Guides — Weather

The fog line

Why one street is sunny and the next over isn't, explained by a gap in the hills.

Checked August 2026

San Mateo has more than one climate, and the difference between them is larger than newcomers expect. On a July afternoon it can be 62°F and grey in the Highlands and 76°F and bright in Shoreview, four miles apart.

The mechanism

Cold ocean water offshore produces a marine layer — a shallow deck of cool, damp air. High pressure inland pulls it east. The Coast Range mostly blocks it, so the marine layer comes through the gaps: the big one at San Bruno to the north, and lower saddles in the ridge west of here.

That air arrives at elevation first. It pours over the hills, thins as it descends, and often runs out before it crosses El Camino. The gradient takes ten minutes to drive through: fog on the ridge, high grey over the slope, sun on the flats.

The neighborhoods on this site are sorted by elevation. Elevation is the variable.

The year, honestly

Winter is the green season. Rain arrives in storm systems rather than drizzle — several wet days, then a week of clear cold. Hills go emerald. Overnight lows in the forties. Anyone arriving from a place with a real winter finds it absurd.

Spring is short and lovely. March and April are the best hiking weeks of the year, with green hills and wildflowers on the trails at Crystal Springs and Sugarloaf. The hills turn gold in May and stay gold until November.

Summer is the surprise. It is not hot. June and July run cool and grey in the mornings, and the afternoon wind off the Bay is dependable enough that kiteboarders schedule around it. Every summer evening event is a jacket event.

Fall is the actual summer. September and October are warm, clear, and still, with the year's highest temperatures. It is also fire-weather season: when an offshore wind sets up, the air is hot and dry and the utility may de-energize lines in the hills. West of the Alameda that is a household planning matter rather than a forecast.

Practical consequences

• Houses here mostly do not have air conditioning, and mostly don't need it — except for a couple of weeks in September, when everyone remembers they don't have it. A window unit or a heat pump for one room is a common fix.
• Older houses have single-pane windows and no insulation, because the climate forgives it. The heating bill does not.
• Microclimate is an address-level fact. Half a mile and a hundred feet of elevation change the answer.
• Air quality is usually excellent and occasionally, during wildfire smoke events elsewhere in the state, is not. A decent air purifier is standard equipment in Bay Area households.