"Welcome to the Greenbelt," the rider said to me, as we waited for a light to change on Skyline. Welcome to the Fogbelt was more like it. We had just ridden about ten miles through pea soup. My glasses needed windshield wipers. Riders barely 50 feet away became ghostly apparitions. It was a classic San Francisco summer day -- wet and chilly.

But not miserable. The Go Greenbelt ride, a seven-day circumnavigation of the San Francisco Bay Area by bicyclists eager to proselytize the preservation of open space, started on Sunday. The contagious exuberance exhibited by the riders from the moment they started assembling at Fort Mason in the Marina district made trifling matters such as the weather irrelevant. The veterans of previous rides were glowing at the prospect before them. One could not but feel their delight rub off.

And then, as happens so often in the hurly-burly of micro-climates that inhabit the Bay Area, we dropped down a few hundred feet in altitude, and it was as if we had passed through some invisible barrier that allowed humans through but forbade passage to fog molecules. Suddenly, it was sunny, warm, and magnificent. We tooled along a busy bike path along the Crystal Springs Reservoir -- I amused myself by counting the foreign languages I heard spoken by strollers on the bike path; Mandarin, Cantonese, Spanish, German, Japanese, and, possibly, Tagalog.

And for an hour or so, I rolled around ideas in my head for a blog post that would try to make sense of this multicultural symphony. But all thoughts of globalization fled at one turn in the road, about 45 miles into the ride.

The hard work of the day was a long climb out of Woodside after lunch, on Old La Honda Road, a beautiful, winding ascent leading ultimately to the top of the Coastal Ridge. You climb and climb and climb, and then you come around a bend and the vast, almost inconceivable beauty and glory that is Northern California spreads out before you. On your right, a valley of rolling, forested hills leads to the Pacific Ocean, which today was a solid mass of fog -- a sea of fog rolling and crashing against Highway 1, but thousands of feet below us.

You think, this is what John Muir exulted in, as he walked across this state. And then you look to your left, and you see down into the south Bay -- you see a thriving hubbub of industry and civilization. You see Silicon Valley; you see the human project.

The point of Greenbelt is to figure out how the human project and the wondrousness of the earth can coexist, without the former carving up the latter into soul-killing sprawl. As I gaped out at the panorama before me, that challenge seemed a kind of manifest destiny. How could anyone behold these two versions of the world and not be inspired to make it work?