California month by month, at a glance
Figures for San Francisco, 1991-2020 series. It is considerably hotter and drier inland and in the south. Hover over each month.
California does not have four seasons: it has two, one wet and one dry. San Francisco takes 111.8 millimetres of rain in January and 0.3 in July. This is not a passing drought, it is the normal regime: between June and September the four months together come to 9.4 millimetres.
The key moments at a glance
In San Francisco these are the two warmest months of the year: mean maxima of 21.2 and 21.0 °C, with 2.5 and 23.9 mm of rain. The fog has withdrawn, the interior has stopped baking and the mountain passes are still open.
40.6 and 17.8 mm of rain, with 309.3 and 325.1 hours of sun, the annual peak. The hills are still green; by June they are gold. The risk is that the high country is still shut.
It practically does not rain — 0.3 mm in July — but the coast fills with fog and the maximum stays at 19 °C while Sacramento passes 33. It is also the busiest season in the parks.
120.9 mm in December and 111.8 in January, with sunshine at its lowest: 160.6 hours. The high passes are closed. In exchange, the desert is at its best.
01 A five-month dry season
The annual total is 581 millimetres over 71 rain days, and almost all of it falls between November and March. December and January take 232 millimetres between them. The four summer months take nine and a half.
The most visible consequence is the colour of the landscape. California's hills are green from January to April and gold from June to November, and that gold is not an aesthetic choice: it is dry annual grass. Anyone travelling in August expecting green fields is reading the wrong calendar.
The other consequence is fire. The vegetation spends half the year drying out, and the fire risk climbs in late summer and autumn, when the fuel is at its driest. You cannot plan it to a date, but it is worth reckoning that September and October can bring smoke, and that roads and trails are sometimes closed at short notice.
02 The July fog
Here is the part that catches almost everyone out: at the height of summer, the California coast is the coolest place in the state, and it is so through a specific, repeatable mechanism.
The water running along the shore is cold, and the air touching it cools and saturates. When the interior heats up and that warm air rises, marine air comes in to fill the gap. Reaching land, it condenses: the marine layer. It climbs the Coast Range hills and spills inland through the low gaps.
In practical terms: the mean July maximum in San Francisco is 19.1 °C, and August does not reach 20. The warmest month of the year is September, at 21.2, and the second is October. In San Francisco, summer arrives in autumn.
The fog is not constant through the day. It tends to be thick in the morning, pull back around midday and return in the afternoon. But it is daily for weeks at a time, and in the western neighbourhoods of the city there are days when it never lifts.
03 A hundred and twenty kilometres, fourteen degrees
The contrast with the interior is the figure that best sums up the state. In Sacramento, some 120 kilometres from the sea, the mean July maximum is 33.7 °C. In San Francisco, 19.1. Fourteen and a half degrees apart, two hours by car, the same day.
The reason is the one that makes the fog. The Coast Ranges block the marine air except through the gaps: where there is a pass, the cool air gets in; where there is not, the valley bakes. Which is why temperature in California depends less on latitude than on distance from the sea and whether there is a breach in between.
Rainfall works just as poorly as a general guide. Sacramento gets 461 millimetres a year and nothing measurable in July. The south of the state is drier still. The rule that does hold everywhere is the seasonal one: either it rains in winter, or it does not rain.
04 The range and the desert run on other clocks
The range has its own calendar, and it is the one that breaks most plans. The high passes close under snow. Tioga, at 3,031 metres, is the highest in the Sierra Nevada and usually shuts in November and does not reopen until late May or early June: in the last cycle it closed on 12 November and opened on 15 May.
That means that between November and May you cannot cross the range there, and the detour costs several hours. If the trip involves getting from the western side to the eastern one, the pass opening date is something to check before booking, not after.
The desert runs the other way. In Death Valley summer makes walking unviable and winter is the good season: mild days, cold nights and low light. November to March is when the desert is visited; June to September is when it is crossed by car with the air conditioning on and little else.
05 What to choose
If you can only choose one date, the answer is September and October. That is when the coast reaches its warmest, when the fog withdraws, when the interior stops being extreme and when the mountain passes are still open. The trade-off is that it coincides with the most delicate part of the fire season.
April and May are the other good option, and the best if what you want is green landscape: the hills have not dried out yet, there are 309 and 325 hours of sun and there are fewer people. The price is that the high passes may still be shut.
July and August work for the range and the northern coast if you go forewarned about the fog, and work badly for nearly everything else: the interior above thirty degrees, the desert unwalkable and the parks at their fullest.
And from December to February the state halves: the high country closes and the coast gets wet, but the desert is at its best and the cities stay mild. It is not a worse California; it is a different one.
