30-year climate normals

San Mateo County Climate and Weather Averages

Hottest month — September

66.4°

average daily high. The coldest month is December, averaging a low of 42.9°F.

San Mateo County gets 26.04" of precipitation a year, and 57% of it falls in Dec, Jan, Feb.

Temperature through the year

Each bar spans the average daily low to the average daily high for that month.

Month	Avg high	Avg low	Precip
January	59.4°	43.4°	5.07"
February	59.5°	43.0°	4.73"
March	60.4°	43.7°	3.67"
April	61.0°	44.7°	1.81"
May	61.7°	48.0°	0.96"
June	63.9°	49.9°	0.36"
July	64.8°	52.7°	0.14"
August	65.6°	53.9°	0.23"
September	66.4°	51.3°	0.21"
October	66.4°	47.9°	1.26"
November	62.6°	44.8°	2.43"
December	59.0°	42.9°	5.17"

Rainfall by month

San Mateo County has a distinct wet season. 57% of the year's precipitation arrives in Dec, Jan, Feb.

January 5.07"

February 4.73"

March 3.67"

April 1.81"

May 0.96"

June 0.36"

July 0.14"

August 0.23"

September 0.21"

October 1.26"

November 2.43"

December 5.17"

Annual total 26.04"

What this climate costs you

Degree days measure how hard a building has to work to stay comfortable. They are the reason two identical houses have very different power bills, and they are the link between the weather here and what you pay every month.

Cooling degree days How much air conditioning the climate demands 12

Heating degree days How much heating the climate demands 3,701

Approximate annual cooling cost At 33.25¢/kWh, California average $4

Approximate annual electric heating cost Lower where heating is gas rather than electric $431

These are rough planning figures from degree days and the state electricity price, not a bill estimate. House size, insulation and fuel type move them a lot. See the full cost of owning a home in San Mateo County, where electricity is one line of eight.

Common questions

What is the weather like in San Mateo County?

San Mateo County averages a high of 66.4°F in September and a low of 42.9°F in December. Annual precipitation is 26.04 inches, with essentially no snow.

When is the rainy season in San Mateo County?

The wettest stretch is Dec, Jan, Feb, which accounts for 57% of the year's precipitation. That is a pronounced wet season rather than rain spread evenly through the year.

How hot does it get in San Mateo County?

The average daily high in September, the hottest month, is 66.4°F. That drives 12 cooling degree days a year, which is what your air conditioning has to work against.

Does it snow in San Mateo County?

Effectively no. Measurable snowfall is rare in San Mateo County.

Sources

Temperature, precipitation and snowfall normals — NOAA National Centers for Environmental Information, 1991–2020 Monthly Climate Normals, station HALF MOON BAY, CA US (USC00043714), 5.6 miles from the county centroid.
Heating and cooling degree days — NOAA, same station and period.
Electricity price — US Energy Information Administration, Electric Power Monthly Table 5.6.A, California residential average.
Wet-season share is the wettest three consecutive months as a percentage of annual precipitation, calculated from the monthly normals.