This time of year, it is common for a marine layer (i.e. clouds) to roll in at night. It sits on top of the inversion layer covering the Los Angeles basin and surrounding valleys.

It's interesting to drive up here because you start below the layer and it's overcast, and then you drive through the clouds, and then you see this -- mountain peaks tens of miles away, floating on the clouds like islands.

This is about 30 feet down from the top of the Mt. Wilson toll road. What you are seeing is the trail itself and some mountains in the background. This is where I take all my "Los Angeles by night" photos.