Clouds and Fog Along the California Coast – June 1st, 200937.7N 122.4W
Here, the coast of California is framed by a blanket of clouds and sea fog. Fog begins to form when water vapor (a colorless gas) condenses into tiny liquid water droplets in the air. Fog normally occurs at a relative humidity near 100%.
Another common type of formation is associated with sea fog (also known as haar or fret), due to the peculiar effect of salt. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves.
Except in areas of storminess, the most common areas of breaking waves are located near coastlines, hence the greatest densities of airborne salt particles are there.
Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast, as shown here. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon.
The close-up focuses on San Francisco, where the combination of cold ocean water and the high heat of the California mainland create the city’s characteristic fog that can cover its western half all day during the spring and early summer. The fog is less pronounced in eastern neighborhoods, in the late summer, and during the fall, which are the warmest months of the year.
The high hills in the geographic center of the city protect neighborhoods directly to their east from the foggy and cool conditions experienced in the Sunset District; for those who live on the eastern side of the city, San Francisco is sunnier, with an average of 260 clear days, and only 105 cloudy days per year.