Flying the San Francisco Bay Area
Weather: Land of Sun and Fog
California's sunny summers are legendary. It is not unusual for inland areas to go months without a single cloud. In contrast, the coastal areas fall under the influence of the Pacific marine layer, responsible for the famous San Francisco fog that picturesquely pours over the Golden Gate.Prior to the widespread use of air conditioning, the fog was actually a tourist attraction. Residents of California's Central Valley would escape the summer heat by travelling to the resort towns of Santa Cruz and Monterey. It is also famously unpredictable. It may arrive as early as 3pm, or perhaps not until after midnight. Once the sun rises, most inland areas clear by 10-11am, although in cooler temperatures of the spring and fall, this can take longer. Pacific coast areas may not clear at all, so the coastal airports of Half Moon Bay, Watsonville, Salinas, and Monterey can remain fogged in for days at a time.
Time lapse movies not withstanding, the fog does not always "move in," but can materialize out of thin air. Cooling resulting from the setting sun can cause fog to condense quickly over large areas, especially near water. San Carlos, Hayward, and Palo Alto are the airports most susceptible to this.
Inland airports such as Santa Rosa, Concord, and Livermore, can see summertime temperatures in the 80s and 90s (F). However, density altitude is rarely a problem because all the Bay Area airports are very near sea level.
Once or twice each summer, a remnant of a Pacific tropical storm may reach as far north as the Bay Area. These storms differ from typical winter storms by arriving from the south or southeast, bringing warm moist air that when mixed with the coastal mountains, can generate convection and even thunderstorms.
The Bay Area's rain season is generally from November to April. It is not unusual for sequential storms to cause rain for weeks at a time. March of 2006 saw 25 days of rain. And IFR may be of little use to light aircraft. Most storms arrive from the north, bringing cold temperatures and freezing levels that because of the surrounding terrain, are below MEAs and minimum vectoring altitudes. IFR departure in an aircraft not equipped for known-icing may be impossible.
The dry days in between winter storms offer some of the best best flying weather of the year. Temperatures are cool but not freezing, and the departing storms have cleared out the haze. The coastlines are free of fog, winter rains have greened the hills, snow caps the distant Sierra Nevada mountains, and visibilities of 50 miles are not uncommon.