What's Up With The Weather Around San Mateo CA?

What's up with the summer weather around here?

People visiting our area for the first time during the summer are often surprised at finding cold weather.  "What's up with the weather around here?" they ask.  Oh, The Fog?  It's 60 degrees!  It's the middle of summer!   Cold winds!  Burr!  Welcome to Northern California and specifically to the San Francisco Bay Area.  Our world is affected by the cold Pacific Ocean and the hot Central Valley of California.

San Francisco is at the tip of a peninsula.  To the west is the ocean.  To the east is the bay.  Across from the bay is a body of land that extends on to the east and the Sierra Mountains.  In our area, you can pick your weather like you pick your clothes.  Some like it warm.  If that's you, you can live anywhere but San Francisco.  Some like it hot.  Is that you?  Go east, or south, or north, my friend.  And, some like it cool.  If that's you, stay near San Francisco and the bay or into the city itself if you complain when it gets hot. The picture above shows fog hugging the coast between Daily City and Half Moon Bay while the tip of the peninsula, San Francisco and the rest of the bay is clear. 

Why is it so cold in the summer? 

The arctic winds can come down with the jet stream, even in summer.  That is what brings the cold air.  This cold air collides with the hot air coming from the central valley and, voilà, fog.  That fog is really our air conditioning.  Think of it as Free air conditioning.

Look at the map above and the numbers.  #1, San Francisco is at the tip of the peninsula and the yellow areas are where the fog generally sits during summer.  As you move down the Peninsula from San Francisco the weather warms up.  What's the Reason?  You're moving further away from the tip of the peninsula where San Francisco sits.  The further away from the ocean you go, the warmer it becomes. the areas between #3 and #4 are pretty wide and the fog sits along the coast but doesn't do much inland during the day.  It will roll back over the hills at night to cool us down, but it burns off in the morning and the bay side warms up. 

We have several fog lines on the peninsula.  The last one sits in San Bruno. San Mateo might be 80 degrees and San Bruno, only about 10 miles north, might be 60 with cold winds.  We have what is known as micro-climates. These are areas within our cities that have warmer or cooler weather.  Where you can go several blocks and get a different kind of weather.   In San Francisco for example, you can live in fog during the summer, yet work in sunshine downtown.  The weather within the city differs because of the hills that stop the fog from flowing in.  This same thing happens for the north bay, the east bay and the south bay.  Our hills direct the flow of the fog and winds throughout the bay area.  In our part of the world, the mid-Peninsula, you can have cool or warm weather.  Other than that, it's pretty consistent with summers anywhere from the low 70's to the mid-80's.

Fog affects our weather daily during the summer. 

Fog can lay on the coast, yet in San Mateo, only 12 miles to the east, it's 80 degrees and sunny.  The difference in temperature from Half Moon Bay and San Mateo can be 20+ degrees.  By evening, the fog will often have rolled in over the hills, and cooled the areas down to the mid-50's.  Cool sleeping weather.  Air conditioning without having to pay for it!

We do get heat waves

Every place gets heat waves, but they generally don't last more than 3 or 4 days around here.  Just as we stop complaining about it being so hot, say at 90 degrees or so, the fog comes rolling in and everyone breathes a sigh.  Why does it take 3 or 4 days before the fog comes back?  Remember what I said about the arctic winds flowing down from Alaska?  Well, the Pacific Ocean off or our coast is generally about 53 degrees.  Burr.  You don't go into that water without a wet suit because it is seriously cold, and could kill you pretty fast if you're in it too long.  Locals don't go into the Pacific around here often because of the strong undertow all along the coast and the cold water.  You can be swept out into the deep waters without realizing it.  It's strong and dangerous.

Collisions of Air Flow

The cold water and the cold air rising from it collide with the hot air from the east bay, and the cold air wins.  Most of the time that is.  There are some summers when El Nino takes over, the warmer southern Pacific waters are brought north via ocean currents and we warm up.  On those occasions we have warm summers, even in San Francisco.  This happens very infrequently and surprises the locals as much as our visitors. See the blue in the ocean?  That's our usual cold water.  The yellow is warm water and this begins around Santa Barbara and southern California about 300 miles to the south.

Take Your Pick

If you're considering a move to our beautiful peninsula, you will be able to not only pick your house and school, but weather, too.  I don't think there is any other place in the US that allows that!  We're lucky to live in an area with nature's free air conditioning.  This is the reason why you see so few homes with air conditioners on the peninsula.  As you go north you see two fog lines, the first is in San Bruno, the second South San Francisco that I mentioned above.  Go south and the weather warms more and more, the fog only comes in late at night.  The winds from the fog come up after 4 PM, providing a welcome cooling.  Go east across the bay and there can be areas with weather is almost as cool as parts of San Francisco because of gaps of hills allowing fog to blow into the valleys and other places where it is consitently hot.  Yet, the same thing takes place there, nature's air conditioner turns on almost every evening of the summer to cool them down, too.  They just don't cool down as much as the peninsula does.