San Franciso is famously cold and foggy during the summer. The central valley heats up, the hot air rises and a few hundred miles to the west in the Sunset district, (where I live), cold air sitting out on the Pacific ocean gets pulled in and settles as dense fog. As Mark Twain said, " the coldest winter I ever spent was a summer in San Francisco".
This year has seen much heavier fog than normal, every morning it looks like it has been raining hard all night. We just had our deck stained when the fog began rolling in. While the drops of water were pretty to look at, the deck hadn't dried fully, and all the varnish bubbled up, meaning it now has to be sanded and restained.