Redwoods peek above a blanket of gray as fog rolls into Bay Area valleys, dripping down to nourish ferns and mosses below. In San Francisco, tourists shiver in the billowing gloom, fooled by the promise of California sunshine as foghorns bellow from the Golden Gate Bridge. Coastal fog is most abundant in the summer, seeding nicknames such as Karl, May Gray, June Gloom, No-Sky July, and Fogust as it envelops coastal forests and cities alike in its damp, chilly embrace.
“August is about the peak of the fog season,” says Daniel Fernandez, fog researcher and environmental science professor at California State University, Monterey Bay.
While sunless coastal summers may seem dreary, fog plays a key role in the Bay Area’s varied microclimates. Dew points and temperatures are closest at night, conjuring nocturnal mists that often last until late morning. This drives a daily ebb and flow of fog keeping temperatures cool.
As the local saying goes, “It’s natural air-conditioning,” explains Peter Weiss-Penzias, an environmental toxicology researcher at UC Santa Cruz who studies fog’s chemical composition and how pollutants might catch a ride on fog droplets. “Having that cooling at night really makes a big, big difference on the livability of our region.”
What precipitates summer fog?
All fog emerges from the same atmospheric recipe. “You need a temperature difference, you need moisture, and you need these tiny particles that the fog can condense upon,” says Fernandez.
In the summer, hotter air over coastal lands hits wet, cold air fresh off the ocean, known as the marine layer. Thicker marine layers form in summer, when the strongest northerly winds of the year drive ocean upwelling. Fog droplets form when moisture from the marine layer condenses around a particle. The particles could be bacteria or dirt or, often, grains of salt tossed up from breaking waves. This suspension of tiny droplets—each smaller than a tenth of the size of a raindrop—coalesces into a cloud that kisses the earth.
The coldest surface water of the year (summoned by enhanced marine upwelling) and the hottest air of the year collide along the coast, creating prime conditions for abundant summer fog. A shallow marine layer of 500 feet or less can create fog right along the coastline, while a layer of 1,000 to 2,000 feet will cover San Francisco Bay. An especially deep marine layer of up to 3,000 feet can creep up valleys, beckoning coastal fog farther inland.
Overcast upside
Fog benefits local ecosystems, especially in the summer. It feeds coastal redwood forests, grasslands, scrub, and chaparral habitats and keeps streams running in the dry summer months. Plants lose less water to evaporation during fog events. For some species, the way sunshine scatters between fog droplets allows more leaves to photosynthesize, boosting growth.
While fog might not provide large amounts of water like rainfall, it is consistent. Native plants have evolved to take advantage of fog by absorbing water directly through their leaves or needles or by collecting droplets that then drip down to the soil. Redwood trees rely on fog for 30 to 40 percent of their yearly water intake.
Human residents benefit from fog as well. Even when inland valleys face brutal heat waves and drought, people living near the coast enjoy cooler weather thanks to fog. An unusually foggy summer in 2025 brought down average temperatures in parts of the Bay Area by a few degrees Fahrenheit and kept landscapes moister and cooler throughout the region. “It really helped limit the impact of fire season,” says Rachel Kennedy, a meteorologist with the National Weather Service.
A nebulous future
Because of fog’s ephemeral nature, studying it is a fickle endeavor. California fog scientists are examining fog formation and how its impacts on local microclimates and ecosystems might shift with climate change.
“Our work is to examine the nuance of coastal fog dynamics and how they may change in the future,” says Sara Baguskas, an ecologist and fog researcher at San Francisco State University.
The future of fog is not necessarily as simple as there being less of it in summers to come. Climate change and fluctuations during El Niño or La Niña years may alter patterns of where, when, and how often fog forms—with different areas seeing more or less fog compared to years past.
For now, summer is the best season to watch the fog drift in, transforming Bay Area landscapes as it muffles noisy cities and brings forest floor smells to life with its haunting mists.
“My appreciation for fog’s beauty is ultimately the root of my entire career,” says Baguskas. “It makes the Bay Area special. How boring would it be without fog?”
