variations in elevation — a maximum elevation change of 297 m; within 80 km, significant variations in elevation — a maximum elevation change of 503 m.

The land around Thunder Bay is covered by artificial surfaces (41%) and trees (26%) within 3 km, trees (43%) and water (39%) within 16 km, and trees (57%) and water (40%) within 80 km.

3 km
20 m
16 km
297 m
80 km
503 m
treesgrasslandcroplandartificial surfacesbare terrainwaterwetland
September weather nearby: Marquette 247 km · Duluth 278 km · Superior 283 km · Hibbing 295 km · Wausau 382 km · Weston 389 km · Kenora 411 km · Marshfield 419 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 Thunder Bay (48.382°, -89.2502°, elevation 188 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. Sea temperatures are the NOAA OISST v2.1 1991–2020 monthly climatology (public domain), sampled at the nearest ocean point. 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).