Why is fall so hot in coastal California?
In many temperate climates around the world, fall is a pretty pleasant season — the leaves change color, and temperatures are cooling down from the hot summer. Except in California for some reason, where it more often than not tends to be a fiery hellscape (or at least, moreso than the rest of the year).
In a lot of coastal California cities like San Francisco or Monterey, September and October are the warmest months, warmer than June or July. The record high of 106°F in San Francisco was recorded on September 1, 2017. October can also see some pretty brutal heat by coastal standards, and the October record in San Francisco is 102°. This might seem pretty unexpected for a city that, whenever I’ve visited, has always been 60 degrees and foggy, even during summer. But there are some unique quirks of California’s geography that make these weird fall heatwaves possible.
California’s weather during summer is dominated by the California current off the Pacific coast. This brings in a steady supply of cold water from the northwest, which is what causes the consistently foggy conditions along the coast for most of this time period. Meanwhile, inland areas heat up much quicker during the summer. The temperature difference between the coast and inland areas drives a strong sea breeze. This allows the ocean moderation to reach quite far inland at times — even areas in the Central Valley such as Sacramento and Stockton occasionally benefit from marine air blowing through the delta of the Sacramento and San Joaquin rivers.
This pattern gets much weaker during fall. The ocean, being slower to warm, reaches its warmest temperature by around September. Inland areas begin to cool down with the reduced sunlight, and the sea breeze weakens as a result. This allows coastal areas to finally see some sun and warmth for the first time in months.
While this is a large part of why the coast tends to be warmer in fall, it isn’t the full story. The Pacific Northwest and Intermountain West also start seeing more storm activity, and these storms produce strong northeasterly winds in their wake. As these winds make their way to the coast, they turn into what are well known as the infamous Santa Ana winds.
The sinking cool air on the backside of the storm results in high pressure to the northeast of California. Since air moves from high pressure to low pressure, this results in a northeasterly wind for California. Because the coast is warmer, the air pressure is also often lower than normal due to rising warm air, strengthening the wind. It might seem like this should be a cool wind, since it originates from a cooler airmass. However, coastal California is surrounded on the north and east by tall mountain ranges, so the air has to descend down these mountains before reaching the coast. As the air descends in elevation, it gets compressed and warmed; compressing the air adds energy to it, which manifests as heat. It’s the same reason why bike pumps get warmer after you use them; the air gets heated as it’s compressed and forced into the tire.
And so, these northeasterly winds become the hot and dry Santa Ana winds we all know and love. Hot and dry air gets forced all the way to the coast, and it can sometimes get warmer at the coast than in inland areas like the Central Valley and the deserts. Local topography can also enhance these effects, with Santa Ana winds getting funneled through narrow passes as an example.
Of course, the temperature and strength of these winds depends a lot on the strength and positioning of the high pressure system, as well as the airmass they originate from. Storms do take a similar track across the Intermountain West during spring as the storm track lifts back north, though northeast winds are often weaker and less common during this time of year due to the cooler coast—of course, this does not entirely rule them out, and early season coastal heatwaves can occur from April to June. However, since ocean temperatures are generally much cooler at this time of year, these heatwaves are less extreme than those during fall. Similarly, Santa Ana winds can also occur during winter, but due to the much cooler origin airmass, these Santa Ana winds are generally cool and dry.
And so, the Santa Ana winds and weaker sea breeze are the primary reasons why coastal California can get so hot during fall. It’s unclear exactly what sort of effects climate change will have on the strength and frequency of Santa Anas, but it is quite likely that hotter and drier conditions will become more commonplace during fall.
