New California Fog Maps Reveal Pictures for Planning




A new series of maps of fog and low clouds covering the northern and central California coasts reveals daytime and nighttime patterns with a level of detail never previously mapped. U.S. Geological Survey scientists used new analyses of satellite images to understand the dynamics of fog.

Fog and low clouds have a significant influence on California’s coastal ecosystem processes and on the local economy for everything from wine production to tourism. The new digital maps can be used for a wide range of applications from siting solar panels to making decisions about what grapes to grow in coastal Calif. The patterns of fog and low clouds that are revealed by the new maps will help ecologists better understand coastal-to-inland plant and animal distribution. The fog and low cloud maps can delineate the commonly used term “fog belt” into zones with increased precision.

This Landsat image of May 22, 1991 shows the marine stratus and stratocumulus cloud layer that moved into the San Francisco Bay-Delta and Monterey Bay. Several cloud patterns can be seen in this image: the eddy-like spiral to the west of the Golden Gate, the darker linear cloud feature that parallels the coast down to Monterey Bay, and fog funneling from Monterey Bay inland.

This Landsat image of May 22, 1991 shows the marine stratus and stratocumulus cloud layer that moved into the San Francisco Bay-Delta and Monterey Bay. Several cloud patterns can be seen in this image: the eddy-like spiral to the west of the Golden Gate, the darker linear cloud feature that parallels the coast down to Monterey Bay, and fog funneling from Monterey Bay inland.

The maps were made using a decade’s (1999 – 2009) worth of summertime satellite weather data. Over 26,000 hourly night and day images were classified to identify the presence of fog and low clouds and compiled for statistical analysis.

“We wanted maps that were as easy to use as the USDA Plant Hardiness Zone Maps and as accurate as National Weather Service images so we compiled thousands of satellite images onto one map layer. People can pinpoint what fog zone their house or field is in. Montara (Calif.) is in the highest zone and gets, on average, more than 14 hours of fog and low clouds each day in the summer,” said Alicia Torregrosa, USGS geographer and lead author on the study.

Map of central California coast showing the average amount of fog and low clouds present during the month of July over a period of 10 years. Areas of darkest blue over the ocean were covered by an average of 17 hours per day of fog and low clouds. Inland areas in darkest red were fogged in or covered by low clouds two hours or less per day, and the narrow band of yellow along the coast was blanketed by fog and low clouds an average of nine hours per day during most Julys.

Map of central California coast showing the average amount of fog and low clouds present during the month of July over a period of 10 years. Areas of darkest blue over the ocean were covered by an average of 17 hours per day of fog and low clouds. Inland areas in darkest red were fogged in or covered by low clouds two hours or less per day, and the narrow band of yellow along the coast was blanketed by fog and low clouds an average of nine hours per day during most Julys.

The fog and low cloud data are freely available for download from the California Landscape Conservation Cooperative Climate Commonswebsite.

The full article, “Goes-derived fog and low cloud indices for Coastal North and Central California ecological analyses,” describing the new digital maps, their uses and creation, was published in “Earth and Space Science,” an open-access journal of the American Geophysical Union.




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