Weather in Silicon Valley and the San Francisco Bay area

November 8, 2011 at 7:20 pm | Posted in Uncategorized | Leave a comment
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Once we decided to move to California, I started looking at the weather in detail to help choose a place to live. This actually turned out to be very complicated (but very interesting) because the Bay area has many microclimates. (According to Wikipedia, a microclimate is a local atmospheric zone where the climate differs from the surrounding area.)

A warning: this whole post is about weather. If you aren’t interested in weather, you may want to just give this one a miss.

For example, let’s look at three different cities that span all of Silicon Valley:

San Francisco – 22 inches of rain per year:

Mountain View – 15 inches of rain per year:

San Jose – 15 inches of rain per year:

The differences are especially noticeable in the summer: in August, San Francisco’s average high is only 69 compared to Mountain View’s 79 and to San Jose’s 87. Which might not seem that unreasonable until you realize how close they all are — SF and SJ are only an hour apart and have a difference of almost twenty degrees!

So what’s going on? Why the microclimates, and how can we use this information to make sure we live someplace warm?

Well, let’s look at a map of the area:

As I mentioned in my last post, the term “Silicon Valley” generally refers to the area between San Francisco and San Jose. Now let’s look at the terrain version of this map with some added notes:

Even though this is labeled, I’ll describe it anyway:

  • The Santa Cruz mountain range (yellow arrows) runs NW and SE. On the map you can see that the southern “side” of it is all green with plants, whereas the northern side is much more brownish gray.
  • Silicon Valley is the area between the red arrows.
  • The San Francisco Bay is specified with blue arrows. The Golden Gate bridge is at its mouth to the Pacific on the west.
  • The area on the NE side of the Bay is generally called the “east bay,” (green arrows) and several people have told us that it’s kind of a crappy area.

Now it just so happens that the weather in this part of the world generally comes up from the ocean to the southwest. So when cold and rain and clouds and etc. are blowing NE towards Silicon Valley, an interesting thing happens: they hit the Santa Cruz mountains. And just like a stunt motorcyclist who rides up a ramp, all the weather and precipitation from the ocean shoots way high up in the atmosphere and continues its northeast-ward path too far up to cause any rain. You can see evidence of this on the mountains themselves: on the ocean side, they’re all green and lush from where the rain constantly hits them. On the land side, though, it’s actually quite dry. (This area is called a rain shadow.)

Over time, of course, the weather drifts back down and begins raining on land, but by this time it has moved even further northeast and has missed Silicon Valley entirely. (A similar thing happens in Seattle, actually — downtown Seattle is in the rain shadow of mountains to the west, so it gets far less rain than many of its more easterly suburbs.)

But this is certainly not the only effect. Let’s look at another one:

As I said, the weather is heading NE from the Pacific. At the bottom of this image, you can see the last effect I discussed: the red arrows show the cold air moving NE and the orange arrows show the weather shooting up when it hits the mountains. But take another look at that mountain range: it pretty much stops at the town of Pacifica (slightly below the blue arrows near the middle of the map). This means that unlike Silicon Valley, San Francisco really has nothing to shield it from the chilly Pacific air. Consequently it is actually a pretty cool city (and it is even colder than Seattle some of the time even though SF is at a much lower latitude).

Once the cold air passes through San Francisco, it continues right across the Bay without any obstacles in its path. Then it hits the east bay and makes them colder and wetter as well. Not as cold, of course, because that weather has been warming up a little over the land, but still much colder than sheltered Silicon Valley.

Along similar lines, once that weather has come inland north of the Santa Cruz mountains, some of it will reach the northern part of Silicon Valley. In particular, San Bruno (where we are staying right now), Burlingame, and San Mateo are much cooler than San Jose (which is at the southeastern edge of Silicon Valley and can be seen two maps up from this one).

There are other effects at work here as well, but these are the main ones I’ve been able to find. I know practically nothing about weather, but this post is what I’ve been able to put together from an evening of reading various weather blogs. And I’m half writing this because nobody seems to have written an article like this before — this is the article I was trying to find and didn’t seem to exist. =)

So to generalize: San Francisco and the east bay are cold and wet. San Jose is warm and dry. The Silicon Valley cities between them vary more or less linearly.

We’re hoping to find an apartment in Foster City, which is still pretty warm despite being close to chilly San Bruno. Driving to Mountain View these days is lovely — I keep my top down all day and drive around without a coat and it almost makes me forget that there even is a cold Seattle where I couldn’t go jacket-free all summer long. It’s nice. =)


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