California and Oregon Humidity and Coastal Fog

In order to better understand fog occurrence along the California and Oregon coast, this study examines meteorological data from thirteen coastal surface stations, seven radar profiler stations, five cooperative observing stations, 700 hPa synoptic maps, and GOES 9 visible satellite images for the months of June to October 1996. Fog exhibits a strong diurnal cycle at all stations, with maximum occurrence in the early morning, and minimum occurrence in the afternoon. When fog is present, surface temperatures and wind speeds are lower, marine atmospheric boundary layer inversion strength is greater, inversion base heights are lower, and inversion top temperatures are higher. At the northern-most stations, the surface wind direction shifts from northerly to southerly during fog events. At approximately 3000 m height, the winds are primarily southerly during fog events and northerly during clear time periods. This corresponds well with 700 hPa synoptic maps, which show a clear correlation of a weak high over California during fog events and a trough passing over the state during clearing, such that fog is about 30% less likely to occur when a trough is in the vicinity. These relationships are demonstrated using summer-long averages and case studies. Current and future efforts to study California coastal fog are also discussed.