Measuring Fog at Cabrillo National Monument: Where Native Plants Get Their Summer Water

San Diego Bay in fog, from Cabrillo National Monument. NPS / Pauline Geisler.

Coastal California's early-summer fog — "May gray," "June gloom" — disappoints sunbathers, but at Cabrillo National Monument, an urban national park in San Diego, it is a lifeline. It keeps a rich community of native plants, many of them rare, hydrated through the region's hot, dry summers. The plants are vital wildlife habitat and a big part of the park's identity: visitors have recorded more than 300 species on the park's iNaturalist page.

Fog also matters for restoring plant communities damaged by invasive plants or unofficial "social" trails — without enough of it, there may not be enough water for restoration to succeed. So Park Service scientists and Scientists in Parks interns built a new system to map fog across the park. Their first results, in the Park Service's Science Report Series, are good news for restoration.

California buckwheat in flower at Cabrillo. NPS / C. Aurrecoechea.

An island of rare plants

Cabrillo sits at the tip of the Point Loma peninsula, hundreds of feet above the Pacific. With water on three sides, it is an "island" of rare plants in busy San Diego. Lying where two species ranges meet, it holds distinct coastal sage scrub and southern maritime chaparral communities:

• Shaw's agave, found mostly in Mexico, reaches the north of its range here;
• lemonade berry reaches the south of its range.

These plants shelter native bees, foxes and many other animals, and unusual birds stop here on the Pacific Flyway. Coastal sage scrub now covers only fifteen percent of its historic range, and urbanization keeps shrinking it.

Yet little is known about fog patterns in the United States, still less at places like Cabrillo — or how climate change might alter fog or the park's microclimates (small areas with their own conditions). Summer fog could drop sharply, or shift in timing and duration, each with different consequences for the park's plants.

Catching fog on a leaf

Fog is unpredictable and hard to measure. Researchers elsewhere have caught it in mesh nets or wires, measured it dripping from big trees onto shrubs in the cloud forests of Channel Islands National Park, or timed fog events from satellites. At Cabrillo the team wanted to measure fog landing directly on leaves.

Scientists in Parks intern Virginia Javier at a fog monitoring station; the weatherproof box holds a data logger, battery and solar controller. NPS.

• Interns Taro Katayama and Brent Wilder spent three months building the system.
• At four stations, leaf-shaped paddles — leaf wetness sensors — sit among the leaves of common plants and record tiny amounts of deposited water, continuously.
• Each station also has a rain collector. When the sensors register water but the rain gauge reads zero, the water came from fog — and vice versa.
• Solar panels and batteries keep each station running for months without maintenance.
• The stations were placed to capture differences in elevation and between the ocean-facing and bay-facing sides of the peninsula.

A leaf wetness sensor in a lemonade berry plant, 100 centimeters (about 39 inches) above the ground. NPS / Taro Katayama.

Microclimates matter

Data from July 2022 to May 2023 show the park has fog microclimates:

• stations facing the Pacific logged more hours of fog than those facing San Diego Bay;
• ocean-side plants seem to get fog all year, bay-side plants only in winter.

For restoration, that points to planting year-round on the ocean side, where fog is plentiful, but perhaps restoring bay-side vegetation only as the winter rains begin. Superintendent Chris Rodriquez said the study shows fog's critical role as a water source for the park's native plants, and will help the park target restoration where its ecosystems can survive and recover.

Scientists in Parks intern Brent Wilder at a station on the monument's bluffs, with its solar panel and rain catcher. NPS / Taro Katayama.

What comes next

Katayama, now the monument's terrestrial biologist, keeps the system running. More data will show current fog patterns and how they change, helping the park choose plantings that will thrive in future. The same approach could work in bigger landscapes — coastal redwood forests or high-elevation Hawaiian rainforests — and the team is working with other parks on a network of fog stations: a model for solar-powered, long-term monitoring of the fog many natural areas depend on.

Sources

Based on Taro Katayama, Linh Anh Cat and Lauren Pandori, "Innovative System Measures Fog That Beloved Plants Need to Thrive," Park Science 39(1), Winter 2024–25, National Park Service; rewritten in hubnx's own words. Cat, deputy superintendent of Kalaupapa National Historical Park, and Pandori, Resource Management and Science lead at Cabrillo, are Park Service scientists. The project was funded in part by the Cabrillo National Monument Foundation, the Scientists in Parks program, Environment for the Americas, Conservation Legacy, the Southern California Research Learning Center and the Ecological Society of America. Photographs: National Park Service.