Revenge of the Fog (San Diego, San Francisco: hotel, living in, floor)

[caligirl2009]

Question to all you weather geeks:

Why is it that every time San Francisco experiences abnormally warm weather and clear skys, like this past Saturday, the fog ALWAYS comes in with a vengeance the next day. The temperature will almost always drop 15-20 degrees in a matter of hours with high winds and thick fog engulfing the city for days. Why is this?

[andyadhi01]

Originally Posted by caligirl2009

Question to all you weather geeks:

Why is it that every time San Francisco experiences abnormally warm weather and clear skys, like this past Saturday, the fog ALWAYS comes in with a vengeance the next day. The temperature will almost always drop 15-20 degrees in a matter of hours with high winds and thick fog engulfing the city for days. Why is this?

What I notice is when the weather is warm in city the fog layer starts to build up few miles away from the coast. I think the warm weather happens when there is high pressure system in the city. As soon as the pressure drops the cold fog rolls in.

Regardless of the science, it sucks big time.

[mayorhaggar]

Warm weather happens when the Pacific stops blowing wind and fog east into the Bay and hot air from the Central Valley blows west. I know that warm air inland is what creates the fog in the first place, so if it's hot as far west as SF, I think it creates a vacuum that sucks the fog in even harder.

[Gentoo]

This happens up and down the CA. coast. At least it also happens in San Diego. The coast cools off first so there's briefly a much bigger temperature difference between the coast and inland areas. The inland air continues to rise as hot air always does. Now the denser fog laden air is the air that first gets sucked in to replace it. Notice the very strong sea breeze today? Lake Merritt had white caps on it and looked like the bay. As this evens out, the sea breeze will return to a more normal flow as the hot air will be farther inland . There will still be a pull on the ocean air but it won't be as powerful. Basically a normal breeze. That is until the next weather system moves in and temporarily disrupts this pattern which is our typical summer pattern.

[rkwalton]

I don't mind the fog as long as I'm not mobile when it rolls in. If I'm mobile it depends on how close I am to the coast on whether I freak out or not. It happened once years ago as I was driving back to the city from Marin just as I was crossing the Golden Gate Bridge and heading down 19th. I.was.terrified.

It happened again this weekend as I was driving in from the city up to the hills in Berkeley. I saw it coming in and the worst of it was when when I got off of the 24 and had to curve around to get to route 13. Once I passed the Claremont Hotel, I'd left it behind as the fog was pretty high up. I just hate when I'm in the thick of it.

It's another reason I'm pleased with choosing the East Bay.

[legal_eagle]

SF is on the coast (duh). When it is warm on the coast, the inland gradually gets even hotter, which may take a couple of days. This causes low air pressure (warm air has lower pressure than cold air) inland, and the colder air over the ocean is pushed or "sucked" into the central valley, depending on your point of reference. There's fog in the cold air from water vapor coming off of the ocean. The same phenomenon happens in So Cal, but not with the intensity as in SF because the ocean air is colder up here. Colder air cannot hold as much water vapor as warm air, so the water vapor in cold air will condensate at form fog or clouds more readily.

[HockeyMac18]

Others have done a good job of explaining it, but here's some more information: San Francisco fog - Wikipedia, the free encyclopedia (sometimes wikipedia isn't the best, but it does a good job of providing decent background on this topic). I don't know if the fog we've had the last few days is exactly similar to the summer fog; but I imagine the atmospheric mechanisms are fairly similar.

Basically, the growing heat inland creates a lower air pressure as compared to the cooler ocean air. PV = nRT applies, but the Earth's atmosphere is not a simple closed-container...because as the air heats up in the atmosphere, the air expands (volume expands), and the overall pressure tends to drop. One way to remember that hot air tends to have lower pressure in the atmosphere is that the lowest pressures have been recorded in hurricanes/typhoons, which originate from warm tropical air, and the highest pressures have usually been recorded on cold days.

This pressure gradient between the warm inland and cooler coast causes a breeze to form (remember, air, like any gas/water/particle, wants to move from areas of high pressure to low pressure, and the intensity of this breeze is often directly correlated with how big a difference there is in relative pressure). Once this breeze has formed, the air from further out in the pacific (which is generally warmer and has more moisture in it) is pulled towards the coast. As it approaches the coast, the air is rapidly cooled by upwelling of the deeper water (which is cold) at the coast and by the California current, which is cooler compared to the outer pacific because this water flows from Alaska. These factors help explain why the entire west coast has such cold waters.

When warm moist air is rapidly cooled, it often falls below the dewpoint (the temperature at which water condenses) and forms clouds/fog. You'll often notice that this "fog" is more of a low-hanging cloud than an actual ground fog (which is what most people think of when they think of "fog"), but this layer can drop to ground-level. And if you live in parts of the city that are higher up in elevation, this low-cloud can be at your elevation, and it will seem like ground fog to you.

This weather pattern usually only occurs in the summer because this is usually the only time of the year that there are drastic differences in temperature/pressure between the inland valleys and the coast. Generally, other times of the year, the temperature gradients are either non-existent or very small (meaning, the temperature at the coast is about the same (or even higher) than the temperatures inland), and as such you don't see this type of fog forming. Those days when it's 90 in walnut creek and 65 at the coast are prime days for coastal fog-formation.

In the winter, the inland valleys get a different type of fog (known as "tule fog" here, but it's more in line with what most people think of when they hear the word "fog" Tule fog - Wikipedia, the free encyclopedia).

Hope this clears up the mysteries of SF fog.

[BayAreaHillbilly]

It really depends on the time of year. During the traditional "Fog Season" (aka the Dry Season aka Summer) there is a definite fog cycle. The inland areas heat up and create a surface thermal low, which draws the sea breeze, bringing the initial surge of fog. Over time, as the upper atmosphere ridge solidifies, the thermal low migrates westward eventually moving out to sea. This brings either still conditions or slight offshore flow, pushing the fog out to sea. All the while inland areas, especially well inland (e.g. Kern County, Inyo County, etc) reach extreme heat, ala 110 - 120 degrees. The thermal gradient brings back the sea breeze, the fog surges inland, the cycle repeats.

What we've had the last few days is not proper summer fog, it's the so called squeegee effect. An incoming trough has created enough of an onshore floor to draw in tropical air, which creates a summer like fog bank over the water. The cyclonic winds are pushing it on shore. As is typical closer to the solstice (as we are now) prior to the trough we had a mini heat wave due to the initial tropical air.

[HockeyMac18]

Article about a cool app that comes in handy in the summer, especially if you're living in the sunset/richmond and want to know how far you have to go to get some sun:

"Where's the Sun?" You Can Ask This iPhone Weather App | 7x7

[Isebiel]

Woah! For lunch I walked west on Geary, through Laurel Heights, and the whole western half of the city was gone! Just a towering lavender-grey wall of fog there. Really cool. Well... from the sunny side, at least.