Fohn Effect in the Bay Area of San Francisco

My two favorite meteorological phenomena are the Fohn Effect and Arctic Sea Smoke.

Known as either Fohn / Foehn / Föhn effect / wind, and described in detail in this book, it occurs when cool moist air is forced over a line of hills, and comes down the other side warmer and dry. In the Bay Area of San Francisco, you can see it in action very often.

This is because the arctic current causes the sea to be very cool, cold even in the summer, when I tested it! Inland the sun warms the ground and this draws the air up, which is replaced by air from the Pacific. The air moves up and over the Santa Clara mountains, cooling as it does so. It soon gets to its dew point (ie is saturated so the air gets cloudy / foggy), and as it continues to climb it continues to cool at 2°C / thousand feet.

When the air comes back down the other side it starts to warm, and soon is no longer saturated. From then on it warms at 3°C / thousand feet.

So imagine air leaves say Half Moon Bay at 15°C, becomes saturated immediately and then climbs 5000ft. At the top it will be 5°C ie 15 - (5 x 2). In the Bay Area, after descending 5000ft it will be 20°C ie 5 + (5 x 3).

Thus around Skyline Boulevard, on the Pacific side it will be cool and cloudy, likely foggy, while inland in, say, Mountain View, it will be warmer and the sky will be clear.

I love the way the clouds tumble down the mountain and dissolve away.

Even better, you can move to high ground, and then look down on the clouds.