Mercer Island weather in September

In September, Mercer Island sees average daytime highs of 69°F and overnight lows near 53°F, with 2.6 in of rain across 8.4 wet days and about 12.4 hours of daylight. It is the 3rd-warmest and 9th-wettest month of the year here.

Highs ease over the month, from about 72°F in the first days to 65°F by the end. The sky is clear or mostly clear about 41% 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 Mercer Island's 3rd-clearest and 10th-windiest month. For the whole year in context, see Mercer Island's year-round climate.

Daytime high: 69°; 3rd warmest
Overnight low: 53°; 10th coldest
Rainfall: 2.6 in; 8.4 wet days
Daylight: 12 h 18 min; 5:53 AM – 6:13 PM
Cloud cover: 48%; 3rd clearest
Wind: 5 mph; 10th windiest
Solar energy: 4.03 kWh/m²
Travel score: 6.8; out of 10

Mercer Island is city in King County, Washington State, USA.

Density: 738/km²; people per km²
Founded: 1960

Air quality: WHO Ambient Air Quality Database. Density & description: Wikidata (CC0), GeoNames.

← August Mercer Island 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 72°F in the first days to 65°F by the end.

A typical September day in Mercer Island reaches about 69°F in the afternoon and slips back to 53°F overnight — a 15°F spread between the day's warmth and the night's chill. On most days the high lands between 62°F and 77°F. Set against its neighbours, September runs about 6°F cooler than August and about 10°F 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 (°F)	72	72	72	72	72	71	71	71	71	70	70	70	70	69	69	69	69	68	68	68	67	67	67	67	66	66	66	65	65	65
Avg low (°F)	56	56	56	55	55	55	55	55	55	55	54	54	54	54	54	53	53	53	53	53	53	52	52	52	52	51	51	51	51	51

	Avg high (°F)	Avg low (°F)
1	72	56
2	72	56
3	72	56
4	72	55
5	72	55
6	71	55
7	71	55
8	71	55
9	71	55
10	70	55
11	70	54
12	70	54
13	70	54
14	69	54
15	69	54
16	69	53
17	69	53
18	68	53
19	68	53
20	68	53
21	67	53
22	67	52
23	67	52
24	67	52
25	66	52
26	66	51
27	66	51
28	65	51
29	65	51
30	65	51

Mercer Island weather by month

How September compares with the rest of the year at a glance. September is the 3rd-warmest and 9th-wettest month here — the current month is highlighted below.

Month	High / Low (°F)	Rainfall (inches)	Wet days	Snow days	Daylight (hours)	Travel score
Jan	46° / 38°	6.9	18.4	2.2	8.7	1.0
Feb	49° / 39°	4.7	14.5	1.8	9.9	1.6
Mar	52° / 40°	5.5	17.5	0.8	11.6	2.3
Apr	57° / 44°	4.5	14.9	0.1	13.3	3.5
May	64° / 49°	3.0	12	0	14.9	5.3
Jun	68° / 53°	2.3	9.5	0	15.7	6.3
Jul	74° / 57°	1.0	5	0	15.4	7.9
Aug	75° / 58°	1.3	4.8	0	14.1	8.0
Sep	69° / 53°	2.6	8.4	0	12.4	6.8
Oct	59° / 47°	5.3	14.1	0	10.6	4.2
Nov	50° / 42°	7.6	18	0.6	9.1	1.9
Dec	45° / 38°	7.0	17.6	2.1	8.3	0.8

Cloud cover in September

ClearMostly clearPartly cloudyMostly cloudyOvercast

The sky is clear or mostly clear about 41% of the time in September.

Beyond those clear spells, partly cloudy skies cover about 16% of September, with mostly cloudy or overcast conditions the remaining 43%.

	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 (%)	60	60	61	61	61	61	60	60	59	58	58	57	56	57	56	56	55	56	56	55	55	55	54	54	54	54	54	52	52	51
Cloudier (%)	40	40	39	39	39	39	40	40	41	42	42	43	44	43	44	44	45	44	44	45	45	45	46	46	46	46	46	48	48	49

	Clearer (%)	Cloudier (%)
1	60	40
2	60	40
3	61	39
4	61	39
5	61	39
6	61	39
7	60	40
8	60	40
9	59	41
10	58	42
11	58	42
12	57	43
13	56	44
14	57	43
15	56	44
16	56	44
17	55	45
18	56	44
19	56	44
20	55	45
21	55	45
22	55	45
23	54	46
24	54	46
25	54	46
26	54	46
27	54	46
28	52	48
29	52	48
30	51	49

Chance of precipitation in September

Any precipitation

Mercer Island gets around 2.6 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 28% and peaks near 31% on the wettest days. The odds climb toward the end of the month.

	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 (%)	23	24	24	24	25	25	25	26	26	27	28	28	28	29	29	29	29	30	30	30	31	31	31	31	31	31	31	31	31	31

	Rain chance (%)
1	23
2	24
3	24
4	24
5	25
6	25
7	25
8	26
9	26
10	27
11	28
12	28
13	28
14	29
15	29
16	29
17	29
18	30
19	30
20	30
21	31
22	31
23	31
24	31
25	31
26	31
27	31
28	31
29	31
30	31

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 71% 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 (%)	58	59	60	60	61	62	63	65	66	67	68	69	69	70	71	71	72	73	73	74	75	76	77	78	79	80	81	82	83	84
Comfortable (%)	39	38	37	37	36	35	34	33	32	31	30	29	29	28	27	27	26	25	25	24	23	22	22	21	20	19	18	17	16	15
Humid (%)	3	3	3	3	3	3	3	2	2	2	2	2	2	2	2	2	2	2	2	2	2	2	1	1	1	1	1	1	1	1

	Dry (%)	Comfortable (%)	Humid (%)
1	58	39	3
2	59	38	3
3	60	37	3
4	60	37	3
5	61	36	3
6	62	35	3
7	63	34	3
8	65	33	2
9	66	32	2
10	67	31	2
11	68	30	2
12	69	29	2
13	69	29	2
14	70	28	2
15	71	27	2
16	71	27	2
17	72	26	2
18	73	25	2
19	73	25	2
20	74	24	2
21	75	23	2
22	76	22	2
23	77	22	1
24	78	21	1
25	79	20	1
26	80	19	1
27	81	18	1
28	82	17	1
29	83	16	1
30	84	15	1

Wind in September

Winds average 5 mph in September.

Across September, daily wind ranges from about 4 mph on the calmest days to 5 mph 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 (mph)	4	4	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5	5
Feels	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm	calm

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

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 5:32 AM to 6:13 AM, and sunset from 6:45 PM to 5:44 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)	13.2	13.2	13.1	13.0	13.0	12.9	12.9	12.8	12.7	12.7	12.6	12.6	12.5	12.5	12.4	12.3	12.3	12.2	12.2	12.1	12.0	12.0	11.9	11.9	11.8	11.7	11.7	11.6	11.6	11.5
Sunrise (AM)	5:32	5:34	5:35	5:36	5:38	5:39	5:41	5:42	5:43	5:45	5:46	5:48	5:49	5:50	5:52	5:53	5:54	5:56	5:57	5:59	6:00	6:01	6:03	6:04	6:06	6:07	6:08	6:10	6:11	6:13
Sunset (PM)	6:45	6:43	6:41	6:39	6:37	6:35	6:32	6:30	6:28	6:26	6:24	6:22	6:20	6:18	6:15	6:13	6:11	6:09	6:07	6:05	6:03	6:01	5:58	5:56	5:54	5:52	5:50	5:48	5:46	5:44

	Daylight (h)	Sunrise (AM)	Sunset (PM)
1	13.2	5:32	6:45
2	13.2	5:34	6:43
3	13.1	5:35	6:41
4	13.0	5:36	6:39
5	13.0	5:38	6:37
6	12.9	5:39	6:35
7	12.9	5:41	6:32
8	12.8	5:42	6:30
9	12.7	5:43	6:28
10	12.7	5:45	6:26
11	12.6	5:46	6:24
12	12.6	5:48	6:22
13	12.5	5:49	6:20
14	12.5	5:50	6:18
15	12.4	5:52	6:15
16	12.3	5:53	6:13
17	12.3	5:54	6:11
18	12.2	5:56	6:09
19	12.2	5:57	6:07
20	12.1	5:59	6:05
21	12.0	6:00	6:03
22	12.0	6:01	6:01
23	11.9	6:03	5:58
24	11.9	6:04	5:56
25	11.8	6:06	5:54
26	11.7	6:07	5:52
27	11.7	6:08	5:50
28	11.6	6:10	5:48
29	11.6	6:11	5:46
30	11.5	6:13	5:44

Solar energy in September

Daily solar energy (kWh/m²)

Average shortwave solar energy reaching the ground each day in September, about 4.03 kWh/m².

For context, solar energy here swings from a low near 0.88 kWh/m² to a high of 6.14 kWh/m² in July across the year — September's 4.03 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)	4.7	4.7	4.6	4.6	4.5	4.5	4.5	4.4	4.3	4.3	4.2	4.2	4.1	4.1	4.0	4.0	3.9	3.8	3.8	3.7	3.7	3.6	3.6	3.5	3.5	3.4	3.4	3.3	3.3	3.2
Level	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Bright	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate	Moderate

	Solar energy (kWh/m²)	Level
1	4.7	Bright
2	4.7	Bright
3	4.6	Bright
4	4.6	Bright
5	4.5	Bright
6	4.5	Bright
7	4.5	Bright
8	4.4	Bright
9	4.3	Bright
10	4.3	Bright
11	4.2	Bright
12	4.2	Bright
13	4.1	Bright
14	4.1	Bright
15	4.0	Bright
16	4.0	Moderate
17	3.9	Moderate
18	3.8	Moderate
19	3.8	Moderate
20	3.7	Moderate
21	3.7	Moderate
22	3.6	Moderate
23	3.6	Moderate
24	3.5	Moderate
25	3.5	Moderate
26	3.4	Moderate
27	3.4	Moderate
28	3.3	Moderate
29	3.3	Moderate
30	3.2	Moderate

Planning a trip in September

Hazard calendar for Mercer Island

In seasonPeak

None of Mercer Island's seasonal hazards are active in September. The full-year calendar below shows when the rainy season, cyclone risk and dust haze do peak.

Mercer Island's rainy season runs October–April, when most of the year's rain falls.

Rainy season

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Is September a good time to visit Mercer Island?

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

6.8

Sightseeing

3rd-best month of the year

3.8

Beach & pool

3rd-best month of the year

September is a reasonable time to visit Mercer Island for sightseeing.

What to pack for Mercer Island in September

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

In September, Mercer Island sees daytime highs around 69°F and nights near 53°F. Worth packing: rain gear. About 8 days see rain.

rain gear

UV index in Mercer Island in September

LowModerateHighVery highExtreme

Clear-sky midday UV in September peaks around 4 — enough to warrant sun protection. The full-year curve, shaded by WHO exposure level, is below.

Under clear skies, the midday UV index in Mercer Island peaks around July 17 at about 8 (very high) — sunscreen and a hat are worth it in the middle of the day. It bottoms out near December 18 at about 1 (low).

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

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
UV index (midday)	1	1	4	5	7	6	8	6	4	3	1	1
Level	Low	Low	Moderate	Moderate	High	High	Very high	High	Moderate	Moderate	Low	Low

	UV index (midday)	Level
Jan	1	Low
Feb	1	Low
Mar	4	Moderate
Apr	5	Moderate
May	7	High
Jun	6	High
Jul	8	Very high
Aug	6	High
Sep	4	Moderate
Oct	3	Moderate
Nov	1	Low
Dec	1	Low

Air quality in Mercer Island 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 Mercer Island is worst in September — around 22 µg/m³ PM2.5 (moderate), about 3.4× the cleanest month (December, 7 µg/m³).

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

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
PM2.5 (µg/m³)	8	9	12	12	11	10	10	18	22	12	8	7
Level	Good	Good	Moderate	Good	Good	Good	Good	Moderate	Moderate	Good	Good	Good

	PM2.5 (µg/m³)	Level
Jan	8	Good
Feb	9	Good
Mar	12	Moderate
Apr	12	Good
May	11	Good
Jun	10	Good
Jul	10	Good
Aug	18	Moderate
Sep	22	Moderate
Oct	12	Good
Nov	8	Good
Dec	7	Good

Location & terrain

Where is Mercer Island?

Mercer Island sits at 47.6°N, 122.2°W, 335 ft above sea level, in United States. The nearest listed city is Columbia City, 2.5 mi away.

Topography around Mercer Island

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

Within 2 miles, the terrain around Mercer Island has only modest variations in elevation — a maximum elevation change of 400 ft around an average of 105 ft above sea level; within 10 miles, large variations in elevation — a maximum elevation change of 2,037 ft; within 50 miles, extreme variations in elevation — a maximum elevation change of 8,179 ft.

The land around Mercer Island is covered by trees (46%) and water (43%) within 2 miles, trees (48%), artificial surfaces (27%) and water (20%) within 10 miles, and trees (75%) within 50 miles.

2 miles

400 ft

trees 46%

10 miles

2,037 ft

trees 48%

50 miles

8,179 ft

trees 75%

treesgrasslandartificial surfacesbare terrainwater

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

September weather nearby: Columbia City 2.5 mi · Bellevue 3.1 mi · Bryn Mawr-Skyway 5.6 mi · West Lake Sammamish 5.6 mi · Seattle 5.6 mi · Renton 6.2 mi · Tukwila 6.8 mi · Kirkland 7.5 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

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 Mercer Island (47.5707°, -122.2221°, elevation 335 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).

	Avg high (°F)	Avg low (°F)
1	72	56
2	72	56
3	72	56
4	72	55
5	72	55
6	71	55
7	71	55
8	71	55
9	71	55
10	70	55
11	70	54
12	70	54
13	70	54
14	69	54
15	69	54
16	69	53
17	69	53
18	68	53
19	68	53
20	68	53
21	67	53
22	67	52
23	67	52
24	67	52
25	66	52
26	66	51
27	66	51
28	65	51
29	65	51
30	65	51

	Clearer (%)	Cloudier (%)
1	60	40
2	60	40
3	61	39
4	61	39
5	61	39
6	61	39
7	60	40
8	60	40
9	59	41
10	58	42
11	58	42
12	57	43
13	56	44
14	57	43
15	56	44
16	56	44
17	55	45
18	56	44
19	56	44
20	55	45
21	55	45
22	55	45
23	54	46
24	54	46
25	54	46
26	54	46
27	54	46
28	52	48
29	52	48
30	51	49

	Rain chance (%)
1	23
2	24
3	24
4	24
5	25
6	25
7	25
8	26
9	26
10	27
11	28
12	28
13	28
14	29
15	29
16	29
17	29
18	30
19	30
20	30
21	31
22	31
23	31
24	31
25	31
26	31
27	31
28	31
29	31
30	31

	Dry (%)	Comfortable (%)	Humid (%)
1	58	39	3
2	59	38	3
3	60	37	3
4	60	37	3
5	61	36	3
6	62	35	3
7	63	34	3
8	65	33	2
9	66	32	2
10	67	31	2
11	68	30	2
12	69	29	2
13	69	29	2
14	70	28	2
15	71	27	2
16	71	27	2
17	72	26	2
18	73	25	2
19	73	25	2
20	74	24	2
21	75	23	2
22	76	22	2
23	77	22	1
24	78	21	1
25	79	20	1
26	80	19	1
27	81	18	1
28	82	17	1
29	83	16	1
30	84	15	1

	Avg high (°F)	Avg low (°F)
1	72	56
2	72	56
3	72	56
4	72	55
5	72	55
6	71	55
7	71	55
8	71	55
9	71	55
10	70	55
11	70	54
12	70	54
13	70	54
14	69	54
15	69	54
16	69	53
17	69	53
18	68	53
19	68	53
20	68	53
21	67	53
22	67	52
23	67	52
24	67	52
25	66	52
26	66	51
27	66	51
28	65	51
29	65	51
30	65	51

	Clearer (%)	Cloudier (%)
1	60	40
2	60	40
3	61	39
4	61	39
5	61	39
6	61	39
7	60	40
8	60	40
9	59	41
10	58	42
11	58	42
12	57	43
13	56	44
14	57	43
15	56	44
16	56	44
17	55	45
18	56	44
19	56	44
20	55	45
21	55	45
22	55	45
23	54	46
24	54	46
25	54	46
26	54	46
27	54	46
28	52	48
29	52	48
30	51	49

	Rain chance (%)
1	23
2	24
3	24
4	24
5	25
6	25
7	25
8	26
9	26
10	27
11	28
12	28
13	28
14	29
15	29
16	29
17	29
18	30
19	30
20	30
21	31
22	31
23	31
24	31
25	31
26	31
27	31
28	31
29	31
30	31

	Dry (%)	Comfortable (%)	Humid (%)
1	58	39	3
2	59	38	3
3	60	37	3
4	60	37	3
5	61	36	3
6	62	35	3
7	63	34	3
8	65	33	2
9	66	32	2
10	67	31	2
11	68	30	2
12	69	29	2
13	69	29	2
14	70	28	2
15	71	27	2
16	71	27	2
17	72	26	2
18	73	25	2
19	73	25	2
20	74	24	2
21	75	23	2
22	76	22	2
23	77	22	1
24	78	21	1
25	79	20	1
26	80	19	1
27	81	18	1
28	82	17	1
29	83	16	1
30	84	15	1

	Avg high (°F)	Avg low (°F)
1	72	56
2	72	56
3	72	56
4	72	55
5	72	55
6	71	55
7	71	55
8	71	55
9	71	55
10	70	55
11	70	54
12	70	54
13	70	54
14	69	54
15	69	54
16	69	53
17	69	53
18	68	53
19	68	53
20	68	53
21	67	53
22	67	52
23	67	52
24	67	52
25	66	52
26	66	51
27	66	51
28	65	51
29	65	51
30	65	51

	Clearer (%)	Cloudier (%)
1	60	40
2	60	40
3	61	39
4	61	39
5	61	39
6	61	39
7	60	40
8	60	40
9	59	41
10	58	42
11	58	42
12	57	43
13	56	44
14	57	43
15	56	44
16	56	44
17	55	45
18	56	44
19	56	44
20	55	45
21	55	45
22	55	45
23	54	46
24	54	46
25	54	46
26	54	46
27	54	46
28	52	48
29	52	48
30	51	49

	Rain chance (%)
1	23
2	24
3	24
4	24
5	25
6	25
7	25
8	26
9	26
10	27
11	28
12	28
13	28
14	29
15	29
16	29
17	29
18	30
19	30
20	30
21	31
22	31
23	31
24	31
25	31
26	31
27	31
28	31
29	31
30	31

	Dry (%)	Comfortable (%)	Humid (%)
1	58	39	3
2	59	38	3
3	60	37	3
4	60	37	3
5	61	36	3
6	62	35	3
7	63	34	3
8	65	33	2
9	66	32	2
10	67	31	2
11	68	30	2
12	69	29	2
13	69	29	2
14	70	28	2
15	71	27	2
16	71	27	2
17	72	26	2
18	73	25	2
19	73	25	2
20	74	24	2
21	75	23	2
22	76	22	2
23	77	22	1
24	78	21	1
25	79	20	1
26	80	19	1
27	81	18	1
28	82	17	1
29	83	16	1
30	84	15	1