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Mercer Island, Washington climate

Csb Warm-summer Mediterranean

Mercer Island has a seasonal cool-temperate climate (Warm-summer Mediterranean, Köppen Csb). The hottest month is August with an average daily high near 25°C, while the coldest is December averaging around 3°C at night. Precipitation totals about 999 mm a year and is strongly seasonal — November alone averages 160 mm while July barely receives 15 mm. Light snow accumulates each winter (16 cm of snow per year on average).

☀️ Summers

Mild

Highs near 25°C in August.

About 4 days a year above 32 °C

❄️ Winters

Chilly

Lows near 3°C in December.

About 25 freezing nights a year

🌧️ Rain & snow

Moderate rainfall

About 999 mm of rain a year, plus 16 cm of snow.

Snow falls through the winter months

⛅ Sky & trend

Often cloudy

Cloudy skies much of the year.

+0.8 °C vs 50 years ago

Temperature & precipitation

Daily high–low temperature range with the monthly average, plus monthly precipitation split into rain and snow · 1991–2020 normals · hover a month for the exact figures

Clear vs cloudy days

Share of days each month that are clear, partly cloudy or cloudy · 1991–2020 · hover a month for the day counts

Mercer Island climate normals (1991–2020)

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Avg high (°C)	9	10	12	15	19	22	25	25	22	16	11	8
Avg temp (°C)	6	7	8	11	14	17	20	20	17	12	8	6
Avg low (°C)	3	3	4	6	9	12	14	14	12	8	5	3
Precipitation (mm)	147	96	106	81	48	37	15	25	41	99	160	145
Snowfall (cm)	5	6	1	—	—	—	—	—	—	—	1	4

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Avg high (°C)	9	10	12	15	19	22
Avg temp (°C)	6	7	8	11	14	17
Avg low (°C)	3	3	4	6	9	12
Precipitation (mm)	147	96	106	81	48	37
Snowfall (cm)	5	6	1	—	—	—

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Avg high (°C)	25	25	22	16	11	8
Avg temp (°C)	20	20	17	12	8	6
Avg low (°C)	14	14	12	8	5	3
Precipitation (mm)	15	25	41	99	160	145
Snowfall (cm)	—	—	—	—	1	4

How often — frequency of weather events

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Days with measurable rain (days)	19	16	17	15	11	9	5	5	8	14	18	18
Days with high ≥ 32°C (days)	—	—	—	—	0.0	0.6	2	1	0.2	—	—	—
Days with low ≤ 0°C (days)	7	6	2	0.1	—	—	—	—	—	0.2	3	7
Days with snowfall ≥ 2.5 cm (days)	0.3	0.4	0.1	—	—	—	—	—	—	—	0.0	0.4

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Days with measurable rain (days)	19	16	17	15	11	9
Days with high ≥ 32°C (days)	—	—	—	—	0.0	0.6
Days with low ≤ 0°C (days)	7	6	2	0.1	—	—
Days with snowfall ≥ 2.5 cm (days)	0.3	0.4	0.1	—	—	—

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Days with measurable rain (days)	5	5	8	14	18	18
Days with high ≥ 32°C (days)	2	1	0.2	—	—	—
Days with low ≤ 0°C (days)	—	—	—	0.2	3	7
Days with snowfall ≥ 2.5 cm (days)	—	—	—	—	0.0	0.4

Sky & atmosphere

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Cloud cover (%)	76	72	74	73	69	66	48	48	51	66	75	75
Clear days (≤ 30% cloud)	3	4	3	3	4	4	11	11	10	5	3	2
Partly cloudy days (30–70%)	7	7	6	8	8	9	10	10	9	9	6	7
Cloudy days (≥ 70%)	22	18	22	19	19	17	10	10	11	17	21	21
Relative humidity (%)	90	87	85	82	80	80	78	76	77	83	89	90
Wind speed (km/h)	3.0	3.0	3.0	2.7	2.3	2.1	1.9	1.9	2.2	2.7	3.1	3.1

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Cloud cover (%)	76	72	74	73	69	66
Clear days (≤ 30% cloud)	3	4	3	3	4	4
Partly cloudy days (30–70%)	7	7	6	8	8	9
Cloudy days (≥ 70%)	22	18	22	19	19	17
Relative humidity (%)	90	87	85	82	80	80
Wind speed (km/h)	3.0	3.0	3.0	2.7	2.3	2.1

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Cloud cover (%)	48	48	51	66	75	75
Clear days (≤ 30% cloud)	11	11	10	5	3	2
Partly cloudy days (30–70%)	10	10	9	9	6	7
Cloudy days (≥ 70%)	10	10	11	17	21	21
Relative humidity (%)	78	76	77	83	89	90
Wind speed (km/h)	1.9	1.9	2.2	2.7	3.1	3.1

30-year delta table (1991–2020 minus 1981–2010, °F)

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Δ Max temp	+0.8	+0.4	+0.5	+0.8	+1.6	+1.2	+1.6	+1.3	+1.1	+0.8	+1.2	+1.3
Δ Avg temp	+0.8	+0.6	+0.6	+1.0	+1.5	+1.1	+1.4	+1.3	+1.3	+1.0	+1.1	+1.4
Δ Min temp	+0.8	+0.8	+0.6	+1.1	+1.4	+1.1	+1.2	+1.3	+1.5	+1.2	+0.9	+1.5

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Δ Max temp	+0.8	+0.4	+0.5	+0.8	+1.6	+1.2
Δ Avg temp	+0.8	+0.6	+0.6	+1.0	+1.5	+1.1
Δ Min temp	+0.8	+0.8	+0.6	+1.1	+1.4	+1.1

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Δ Max temp	+1.6	+1.3	+1.1	+0.8	+1.2	+1.3
Δ Avg temp	+1.4	+1.3	+1.3	+1.0	+1.1	+1.4
Δ Min temp	+1.2	+1.3	+1.5	+1.2	+0.9	+1.5

Methodology & sources

Temperature & precipitation — the official 1991–2020 climate normals from NOAA's U.S. Climate Normals, measured at Seattle Tacoma AP (NOAA GHCN station USW00024233), about 16 km from the city centre.

Cloud, humidity, wind & sunshine — modelled estimates from NASA POWER, NASA's satellite-and-reanalysis climatology. This is the standard global source for atmospheric variables, which are not measured at most weather stations.

How we build these numbers →