THE MEDITERRANEAN YEAR
Dry summers, wet winters, and a state that bets everything on a handful of storms.
Why the summer is dry
A HIGH THAT MOVES NORTH AND BACKMost of the world's mid-latitudes get rain year-round. California does not, and the reason is a semi-permanent area of high pressure over the eastern Pacific. In summer it strengthens and shifts north, parking off the coast, and it does two things at once: it steers the storm track into the Pacific Northwest and Canada, well north of California, and it forces air to sink over the state. Sinking air warms and dries, which suppresses cloud outright.
In autumn the high weakens and slides south, the storm track drops with it, and systems that have spent all summer hitting Oregon start reaching San Francisco, then Los Angeles. That migration is California's entire wet season, and its timing is why the state's water year is measured from October rather than January.
The same high is responsible for the coast being cold. Its clockwise circulation drives a persistent northwesterly along a north–south shoreline, which pushes surface water offshore and pulls 50°F water up to replace it. That is why Monterey Bay is colder in July than in January, and why the summer fog exists at all.
How concentrated the wet season is
SHARE OF ANNUAL PRECIPITATION BY MONTHEach bar is one place's year, split into twelve months by how much of its annual precipitation falls in each. The pattern is the argument: almost everywhere in California, the year's water arrives between November and April — across the 57 places on this site that window averages 86% of the annual total, and in 50 of them half the year falls in just three consecutive months. Watch the desert stations, though: they break the pattern, because the summer monsoon reaches them and nothing else in the state gets it.
Nine climate types in one state
KÖPPEN–GEIGERCalifornia carries more Köppen types than any other US state, and they are not scattered — they are stacked by elevation and strung along the coast in a way you can drive through in a morning.
The three gradients
RAIN SHADOW · MARINE REACH · LAPSE RATENearly every difference between two California places is one of three gradients, and each is measurable from the normals. Below, each is computed across the whole fifty-seven-place table rather than asserted.
The variability is the climate
A NORMAL IS A POOR DESCRIPTION OF A CALIFORNIA YEARAn average year is rare
California has the highest year-to-year precipitation variability of any state. A "normal" 18 inches in Sacramento is the midpoint of a distribution that runs from under 8 to over 35 — so the normals on this site describe the centre of a wide spread, not a year you should expect.
A few storms decide it
Roughly half of the state's precipitation arrives in the wettest 5–10 days of the year. Whether a season is a drought or a flood usually comes down to how many atmospheric rivers made landfall and where — not to a season being broadly wetter.
Whiplash, not trend
The clearest signal in the modern record is not steadily wetter or drier but faster swings between the two: severe drought and record wet years arriving back to back, which is harder to manage than either extreme on its own.
ENSO explains less than people expect
El Niño tilts the odds wetter in Southern California and La Niña drier, but the relationship is weak in the north and unreliable everywhere in any single year. It is a thumb on the scale, not a forecast.
The snowpack is the reservoir
What matters is not only how much fell but how much fell as snow. The same storm total arriving warm runs off in days; arriving cold, it releases through spring. See Water.
Tule fog is measurably declining
The Central Valley's dense winter radiation fog has fallen sharply over four decades — one of the clearest and least-disputed climate signals in the state, and one with real consequences for the fruit and nut crops that need winter chill.