Fog mapping

One of TBC3’s efforts is focused on mapping and understanding summertime fog and low cloud patterns.  Terrestrial biodiversity in California, and in Mediterranean climate regions worldwide, is strongly influenced by the summertime stratus and stratocumulus clouds that form over the ocean and advect onshore.  Fog is the name given to these low clouds when they touch the earth’s surface.   The fog droplets of water, nutrients, and other minute particles are too small and light to fall out of the air, instead collecting on needles, spiderwebs and other surfaces, and then dripping into coastal ecosystems.   The overcast low clouds reflect solar radiation, shading the earth’s surface and reducing air and stream temperatures.  This in turn reduces the levels of plant evapotranspiration.  Fog and low clouds reduce the stress of arid conditions that can be fatal to fish, plants and other species during the driest months of California summers.

Some studies suggest that fog has declined by 33% along the California coast over the last century.  Other studies based on airport records have found that the base of low cloud layers has been lifting higher, away from ground level, in urban areas with greater heat trapping impervious surfaces.  The result is less fog in these areas (but not necessarily less low clouds).

These changes are expected to strongly impact the distribution of plants and animals in the San Francisco Bay Area and in other foggy regions with Mediterranean climates. Temporal or spatial alterations in fog distribution will likely be a major driver of landscape change and impact California water budgets and energy use.  TBC3 is preparing for these changes by creating maps of fog and low cloud cover derived from ~26,000 hours of day and night time weather satellite images.  With funding support from the California Landscape Conservation Cooperative and the Gordon and Betty Moore Foundation, these digital datasets and analytical results are available for download from the California Climate Commons.

More information about the project is available at http://geography.wr.usgs.gov/fog.

Contact Info:
Alicia Torregrosa
atorregrosa@usgs.gov
650-329-4091