Jessica Lundquist is a Professor of Civil and Environmental Engineering at the University of Washington, where she has been teaching and doing research since 2006. She earned her B.S. in Atmospheric Sciences from the University of California, Davis, and her graduate degrees from Scripps Institution of Oceanography at the University of California, San Diego, with a master’s thesis on coastal fog and a PhD thesis on diurnal cycles in streamflow. She received a FHL New Faculty Research Fellowship to begin new research at FHL this past summer.
My family and I arrived at FHL on June 26, 2021, just before Seattle recorded its hottest day on record (42°C or 108°F at Sea-Tac on June 28). We came to study coastal fog and potential climate refugia, and we felt like climate refugees ourselves. The FHL main weather station peaked at 35°C (95°F) on the 28th: still hot but nowhere near as bad. However, we did not see fog that weekend, and the relatively cooler temperatures were still
too hot for many marine organisms around the Salish Sea.
Fog is a cloud that extends to the ground and forms when air temperature either cools or moistens enough that it cannot hold more water vapor and droplets form. This is called the dew point temperature because at this temperature we see dew on the ground. The most common form is radiation fog, which forms from near-surface air temperatures cooling on clear nights. Because moist air is less dense than dry air, a capping inversion (where air temperatures increase with height) is required to keep the fog at the surface. Inversions are associated with high pressure and sinking air, both of which are common along the western U.S. in the summer.
Marine and coastal fog add an extra layer of complexity, as fog can form by increasing the moisture content of the air from evaporation (latent heat flux) from the ocean. Fog can also form from cooling the already moist air from below, which occurs over cold sea surface temperatures associated with the upwelling of colder, deeper water to the surface. The persistent summer fog along the California and Oregon coast is associated with the strong ocean upwelling and consistent high atmospheric pressure leading to a capping inversion over a layer of cool, moist air.
In the Salish Sea and around FHL, these ingredients combine to create more variable and, in my opinion, more beautiful fog. Air temperature fluctuates with the solar cycle, so fog is most common just before dawn, but around the San Juan Islands, surface currents and upwelling are
forced by the tides. Rising tides not only bring in colder surface water from the Strait of Juan de Fuca but also, through strong currents over narrow sills, generate upwelling of colder water from below. Thus, fog is more likely at times of rising tides and at locations of current-driven upwelling. Fog that forms over this cold water can also be advected by the breeze from one location to another.