NOAA's GOES-15 satellite captured the southern California "June Gloom" on June 10, 2013. That's a weather pattern that creates cloudy, overcast skies and cool temperatures. The "June Gloom" of low lying stratus clouds form over the ocean and can be pushed to coastal areas by wind. It usually happens off the west coast of California during the late spring and early summer.
As for the "seam" of blue within the "June Gloom," it appears to be actinoform clouds, a seam in the marine stratocumulus aka "June Gloom" of southern California. Actinoform clouds and marine stratus in general are only marginally stable. Sometimes the cloud deck spontaneously dissolves along a line by drizzling out the moisture. This seam is an unusually long curve that is not identified with a coastline or a weather front.
Dennis Chesters/Rob Gutro
NASA's Goddard Space Flight Center, Greenbelt, Md.
REFERENCES
en.wikipedia.org/wiki/Actinoform_cloud
en.wikipedia.org/wiki/June_Gloom
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