What causes our summer fog?

One question I've been getting a lot this spring is, "when will we have good fog?" 

The answer is, well, almost every day! It just depends what you're looking for. 

Fog chasing, even more than sunrise/sunset, is going to require you to spend some time researching and scouting. There is no way around this. It's part of the journey! 


Normally, our part of the atmosphere gets colder as you gain elevation. (When you drive up into the mountains, it gets cold, right?) But anyone who has ever stood above bay area fog will tell you just the opposite -- it was nice and warm above the fog, and cold and miserable below. 

That weird foggy scenario is called a temperature inversion, when we have warmer air above cooler air. It's an inversion because it's the opposite of what we'd normally expect to see in the atmosphere, cooler air on top of warmer air. The cooler air is also very moist, because it's coming off the ocean. 

But there's a little more to it. As you probably know (break out the 5th grade science textbooks), warm air rises and cold air sinks. This is because cold air is more dense, so it "wants" to sink down. In a normal situation, the air near the surface warms during the day, and rises, leading to nice mixing. But in our foggy situation, the cool air near the surface wants to stay where it is, beneath the less dense warmer air. Very little mixing happens with the warmer air, since everyone is where they want to be. You might say it's trapped. This is our bay area marine layer: cool, moist ocean air trapped beneath warm, drier summer air. 

So, let's review. We have cool, moist air (the fog layer) trapped underneath warmer, drier air. The depth of this cool, moist layer is what determines the fog height. It varies with the weather patterns. Some of you may already know that warm weather is generally associated with a shallower marine layer and lower fog, for example. Likewise, cooler weather is associated with a deeper marine layer and higher fog. 

Fog will push inland when pressure conditions favor it to do so. Conditions are most favorable when the land cools off, which is generally at night. Sunrise is usually the coldest part of the night, since nothing has been heating the surface all night, but it continues to cool -- this is why fog is usually most extensive at sunrise. It then retreats as the land heats up during the day. 

Summer fog almost always comes in from the ocean. As you (hopefully) know, we have several mountain ranges in the bay area. The fog will generally try to push as far inland as it can overnight, but because it's trapped in its layer, it can't get above big mountain barricades.

That's why low fog is usually confined to the immediate coast -- it can't get past the coastal mountains, or even the 900-foot hills of SF. As the fog gets higher, it's now able to pour into parts of the bay through lower areas of the coastal mountains. So long as the fog isn't too high, though, the east bay hills and Santa Cruz mountains will still block further progress. Once the fog gets closer to 1800-2000 feet, it begins to spill over more areas of the Santa Cruz mountains, and is able to fill the south bay. 

Let's review the approximate topography of the bay area. 

We note from the map:

Red arrows denote the onshore component of the fog flow. (The fog can actually be coming from the north or south, but it will still be pushing in from the ocean.)

(1) North bay coastal mountains. With the exception of a few high peaks like Tam, elevations max out around 1000-1200'. (Tam is 2460'.) Any fog higher than ~1000' will be able to push all the way through these mountains. 

2) Hills of San Francisco, around 900'. Fog can pour in through the Golden Gate to the north (as you know), but won't make it over the SF hills unless it's >900'. 

3) San Bruno/Pacifica mountains, around 1300'. Fog higher than this will pour right over the mountains, but lower fog will only come in through small gaps to the north and south. 

4) Mid-Santa Cruz mountains, around 1600-2000'. Fog higher than this  will pour over, lower fog only through small gaps to north and south. 

5) Southern Santa Cruz mountains, 2200-3000+. This poses a major barricade for fog entering the south bay. It should now be easy to see why the south bay/San Jose area fogs up last -- most fog can't make it over these mountains, so it has to through the gaps way to the north, and fill in the entire valley -- all before sunrise, or the heating begins and it retreats. 

6) East bay hills (Grizzly Peak region), up to 1600'. Fog can spread around the bays to the north and south, but won't make it past these hills unless it's high. 

7) East bay hills (Union City/Fremont), around 1600-1800'. These mountains are the reason why fog usually won't make it into the Diablo or Tri-Valley area unless it's rather high. 

8) East San Jose hills (1800-4000+). It takes quite a lot to make it past this! 

9) This low-elevation plain allows fog above ~1000' to spill into the Highway 25 valley, labeled (10) on the map, and Gilroy/Hollister area.



The above applies to marine-based fog, which is by far our most common setup, especially in the summer months. In the winter, Tule fog (valley fog) forms inland, which turns the tables upside down. If you see low fog in San Jose, or Diablo, or Tri-Valley, Petaluma, etc, you can be pretty sure it is valley fog -- because there's no other way low fog could have gotten there! 


By now, most of you are probably familiar with our Fog Chaser's Guide. Use it, but also combine in your knowledge of bay area geography and places in your area of sufficient elevation! 

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