File:Marine Stratocumulus Clouds off of California (MODIS 2021-07-04).jpg

English: A thick layer of marine stratocumulus clouds hugged the coast of California on July 1, 2021, bringing thick fingers of fog onshore, especially over San Francisco. Local media confirmed that the city was entrenched on fog on Thursday morning, quoting the weather service’s Bay Area office that had quipped, “We just came out of June Gloom and entered No Sky July”. The fog has continued through July 3, leaving some questioning if skies would clear in time for traditional Independence Day firework celebrations on July 4.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of the massive cloud bank on July 1. The finger of fog can be seen hanging over much of San Francisco Bay and travelling as far as 40 miles (64.4 km) inland.

Large banks of stratocumulus clouds often form near the West Coast in spring and summer, usually during periods of upwelling. Upwelling occurs when surface waters are displaced by strong winds, causing cold, nutrient-rich waters to be pulled up from the depths to the surface. Such clouds form in the atmosphere’s marine layer, which extends from the ocean surface up to an altitude between 1000-2000 feet. The eddies in the marine layer clouds are caused by wind moving around obstructions above the surface of the ocean, such as tall islands.