Suomenlinna weather in September

In September, Suomenlinna sees average daytime highs of 15°C and overnight lows near 9°C, with 58 mm of rain across 10.5 wet days and about 12.6 hours of daylight. It is the 4th-warmest and 6th-wettest month of the year here.

Highs ease over the month, from about 17°C in the first days to 13°C by the end. The sky is clear or mostly clear about 20% of the time in September, and the air most often feels dry.

Daylight stretches from about 13 h 54 min at the start of September to 11 h 12 min by month's end. Set against the rest of the year, September is Suomenlinna's 7th-clearest and 6th-windiest month. For the whole year in context, see Suomenlinna's year-round climate.

Daytime high: 15°; 4th warmest
Overnight low: 9°; 9th coldest
Rainfall: 58 mm; 10.5 wet days
Daylight: 12 h 30 min; 5:58 AM – 6:31 PM
Cloud cover: 68%; 7th clearest
Wind: 14 km/h; 6th windiest
Solar energy: 2.57 kWh/m²
Travel score: 4.4; out of 10

← August Suomenlinna year-round climate October →Best places to visit in September

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

Temperature in September

Average highAverage lowShaded bands: 10–90% of days

Highs ease over the month, from about 17°C in the first days to 13°C by the end.

A typical September day in Suomenlinna reaches about 15°C in the afternoon and slips back to 9°C overnight — a 7°C spread between the day's warmth and the night's chill. On most days the high lands between 11°C and 19°C. Set against its neighbours, September runs about 5°C cooler than August and about 7°C warmer than October.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
Avg high (°C)	18	18	18	17	17	17	17	17	16	16	16	16	16	15	15	15	15	15	14	14	14	14	14	13	13	13	13	13	12	12
Avg low (°C)	11	11	11	10	10	10	10	10	10	10	9	9	9	9	9	8	8	8	8	8	8	8	7	7	7	7	7	7	7	6

	Avg high (°C)	Avg low (°C)
1	18	11
2	18	11
3	18	11
4	17	10
5	17	10
6	17	10
7	17	10
8	17	10
9	16	10
10	16	10
11	16	9
12	16	9
13	16	9
14	15	9
15	15	9
16	15	8
17	15	8
18	15	8
19	14	8
20	14	8
21	14	8
22	14	8
23	14	7
24	13	7
25	13	7
26	13	7
27	13	7
28	13	7
29	12	7
30	12	6

How September really feels in Suomenlinna

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

heat index over the day's high

Days feeling ≤ 0°C

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.

Suomenlinna 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.

Month	High / Low (°C)	Rainfall (mm)	Wet days	Snow days	Daylight (hours)	Travel score
Jan	-2° / -7°	47	10.9	10.1	6.4	0.0
Feb	-3° / -8°	42	9.5	8.6	8.6	0.0
Mar	1° / -5°	36	8.4	7	11.3	0.0
Apr	8° / 0°	37	8.5	2.3	14.2	1.4
May	14° / 6°	46	9.3	0.1	16.8	4.3
Jun	19° / 11°	74	11.8	0	18.4	5.6
Jul	22° / 14°	78	12.1	0	17.7	6.3
Aug	20° / 13°	74	12	0	15.4	6.0
Sep	15° / 9°	58	10.5	0	12.6	4.4
Oct	9° / 4°	60	11.4	0.7	9.7	1.5
Nov	4° / 0°	58	11.6	4.4	7.1	0.0
Dec	0° / -4°	56	12.1	8.8	5.6	0.0

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 20% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 15% of September, with mostly cloudy or overcast conditions the remaining 65%.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
Clearer (%)	33	33	34	35	36	36	36	37	37	37	38	38	38	37	37	37	36	36	35	35	35	34	34	33	33	33	33	33	33	32
Cloudier (%)	67	67	66	65	64	64	64	63	63	63	62	62	62	63	63	63	64	64	65	65	65	66	66	67	67	67	67	67	67	68

	Clearer (%)	Cloudier (%)
1	33	67
2	33	67
3	34	66
4	35	65
5	36	64
6	36	64
7	36	64
8	37	63
9	37	63
10	37	63
11	38	62
12	38	62
13	38	62
14	37	63
15	37	63
16	37	63
17	36	64
18	36	64
19	35	65
20	35	65
21	35	65
22	34	66
23	34	66
24	33	67
25	33	67
26	33	67
27	33	67
28	33	67
29	33	67
30	32	68

Chance of precipitation in September

Any precipitation

Suomenlinna gets around 58 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 36% and peaks near 38% on the wettest days.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
Rain chance (%)	38	38	38	38	38	38	38	38	38	38	38	38	38	37	37	36	36	35	34	34	33	33	32	32	32	32	32	33	33	34

	Rain chance (%)
1	38
2	38
3	38
4	38
5	38
6	38
7	38
8	38
9	38
10	38
11	38
12	38
13	38
14	37
15	37
16	36
17	36
18	35
19	34
20	34
21	33
22	33
23	32
24	32
25	32
26	32
27	32
28	33
29	33
30	34

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 87% of days. It rarely turns muggy — the humid and sticky bands barely register.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
Dry (%)	71	72	73	74	76	77	78	79	80	82	84	85	86	87	89	90	91	92	93	94	95	96	96	96	97	97	97	98	98	98
Comfortable (%)	27	26	25	24	23	22	21	20	19	18	16	15	14	13	11	10	9	8	7	6	5	4	4	4	3	3	3	2	2	2
Humid (%)	2	2	2	2	1	1	1	1	1	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0

	Dry (%)	Comfortable (%)	Humid (%)
1	71	27	2
2	72	26	2
3	73	25	2
4	74	24	2
5	76	23	1
6	77	22	1
7	78	21	1
8	79	20	1
9	80	19	1
10	82	18	0
11	84	16	0
12	85	15	0
13	86	14	0
14	87	13	0
15	89	11	0
16	90	10	0
17	91	9	0
18	92	8	0
19	93	7	0
20	94	6	0
21	95	5	0
22	96	4	0
23	96	4	0
24	96	4	0
25	97	3	0
26	97	3	0
27	97	3	0
28	98	2	0
29	98	2	0
30	98	2	0

Wind in September

Winds average 14 km/h in September.

Across September, daily wind ranges from about 13 km/h on the calmest days to 15 km/h at its breeziest. It stays fairly steady from day to day.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
Wind speed (km/h)	13	13	13	13	13	13	14	14	14	14	14	14	14	14	14	14	14	14	14	14	14	14	14	14	14	14	15	15	15	15
Feels	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze	light breeze

	Wind speed (km/h)	Feels
1	13	light breeze
2	13	light breeze
3	13	light breeze
4	13	light breeze
5	13	light breeze
6	13	light breeze
7	14	light breeze
8	14	light breeze
9	14	light breeze
10	14	light breeze
11	14	light breeze
12	14	light breeze
13	14	light breeze
14	14	light breeze
15	14	light breeze
16	14	light breeze
17	14	light breeze
18	14	light breeze
19	14	light breeze
20	14	light breeze
21	14	light breeze
22	14	light breeze
23	14	light breeze
24	14	light breeze
25	14	light breeze
26	14	light breeze
27	15	light breeze
28	15	light breeze
29	15	light breeze
30	15	light breeze

Sunrise & sunset in September

DaylightNight

Daylight runs from about 13 h 54 min at the start of September to 11 h 12 min by month's end. The lit band spans sunrise to sunset each day.

Days shrink by about 162 minutes over September. Sunrise moves from 5:22 AM to 6:33 AM, and sunset from 7:18 PM to 5:46 PM.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
Daylight (h)	13.9	13.8	13.8	13.7	13.6	13.5	13.4	13.3	13.2	13.1	13.0	12.9	12.8	12.7	12.6	12.5	12.4	12.4	12.3	12.2	12.1	12.0	11.9	11.8	11.7	11.6	11.5	11.4	11.3	11.2
Sunrise (AM)	5:22	5:24	5:27	5:29	5:31	5:34	5:36	5:39	5:41	5:44	5:46	5:48	5:51	5:53	5:56	5:58	6:01	6:03	6:06	6:08	6:10	6:13	6:15	6:18	6:20	6:23	6:25	6:28	6:30	6:33
Sunset (PM)	7:18	7:15	7:12	7:09	7:05	7:02	6:59	6:56	6:53	6:50	6:46	6:43	6:40	6:37	6:34	6:31	6:27	6:24	6:21	6:18	6:15	6:11	6:08	6:05	6:02	5:59	5:56	5:52	5:49	5:46

	Daylight (h)	Sunrise (AM)	Sunset (PM)
1	13.9	5:22	7:18
2	13.8	5:24	7:15
3	13.8	5:27	7:12
4	13.7	5:29	7:09
5	13.6	5:31	7:05
6	13.5	5:34	7:02
7	13.4	5:36	6:59
8	13.3	5:39	6:56
9	13.2	5:41	6:53
10	13.1	5:44	6:50
11	13.0	5:46	6:46
12	12.9	5:48	6:43
13	12.8	5:51	6:40
14	12.7	5:53	6:37
15	12.6	5:56	6:34
16	12.5	5:58	6:31
17	12.4	6:01	6:27
18	12.4	6:03	6:24
19	12.3	6:06	6:21
20	12.2	6:08	6:18
21	12.1	6:10	6:15
22	12.0	6:13	6:11
23	11.9	6:15	6:08
24	11.8	6:18	6:05
25	11.7	6:20	6:02
26	11.6	6:23	5:59
27	11.5	6:25	5:56
28	11.4	6:28	5:52
29	11.3	6:30	5:49
30	11.2	6:33	5:46

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 2.57 kWh/m².

For context, solar energy here swings from a low near 0.16 kWh/m² to a high of 5.82 kWh/m² in June across the year — September's 2.57 kWh/m² falls in the middle.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30
Solar energy (kWh/m²)	3.3	3.2	3.2	3.1	3.1	3.0	3.0	2.9	2.9	2.8	2.8	2.7	2.7	2.6	2.6	2.5	2.5	2.5	2.4	2.4	2.3	2.3	2.2	2.2	2.1	2.1	2.0	1.9	1.9	1.9
Level	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Dim	Dim	Dim

	Solar energy (kWh/m²)	Level
1	3.3	Moderate
2	3.2	Moderate
3	3.2	Moderate
4	3.1	Moderate
5	3.1	Moderate
6	3.0	Moderate
7	3.0	Moderate
8	2.9	Moderate
9	2.9	Moderate
10	2.8	Moderate
11	2.8	Moderate
12	2.7	Moderate
13	2.7	Moderate
14	2.6	Moderate
15	2.6	Moderate
16	2.5	Moderate
17	2.5	Moderate
18	2.5	Moderate
19	2.4	Moderate
20	2.4	Moderate
21	2.3	Moderate
22	2.3	Moderate
23	2.2	Moderate
24	2.2	Moderate
25	2.1	Moderate
26	2.1	Moderate
27	2.0	Moderate
28	1.9	Dim
29	1.9	Dim
30	1.9	Dim

Planning a trip in September

Is September a good time to visit Suomenlinna?

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

4.4

Sightseeing

4th-best month of the year

0.5

Beach & pool

4th-best month of the year

September is a reasonable time to visit Suomenlinna for sightseeing.

What to pack for Suomenlinna in September

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

In September, Suomenlinna sees daytime highs around 15°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 Suomenlinna 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 17°C and coldest in March near 1°C.

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

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Sea temperature (°C)	2	1	1	2	6	11	16	17	14	10	7	4
Comfort	Bracing	Bracing	Bracing	Bracing	Bracing	Bracing	Cool	Cool	Bracing	Bracing	Bracing	Bracing

	Sea temperature (°C)	Comfort
Jan	2	Bracing
Feb	1	Bracing
Mar	1	Bracing
Apr	2	Bracing
May	6	Bracing
Jun	11	Bracing
Jul	16	Cool
Aug	17	Cool
Sep	14	Bracing
Oct	10	Bracing
Nov	7	Bracing
Dec	4	Bracing

UV index in Suomenlinna in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 3. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Suomenlinna peaks around June 18 at about 5 (high) — a little sun protection helps at midday. It bottoms out near January 1 at about 0 (low).

The index reaches 3 or more — the level where the WHO recommends sun protection — in 4 months of the year.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
UV index (midday)	0	1	2	3	4	5	5	5	3	1	0	0
Level	Low	Low	Low	Moderate	Moderate	High	Moderate	Moderate	Moderate	Low	Low	Low

	UV index (midday)	Level
Jan	0	Low
Feb	1	Low
Mar	2	Low
Apr	3	Moderate
May	4	Moderate
Jun	5	High
Jul	5	Moderate
Aug	5	Moderate
Sep	3	Moderate
Oct	1	Low
Nov	0	Low
Dec	0	Low

Location & terrain

Where is Suomenlinna?

Suomenlinna sits at 60.1°N, 25.0°E, 0 m above sea level, in Finland. The nearest listed city is Helsinki, 4.0 km away.

Topography around Suomenlinna

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

Within 3 km, the terrain around Suomenlinna has only modest variations in elevation — a maximum elevation change of 35 m around an average of 3 m above sea level; within 16 km, only modest variations in elevation — a maximum elevation change of 83 m; within 80 km, only modest variations in elevation — a maximum elevation change of 193 m.

The land around Suomenlinna is covered by water (77%) within 3 km, water (62%) and trees (21%) within 16 km, and water (50%) and trees (38%) within 80 km.

3 km

35 m

water 77%

16 km

83 m

water 62%

80 km

193 m

water 50%

treesgrasslandcroplandartificial surfaceswater

← August Suomenlinna year-round climate October →Best places to visit in September

September weather nearby: Helsinki 4.0 km · Laajasalo 4.0 km · Kallio 5.0 km · Taka-Töölö 5.0 km · Herttoniemi 6.0 km · Lauttasaari 7.0 km · Munkkiniemi 8.0 km · Viikki 8.0 km

Finland travel facts

Travel advisory: Level 1; Exercise Normal Precautions
Power plugs: Type C / F; 230 V · 50 Hz
Prices: ≈ 85% of US; price level, 2025
GDP per capita: $56,149; 2025
Happiness: 7.8 / 10; World Happiness, 2025
Safe drinking water: 100%; of population, 2024
Homicide rate: 1; per 100k, 2023
Road deaths: 3.9; per 100k/yr, 2019
Income group: High; World Bank
English: Very high; EF EPI 603
Religion

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 Suomenlinna (60.1478°, 24.9856°, elevation 0 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).

	Avg high (°C)	Avg low (°C)
1	18	11
2	18	11
3	18	11
4	17	10
5	17	10
6	17	10
7	17	10
8	17	10
9	16	10
10	16	10
11	16	9
12	16	9
13	16	9
14	15	9
15	15	9
16	15	8
17	15	8
18	15	8
19	14	8
20	14	8
21	14	8
22	14	8
23	14	7
24	13	7
25	13	7
26	13	7
27	13	7
28	13	7
29	12	7
30	12	6

	Clearer (%)	Cloudier (%)
1	33	67
2	33	67
3	34	66
4	35	65
5	36	64
6	36	64
7	36	64
8	37	63
9	37	63
10	37	63
11	38	62
12	38	62
13	38	62
14	37	63
15	37	63
16	37	63
17	36	64
18	36	64
19	35	65
20	35	65
21	35	65
22	34	66
23	34	66
24	33	67
25	33	67
26	33	67
27	33	67
28	33	67
29	33	67
30	32	68

	Rain chance (%)
1	38
2	38
3	38
4	38
5	38
6	38
7	38
8	38
9	38
10	38
11	38
12	38
13	38
14	37
15	37
16	36
17	36
18	35
19	34
20	34
21	33
22	33
23	32
24	32
25	32
26	32
27	32
28	33
29	33
30	34

	Dry (%)	Comfortable (%)	Humid (%)
1	71	27	2
2	72	26	2
3	73	25	2
4	74	24	2
5	76	23	1
6	77	22	1
7	78	21	1
8	79	20	1
9	80	19	1
10	82	18	0
11	84	16	0
12	85	15	0
13	86	14	0
14	87	13	0
15	89	11	0
16	90	10	0
17	91	9	0
18	92	8	0
19	93	7	0
20	94	6	0
21	95	5	0
22	96	4	0
23	96	4	0
24	96	4	0
25	97	3	0
26	97	3	0
27	97	3	0
28	98	2	0
29	98	2	0
30	98	2	0

	Avg high (°C)	Avg low (°C)
1	18	11
2	18	11
3	18	11
4	17	10
5	17	10
6	17	10
7	17	10
8	17	10
9	16	10
10	16	10
11	16	9
12	16	9
13	16	9
14	15	9
15	15	9
16	15	8
17	15	8
18	15	8
19	14	8
20	14	8
21	14	8
22	14	8
23	14	7
24	13	7
25	13	7
26	13	7
27	13	7
28	13	7
29	12	7
30	12	6

	Clearer (%)	Cloudier (%)
1	33	67
2	33	67
3	34	66
4	35	65
5	36	64
6	36	64
7	36	64
8	37	63
9	37	63
10	37	63
11	38	62
12	38	62
13	38	62
14	37	63
15	37	63
16	37	63
17	36	64
18	36	64
19	35	65
20	35	65
21	35	65
22	34	66
23	34	66
24	33	67
25	33	67
26	33	67
27	33	67
28	33	67
29	33	67
30	32	68

	Rain chance (%)
1	38
2	38
3	38
4	38
5	38
6	38
7	38
8	38
9	38
10	38
11	38
12	38
13	38
14	37
15	37
16	36
17	36
18	35
19	34
20	34
21	33
22	33
23	32
24	32
25	32
26	32
27	32
28	33
29	33
30	34

	Dry (%)	Comfortable (%)	Humid (%)
1	71	27	2
2	72	26	2
3	73	25	2
4	74	24	2
5	76	23	1
6	77	22	1
7	78	21	1
8	79	20	1
9	80	19	1
10	82	18	0
11	84	16	0
12	85	15	0
13	86	14	0
14	87	13	0
15	89	11	0
16	90	10	0
17	91	9	0
18	92	8	0
19	93	7	0
20	94	6	0
21	95	5	0
22	96	4	0
23	96	4	0
24	96	4	0
25	97	3	0
26	97	3	0
27	97	3	0
28	98	2	0
29	98	2	0
30	98	2	0

	Avg high (°C)	Avg low (°C)
1	18	11
2	18	11
3	18	11
4	17	10
5	17	10
6	17	10
7	17	10
8	17	10
9	16	10
10	16	10
11	16	9
12	16	9
13	16	9
14	15	9
15	15	9
16	15	8
17	15	8
18	15	8
19	14	8
20	14	8
21	14	8
22	14	8
23	14	7
24	13	7
25	13	7
26	13	7
27	13	7
28	13	7
29	12	7
30	12	6

	Clearer (%)	Cloudier (%)
1	33	67
2	33	67
3	34	66
4	35	65
5	36	64
6	36	64
7	36	64
8	37	63
9	37	63
10	37	63
11	38	62
12	38	62
13	38	62
14	37	63
15	37	63
16	37	63
17	36	64
18	36	64
19	35	65
20	35	65
21	35	65
22	34	66
23	34	66
24	33	67
25	33	67
26	33	67
27	33	67
28	33	67
29	33	67
30	32	68

	Rain chance (%)
1	38
2	38
3	38
4	38
5	38
6	38
7	38
8	38
9	38
10	38
11	38
12	38
13	38
14	37
15	37
16	36
17	36
18	35
19	34
20	34
21	33
22	33
23	32
24	32
25	32
26	32
27	32
28	33
29	33
30	34

	Dry (%)	Comfortable (%)	Humid (%)
1	71	27	2
2	72	26	2
3	73	25	2
4	74	24	2
5	76	23	1
6	77	22	1
7	78	21	1
8	79	20	1
9	80	19	1
10	82	18	0
11	84	16	0
12	85	15	0
13	86	14	0
14	87	13	0
15	89	11	0
16	90	10	0
17	91	9	0
18	92	8	0
19	93	7	0
20	94	6	0
21	95	5	0
22	96	4	0
23	96	4	0
24	96	4	0
25	97	3	0
26	97	3	0
27	97	3	0
28	98	2	0
29	98	2	0
30	98	2	0