Freezing fog is fog that forms when the air is at or below 32°F. Its tiny water droplets stay liquid in the air, then freeze the moment they touch a surface. Freezing fog is rare in the Bay Area, because mild Pacific air keeps the coast and lowlands above freezing on most nights. When it does happen, it creates one of the region's more striking winter sights. Trees, fences and grass turn white with rime, a crust of ice built entirely from fog rather than from rain or dew. The best chances come on the highest peaks and in the coldest inland valleys during strong cold snaps.
How Freezing Fog Forms
Ordinary fog is made of tiny liquid water droplets floating in the air. Surprisingly, those droplets can stay liquid even when the air is a little below freezing. Water needs something to freeze onto, such as a speck of ice or a solid surface, and tiny droplets floating in clean air often lack one. Droplets in this state are called supercooled.
When supercooled droplets touch something solid, such as a branch, a car, a power line or the ground, they freeze at once. Over hours, millions of them build up a rough, white, opaque layer of ice called rime. Rime grows thickest on the side of objects facing into the breeze, because that side meets the most fog. Long, feathery spikes of rime can point straight into the wind.
Rime is different from hoarfrost, which forms when water vapor turns straight into ice crystals on a cold surface without becoming liquid first. Hoarfrost tends to look delicate and sparkly. Rime is usually whiter and more crusty. Both are different from freezing rain, which falls from clouds as rain and freezes on impact. Freezing fog builds ice slowly from the fog itself, with nothing falling from the sky.
Where Freezing Fog Occurs in the Bay Area
Because the Bay Area is so mild, temperatures along the coast and around the bay rarely reach freezing, even on the coldest winter nights. Freezing fog needs both fog and air below 32°F, so it shows up mainly in two kinds of places, high peaks and cold inland valleys.
Mount Hamilton, at about 4,265 feet, and the upper slopes of Mount Diablo, at up to 3,849 feet, drop below freezing on many winter nights. When low clouds wrap around these peaks during cold weather, the summits sit inside freezing fog. The cloud itself works just like fog on the ground, and the droplets coat trees, signs and railings with rime. The roads to the top, such as Mount Hamilton Road, can turn slick when that happens.
In the inland valleys, the Livermore Valley and parts of the Napa and Sonoma valleys can fall below 32°F on the coldest clear nights. If radiation fog forms on one of those nights, the result is freezing fog. The fog may look thin, but by morning it can leave a noticeable coat of rime on cars, plants and fences.
Freezing fog is more common farther inland, in the Central Valley. During long winter spells of tule fog, temperatures under the fog can hover near or below freezing for days. The Weather Service offices that cover the valley sometimes issue Freezing Fog Advisories, which warn of both low visibility and icy surfaces.
A few clues in the forecast hint that freezing fog is possible. The best setup is a clear, calm night after a recent rain, when the ground is wet and the air near the surface is moist. If overnight lows are forecast near or below freezing and fog is also mentioned, the stage is set. These nights usually come in December and January, when the nights are longest and a dry spell follows a storm. Even then, the freezing fog tends to be patchy, filling low spots and valley floors while nearby hillsides stay clear.
Freezing Fog Hazards and Safety
Freezing fog makes roads dangerous in a way that is easy to miss. Nothing falls from the sky, so the road may look merely damp. Bridges, overpasses and exposed stretches of road can still collect a thin, clear glaze of ice from the fog. This black ice is hard to see, and drivers often have no warning until their tires start to slide. Bay Area drivers, who rarely face icy roads, are especially at risk.
The Weather Service issues Dense Fog Advisories when visibility drops very low, and a Freezing Fog Advisory when the fog is also icing surfaces. Whenever fog and temperatures at or below 32°F come together, treat bridges and raised roadways as icy, whatever they look like. Slow down, leave extra room and avoid sudden braking. Take special care on the summit roads of Mount Hamilton and Mount Diablo on cold, foggy mornings.
Deep Dive: How Can Water Stay Liquid Below Freezing?
Water does not freeze just because it reaches 32°F. Ice has to start somewhere, from a tiny seed crystal, and small, clean droplets often have nothing to start from. Fog droplets can stay liquid far below freezing as a result, waiting for something to trigger the change.
Why Does Ice Need a Seed?
For ice to form, water molecules must lock into a small, orderly crystal that is stable enough to grow. In pure water, that happens by chance only at very low temperatures. Laboratory studies put the temperature at which tiny pure droplets freeze on their own near minus 38°C, or minus 36°F.
Above that, droplets need help from a particle that acts as a template. Scientists call these ice-nucleating particles. Certain mineral dusts, bacteria and bits of plant material work at relatively mild temperatures, but they are scarce. Most of the particles that fog droplets grow on are salts and pollution, which are good at gathering water but poor at starting ice.
That is why freezing fog in the Bay Area valleys is nearly always a fog of supercooled liquid droplets. The droplets freeze only when they touch a solid surface, such as a twig, fence wire or road, which acts as the seed.
Why Does Rime Grow Toward the Wind?
When a supercooled droplet hits a cold surface, it freezes almost instantly, trapping tiny pockets of air. Millions of these frozen droplets pile up into rime, which looks white and opaque because of the trapped air. Larger drops, as in freezing rain, spread out before freezing and form clear glaze ice instead.
Droplets carried by the breeze strike the upwind side of an object first, so rime builds there. As the deposit grows, its tips catch the most droplets, and the rime extends into feathers that point into the wind. A branch coated with rime therefore records which way the fog drifted overnight.
If ice crystals do start to form in the fog, they grow quickly at the droplets' expense. Near 14°F, by our calculation, air saturated for liquid water holds about 10 percent more vapor than ice can tolerate. Vapor moves from the droplets onto the crystals, and the fog can turn into glittering ice fog. That process is common in the far colder valleys of the interior West, but rare in the Bay Area.
What Freezing Fog Looks Like in the Bay Area
After a night of freezing fog, the landscape can look dusted with snow. Grass, branches, fence posts and any surface facing the breeze wear a white, crystal-like coat. In the Napa and Livermore valleys, a morning after freezing fog can turn bare winter vines into white sculptures, with every wire and post outlined in ice. The patterns show which way the fog drifted through the night.
Once the sun climbs and the air warms above freezing, the rime melts quickly. Surfaces drip as though it had rained, even though no rain fell. The change from a white, frosted landscape to a damp, green one can take less than an hour.
It is one of the more unusual and beautiful sequences of a Bay Area winter. It happens almost only in the inland valleys and on the high peaks. If you want to see it, watch for a cold, clear forecast with calm wind and patchy fog. Then head out early, before the sun has a chance to melt the ice.
