Palomar Observatory's founders made a great choice in picking Palomar Mountain as the site for the 200-inch telescope. We often have clear skies and a stable atmosphere to look through (giving us sharper images of the universe).
What our founders did not anticipate was the almost exponential growth of the population of Southern California. With the increased number of people has come increased sky brightness, what most people call light pollution. Thankfully the founders did choose a site that from time to time has a natural shield from the city lights. When the low marine layer clouds blow in off of the ocean, like they are doing today, the cities and their many lights are below the clouds and the observatory is above them. The result is dark skies, making this site almost as dark as it was back in the 1930s.
The picture above was taken from the catwalk of the 200" Hale Telescope earlier today. Tonight all of lights in Riverside County (and San Diego County too, you just can't see it in this shot) will lie below the clouds. "June Gloom" is gray for those below, but wonderful for astronomers.