Innovative System Measures Fog That Beloved Plants Need to Thrive

Fog is a critical but mysterious water source for native plants during hot, dry summers at Cabrillo National Monument. Researchers used new technology to uncover the park’s fog patterns, showing the best places to restore damaged habitat.

By Taro Katayama, Linh Anh Cat, and Lauren Pandori

About this article

This article was originally published in the "In Brief" section of Park Science magazine, Volume 39, Number 1, Winter 2024-25 (January 15, 2025).

A foggy view of San Diego Bay from Cabrillo National Monument.

Image credit: NPS / Pauline Geisler

May gray, June gloom—it goes by many names. Whichever you prefer, fog is a regular presence along the coasts of sunny California. It may frustrate tourists planning to sunbathe, but at Cabrillo National Monument, an urban national park in San Diego, fog is a lifeline for a rich community of native plants. It helps them stay hydrated during the region’s hot, dry, summer months. These plants—many of them rare—aren’t just vital wildlife habitat; they’re an iconic element of the park’s identity. They’re so beloved by visitors that over 300 species of them are confirmed on the park’s iNaturalist site, with new observations added daily.

Fog is also crucial for restoring Cabrillo’s plant communities that have been damaged by things like invasive plants or unofficial “social” trails. But when there isn’t enough fog, there may not be enough water for restoration to work. So we, Cat and Pandori, National Park Service scientists, along with Scientists in Parks interns Taro Katayama and Brent Wilder, devised a technologically innovative system to study fog patterns across the park. Our early results, recently published in the National Park Service’s Science Report Series, has good news for restoring the park’s plants.

California buckwheat plant flowers at Cabrillo National Monument.

Image credit: NPS / C. Aurrecoechea

Iconic Plants Face a Fog of Uncertainty

Cabrillo National Monument lies at the tip of California’s Point Loma peninsula, hundreds of feet over the Pacific Ocean. Three sides of the park are surrounded by water, so the park acts as an “island” for rare plants and unique plant communities in busy, urban San Diego. Cabrillo has distinct coastal sage scrub and southern maritime chaparral plant communities because the park is at the intersection between two species ranges.

Plant species like Shaw’s agave are mostly found in Mexico; in Cabrillo, they’re at the northern part of their range. Plants like lemonade berry are at the southern part of their range. These plants provide habitat for native bees, foxes, and many other animals. Unusual bird species use Cabrillo as a rest stop along the Pacific Flyway. Coastal sage scrub habitats like this one currently occupy only fifteen percent of their historic range, and urbanization is making them even rarer.

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Despite fog’s importance to plant communities like these, we know little about fog patterns in the United States.

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Despite fog’s importance to plant communities like these, we know little about fog patterns in the United States. How fog behaves in places like Cabrillo is even more mysterious. Specifically, we don’t know how climate change may alter fog dynamics or microclimates—localized areas with unique environmental conditions—at the park. For example, fog could dramatically decrease during the summer months. Or climate change could shift when fog occurs and for how long. Different changes could have different implications for how the park manages its unique native plant communities. Our work will help scientists and resource managers understand changing fog patterns and better predict when and where moisture levels are sufficient to support revegetation.

Assessing the Unpredictable

Part of why fog is so poorly understood is that it’s unpredictable and notoriously hard to measure. Some scientists have tried to quantify it by putting up mesh nets or wires to catch it and measuring its volume. Researchers have also measured moisture collecting on large trees as it drips down to smaller shrubs, like in the cloud forests at Channel Islands National Park. Some have used satellite imagery to quantify the duration of fog events over time. At Cabrillo, we wanted to measure fog landing directly on leaves, so we needed a different approach.

Scientist in Parks intern Virginia Javier kneels next to a fog monitoring station. The station has a data logger, battery, and solar controller inside a weatherproof box.

Image credit: NPS

Katayama and Wilder spent three months building a fog monitoring system for the park. The solar-powered system continuously captures data from a set of leaf-shaped paddles. At four monitoring stations, they placed the paddles, also known as leaf wetness sensors, among the leaves of common plants. The paddles detect fog by recording trace amounts of deposited water.

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If the weather station registers zero rain at the same time as the leaf wetness sensors detect water, we know fog must have been the source.

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We can tell whether water came from rain or fog, because each monitoring station has a weather station with a rain collector. If the weather station registers zero rain at the same time as the leaf wetness sensors detect water, we know fog must have been the source—and vice versa. Solar panels and batteries power each self-sustaining unit, which can operate for months without maintenance. We strategically located the fog monitoring stations to capture elevation differences as well as differences between the ocean-facing and bay-facing sides of the peninsula.

A leaf wetness sensor among a lemonade berry plant. The measuring stick shows the sensor is 100 centimeters (about 39 inches) above the ground.

Image credit: NPS / Taro Katayama

Microclimates Matter

Our early data show that the park has fog microclimates. For example, stations on the peninsula that face the Pacific Ocean recorded more hours of fog than those facing San Diego Bay. The data, collected from July 2022 to May 2023, also could suggest seasonal fog patterns: oceanside plants receive fog throughout the year, but bayside plants only receive fog during the winter months.

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“By uncovering distinct fog patterns, we can better target our restoration efforts, ensuring the survival and resilience of our unique ecosystems."

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These results demonstrate that the new stations are working. They can detect changes in fog quickly and give valuable insights for native plant restoration. Our work shows, for example, that restoring native plant species year-round on the ocean side of the peninsula—where there’s more fog—will be most successful. But perhaps bayside vegetation should only be restored at the beginning of the winter rains to ensure plant survival.

“This study reveals the critical role of fog as a water source for native plants in Cabrillo National Monument and enhances our understanding of habitat restoration,” said park superintendent Chris Rodriquez. “By uncovering distinct fog patterns, we can better target our restoration efforts, ensuring the survival and resilience of our unique ecosystems.”

Scientist in Parks intern Brent Wilder works on a fog monitoring station on the bluffs in the monument. The station has a solar panel and rain catcher.

Image credit: NPS / Taro Katayama

A Blueprint for the Future

Katayama, now Cabrillo National Monument’s terrestrial biologist, continues to maintain the fog monitoring system. As it collects more data, the system will help clarify current fog patterns and show how fog dynamics change over time. The more we learn about local fog conditions, the more we can help the park design plantings that will thrive into the future. Our monitoring system could also be used in larger areas, to analyze fog conditions in coastal California redwood forests or high-elevation Hawaiian rainforests for example. That’s why we’re working with other national parks to create a network of fog monitoring stations.

Our research could be a blueprint for solar-powered, long-term monitoring of the critical but poorly understood fog that sustains the unique plant and animal communities in many natural areas. And of course, it’ll help keep the park’s spectacularly photogenic flowers blooming.

Acknowledgments

This project was funded in part by the Cabrillo National Monument Foundation, the Scientists in Parks program, Environment for the Americas, Conservation Legacy, Southern California Research Learning Center, and the Ecological Society of America. We thank these organizations for their support of the National Park Service and for providing opportunities for future generations of scientists.

We would also like to thank Jay Simpson, Virginia Javier, Brent Wilder, and the wonderful volunteers for their stellar contributions to this project. We express sincere thanks to those who have made this program outstanding and contributed to its success.

About the authors

^{Taro Katayama is a biologist with Cabrillo National Monument. Image courtesy of Taro Katayama.}

^{Linh Anh Cat, PhD, is the deputy superintendent of Kalaupapa National Historical Park. Image credit: NPS}

^{Lauren Pandori, PhD, is the Resource Management and Science lead at Cabrillo National Monument. Image credit: NPS}