Kelvin-Helmholtz wave clouds

Clouds resembling breaking waves in the ocean are unusual and beautiful, and they've occurred on at least two occasions in recent months in Pacifica, less than 10 miles south of San Francisco, catching the eyes of hobbyist photographers. 

SF resident Tara Cronin captured an image of a cresting wave cloud rising above the mountains in the coastal town on May 27 at 3:40 p.m. and posted it on Instagram. Cronin said the American Meteorological Society shared her photo and it went viral with more than 1 million impressions.