There is nothing more beautiful than watching the fog roll in over San Francisco. Often it looks like water flowing in slow motion. And now, in this mesmerizing time-lapse film, you can see just how similar the fog's motion really is to an airborne flood.

Writes filmmaker Simon Christen:

The weather conditions have to be just right for the fog to glide over the hills and under the bridge. I developed a system for trying to guess when to make the drive out to shoot, which involved checking the weather forecast, satellite images and webcams multiple times a day. For about 2 years, if the weather looked promising, I would set my alarm to 5am, recheck the webcams, and then set off on the 45-minute drive to the Marin Headlands.

I spent many mornings hiking in the dark to only find that the fog was too high, too low, or already gone by the time I got there. Luckily, once in a while the conditions would be perfect and I was able to capture something really special. Adrift is a collection of my favorite shots from these excursions into the ridges of the Marin Headlands.

See more of Christen's work on his website.