MARINE & COAST
840 miles of shoreline, almost all of it facing a cold current.
The single most important fact about the California coast is that its ocean is cold, and coldest in summer. That one inversion explains the fog, the wind, the microclimates, and why a July afternoon in San Francisco is cooler than a January afternoon in San Diego.
The mechanism is upwelling. A semi-permanent high sits over the eastern Pacific in summer and drives a persistent northwesterly along a shoreline that runs north–south. That wind pushes surface water offshore, and cold water rises from below to replace it. Air crossing it condenses — which is the marine layer, and everything that follows from it.
Where that layer can reach inland, it does. Where a range stops it, the climate changes within a few miles. Almost every coastal microclimate in California is a question about how far the marine air got.
How maritime is it, really?
ANNUAL RANGE — THE CLEANEST MARINE INDEX THERE ISThe best single measure of maritime influence is not temperature but annual range: the gap between the warmest month's mean and the coldest month's. Water has enormous thermal inertia, so a place genuinely governed by the ocean barely moves through the year, while a continental place swings wildly. The bars below are every place on this site, shortest range first — and the top of the list is a near-perfect map of the coastline.
The coastal transect
HOW FAR INLAND THE OCEAN REACHESThree pairs, each one a coastal station against an inland station within about thirty miles of it. The July gap is the marine layer's reach, measured.
| Coastal | Jul high | Range | Inland | Jul high | Range | Jul gap |
|---|
What the coast does, and where
Point Conception
The sharpest break on the West Coast. North of it the shore runs north–south into the full northwesterly and the cold current; east of it the coast turns and shelters the Southern California Bight, which is warmer, calmer and a different marine climate entirely.
The Catalina eddy
A counter-clockwise coastal circulation that spins up in the Bight, deepens the marine layer and can hold the Southern California beaches grey for days. It is a genuine local circulation, not just fog — and it is why San Diego and Los Angeles can be overcast while the deserts behind them bake.
May Gray, June Gloom
The marine layer is deepest in late spring, because the interior has warmed while the ocean has not. The temperature difference driving the onshore flow is at its maximum before the water catches up — which is why the greyest months are not the coldest ones.
The Golden Gate
The only sea-level gap in six hundred miles of Coast Ranges, and therefore the nozzle every bit of marine air bound for the Central Valley passes through. See the wind regimes.
Sea fog vs. valley fog
Two completely different things that share a name. Coastal stratus is advection fog — warm moist air moving over cold water — and is a summer phenomenon. Tule fog in the Central Valley is radiation fog under a winter inversion. Neither predicts the other.
The islands
The Channel Islands straddle the temperature break itself. The northern islands sit in the cold side and are foggy and wind-blasted; the southern islands sit in the warm side and are neither. The region page has the detail.
Analysis
FILED UNDER MARINE & COASTGoing on the water?
This page explains the coastal climate. It is not a marine forecast and carries no observations. For sea state, swell and wave height, our sister site Forecast Waves draws four global wave models by ocean basin.
For Small Craft Advisories, Gale Warnings, and coastal and offshore forecasts, use the National Weather Service Marine Forecasts and the National Data Buoy Center for live buoy observations. Tides and currents come from NOAA CO-OPS.