I had to drive up to San Francisco early this afternoon. When I left Palo Alto, it was sunny and warm. Heading north on highway 101, when I got to San Mateo I started seeing low clouds to the north. By the time I got to San Francisco, the sky was gray, and some people were driving with their headlights on.
In San Francisco, it was cloudy, damp, and down to 60 degrees, a good ten degrees cooler than it had been in Palo Alto, with a bracing northwest wind. You could sense the huge old mass of water in the Pacific Ocean just a few miles away.
At 4:30 I drove back to San Mateo along Interstate 280, around the Pacific Ocean side of San Bruno Mountain, and then up into the hills of the Coastal Range. Fingers of fog were creeping over the mountains, winding down through the tree-covered hills around Crystal Springs, but the sun evaporated them before they got very far.
When I arrived home in San Mateo, it was sunny and warm. But almost as soon as the sun set, the fog drifted over the Coastal Range, and became low clouds that now cover the sky above us. The temperature is down to 60 degrees, and outside it feels like it did in San Francisco this afternoon: cloudy, damp, and cool.