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Foster City, California climate

Csb Warm-summer Mediterranean

Foster City has a mildly seasonal mild-temperate climate (Warm-summer Mediterranean, Köppen Csb). The hottest month is July with an average daily high near 81°F, while the coldest is December averaging around 40°F at night. Precipitation totals about 19 inches a year and is strongly seasonal — December alone averages 3.9 inches while July barely receives 0.00 inches.

☀️ Summers

Warm

Highs near 81°F in July.

About 17 days a year above 90 °F

❄️ Winters

Cool

Lows near 40°F in December.

About 6 freezing nights a year

🌧️ Rain

Fairly dry

About 19 in of rain a year.

Wettest in December

⛅ Sky & trend

Partly cloudy

A roughly even mix of sun and cloud.

+0.0 °F vs 50 years ago

Temperature & precipitation

Daily high–low temperature range with the monthly average, plus monthly precipitation · 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

Foster City climate normals (1991–2020)

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Avg high (°F)	59	62	65	69	73	79	81	81	79	75	65	59
Avg temp (°F)	50	52	55	58	62	66	69	69	67	62	54	49
Avg low (°F)	41	43	45	47	50	54	57	57	54	50	44	40
Precipitation (in)	3.8	3.7	3.0	1.2	0.5	0.2	0.0	0.0	0.1	0.9	1.9	3.9

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Avg high (°F)	59	62	65	69	73	79
Avg temp (°F)	50	52	55	58	62	66
Avg low (°F)	41	43	45	47	50	54
Precipitation (in)	3.8	3.7	3.0	1.2	0.5	0.2

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Avg high (°F)	81	81	79	75	65	59
Avg temp (°F)	69	69	67	62	54	49
Avg low (°F)	57	57	54	50	44	40
Precipitation (in)	0.0	0.0	0.1	0.9	1.9	3.9

How often — frequency of weather events

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Days with measurable rain (days)	8	8	9	5	3	0.9	0.0	0.2	0.5	2	6	9
Days with high ≥ 90°F (days)	—	—	—	0.3	1	4	3	3	4	1	—	—
Days with low ≤ 32°F (days)	2	0.6	—	—	—	—	—	—	0.0	—	0.4	2

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Days with measurable rain (days)	8	8	9	5	3	0.9
Days with high ≥ 90°F (days)	—	—	—	0.3	1	4
Days with low ≤ 32°F (days)	2	0.6	—	—	—	—

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Days with measurable rain (days)	0.0	0.2	0.5	2	6	9
Days with high ≥ 90°F (days)	3	3	4	1	—	—
Days with low ≤ 32°F (days)	—	—	0.0	—	0.4	2

Sky & atmosphere

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Cloud cover (%)	60	61	60	55	54	48	58	59	51	47	51	57
Clear days (≤ 30% cloud)	7	6	7	8	8	10	4	4	9	11	10	8
Partly cloudy days (30–70%)	9	8	11	11	11	11	15	14	11	11	10	9
Cloudy days (≥ 70%)	15	14	14	11	12	9	12	13	10	9	10	14
Relative humidity (%)	84	83	84	84	85	85	86	86	83	80	80	81
Wind speed (mph)	8.0	8.6	8.5	9.0	8.9	8.7	7.5	7.3	6.7	6.6	7.2	8.4

Jan – Jun	Jan	Feb	Mar	Apr	May	Jun
Cloud cover (%)	60	61	60	55	54	48
Clear days (≤ 30% cloud)	7	6	7	8	8	10
Partly cloudy days (30–70%)	9	8	11	11	11	11
Cloudy days (≥ 70%)	15	14	14	11	12	9
Relative humidity (%)	84	83	84	84	85	85
Wind speed (mph)	8.0	8.6	8.5	9.0	8.9	8.7

Jul – Dec	Jul	Aug	Sep	Oct	Nov	Dec
Cloud cover (%)	58	59	51	47	51	57
Clear days (≤ 30% cloud)	4	4	9	11	10	8
Partly cloudy days (30–70%)	15	14	11	11	10	9
Cloudy days (≥ 70%)	12	13	10	9	10	14
Relative humidity (%)	86	86	83	80	80	81
Wind speed (mph)	7.5	7.3	6.7	6.6	7.2	8.4

Methodology & sources

Temperature & precipitation — the official 1991–2020 climate normals from NOAA's U.S. Climate Normals, measured at Redwood City (NOAA GHCN station USC00047339), about 10 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 →