Fog as an ecosystem service: Quantifying fog-mediated reductions in maximum temperature across coastal to inland transects in northern California

Background/Question/Methods

Several studies have documented the human benefits of temperature cooling derived from coastal fog such as the reduction in the number of hospital visits/emergency response requests from heat stress-vulnerable population sectors or decreased energy consumption during periods when summer maximum temperatures are lower than normal.  In this study we quantify the hourly, daily, monthly and seasonal thermal effect of fog and low clouds (FLC) hours on maximum summer temperatures across a northern California landscape.  The FLC data summaries are calculated from the CIRA (Cooperative Institute for Research in the Atmosphere) 11 year archive (1999 – 2009) that were derived from hourly night and day images using channels 1 (Visible), 2 (3.6 μm) and 4 (10.7 μm)  NOAA GOES (Geostationary Operational Environmental Satellite).  The FLC summaries were analyzed with two sets of site-based data, meteorological (met) station-based measurements and down-scaled 270m grid-based interpolated PRISM data for selected point locations spanning a range of coastal to inland geographic conditions and met station locations.  Data for fog hours by site, month, and year and maximum temperatures at sample sites were factored into a nested regression (by site) of maximum temperature against fog hours.

Results/Conclusions

We found that increases in fog cover reduce maximum temperature by -0.7 to -0.4 ^oC per fog hour, once site means and year and months are taken into account.  Our results suggest variability related to site specific thermal response.  For example, sites closest to the coast have less thermal variability between low cloud and sunny days than sites further from the coast suggesting a much stronger influence of ocean thermal capacity than of FLC thermal dynamics. An inland site like Santa Rosa that gets on average 4 hours of fog cover per day benefits by a 2.4^oC cooling.  The thermal relief provided by summertime FLC for inland sites is equivalent in magnitude to the temperature increase projected by the driest and hottest of regional downscaled climate models using the A2 (“worst”) IPCC scenario.  Extrapolating these thermal calculations can facilitate future quantifications of the ecosystem service provided by summertime low clouds and fog.  Loss of coastal fog or reduced inland penetration could well exacerbate climate warming.  Conversely, an increase in coastal fog will ameliorate climate warming but the effects might be constrained to inland costal sites.