The Summertime Stratus over the Offshore Waters of California

The distribution of stratus clouds over the Pacific Ocean near California is studied via satellite photographs during the summer. A line of minimum cloudiness, north-south oriented, is found off the northern and central coast. This line is particularly well defined during July, and it may be caused by a strong surface wind divergence. Clouds to the west of the line are primarily stratocumulus, forming in air streaming southward over warmer water, while those to the east are mainly smooth-topped stratus, forming in air cooled by the upwelled water near the coast. A strong diurnal variation in low cloudiness is noted, with maximum cloud cover near sunrise and minimum in late afternoon. These findings suggest that the variation is caused by the clouds' net radiative flux. Upper-level synoptic-scale flow patterns seem to be poorly related to the low-cloud patterns over the ocean except in extreme cases.