On a cold, clear Bay Area morning after a frosty night, wisps of mist sometimes rise off the bay, a lake or a pond at sunrise. The water looks as if it is steaming, with thin curls of fog drifting upward and vanishing a few feet above the surface. This is steam fog, also called evaporation fog, or sea smoke when it forms over the ocean. It works the opposite way from advection fog, the Bay Area's famous summer fog. Summer fog forms when moist air cools over cold water, while steam fog forms when cold air sits over warmer water. In the Bay Area it is a small, short-lived sight, but a beautiful one, seen mostly on the coldest winter mornings.
How Steam Fog Forms
Steam fog forms through quick evaporation followed by quick condensation. When cold, dry air moves over water that is much warmer than the air, water evaporates rapidly from the surface. That adds moisture to the thin layer of air right above the water.
Cold air cannot hold much water vapor, so the added moisture soon pushes it past its limit. The extra vapor condenses into tiny droplets almost at once, forming a thin layer of fog just above the surface. It is the same effect you see when your breath turns to mist on a cold morning. Steam rising from a hot bath in a cool room works the same way.
The fog rises because the water also warms the air touching it, and warm air floats upward. As the wisps climb, they mix with the drier air above and evaporate again. That cycle of forming, rising and vanishing gives steam fog its wispy, curling look, so different from the flat gray sheets of summer fog.
Over the ocean in very cold places, such as the North Atlantic in winter, sea smoke can be thick and widespread enough to hide ships. In the Bay Area, the gap between water and air temperature is usually small, so steam fog stays thin and rarely rises more than a few feet.
When Steam Fog Occurs in the Bay Area
Steam fog needs two things at once, water that is clearly warmer than the air and cold, dry air moving over it. San Francisco Bay changes temperature slowly, and in winter its water usually sits in the low to mid 50s. On a cold morning, the air over it can fall into the 30s or low 40s.
The best setups come right after a cold, dry storm passes. Behind the storm, clear skies and cold air from the north let temperatures drop sharply overnight. Steam fog forms when the air turns much colder than the water, often by 15 degrees or more. A light breeze that moves the cold air across the surface helps. Early morning is the time to look, before the sun warms the air and the contrast fades.
Inland lakes and reservoirs can steam too, and often more strongly, because the air around them gets colder. Reservoirs in Napa County, Lake Berryessa to the north and the reservoirs of Contra Costa County sometimes show steam fog on the coldest winter mornings. Even then, it is usually a thin veil of mist on the water rather than the thick sea smoke of colder climates.
Steam Fog vs. Other Bay Area Fog Types
Steam fog is a minor player next to the Bay Area's two main fog types. The summer marine layer, which is advection fog, and winter radiation fog cause nearly all the fog that affects daily life in the region. Steam fog is small and short-lived, tied to water and to the coldest mornings.
It is easy to tell apart by its look. Steam fog rises from the water in wisps and curls, as if the water were hot. Advection fog and radiation fog form flat, even layers instead. Steam fog rarely cuts visibility over a wide area and almost never leads to a Dense Fog Advisory. For most people it is simply a lovely sight for an early walk, a paddle or a ferry ride.
Sea smoke over the Pacific is rare off the Bay Area coast. The cold California Current and upwelling keep the ocean cool all year. For the ocean to steam, the air would need to get far colder than it usually does. The bay and inland lakes, which sit beside colder land, offer much better odds.
Deep Dive: Why Can Mixing Two Clear Kinds of Air Make Fog?
Steam fog depends on a quirk of water vapor. The amount of vapor air can hold rises faster and faster as the air warms, so the curve of that limit bends upward. Mixing warm, damp air with cold air can therefore produce a blend that holds more vapor than its temperature allows, even though neither starting air was foggy.
Why Does a Curved Limit Matter?
The limit on water vapor follows the Clausius-Clapeyron relation, rising by about 7 percent for each degree Celsius. When two kinds of air mix, the temperature and the vapor of the mixture both land roughly halfway between the two. The vapor limit at that halfway temperature is lower than the halfway point between the two limits, because the curve bends.
By our calculation, consider air right above a pond at 55°F, saturated by evaporation from the water. Mix it equally with outside air at 25°F and 80 percent humidity. The blend comes out near 40°F, holding about 10 percent more vapor than air at that temperature can keep. The extra condenses at once, and a wisp of fog appears.
Your breath on a cold morning works exactly the same way. Air leaving your lungs is warm and nearly saturated. As it mixes with cold outside air, the blend passes the vapor limit and turns briefly into a small cloud.
Why Does Steam Fog Need Such a Big Temperature Gap?
The calculation also shows why steam fog is rare here. The bend in the curve is gentle over small temperature ranges, so the mixture lands above saturation only when the water is much warmer than the air. By our calculation, the same pond with outside air at 30°F and 50 percent humidity gives a blend at only about 96 percent humidity, and no fog forms.
The Bay Area's mild winters rarely produce that kind of gap, except on the coldest, clearest mornings. Steam fog over the bay and inland reservoirs is usually thin and short-lived for that reason. It vanishes once the sun warms the air above the water.
In very cold places the gap is huge. Air at well below 0°F flowing over open water near freezing creates sea smoke thick enough to hide ships. The physics is the same as a wisp over a Bay Area reservoir, just at a much larger scale.
Where to See Steam Fog in the Bay Area
The best places to look are spots with a clear view across calm water on cold, clear winter mornings. The Berkeley Marina, the Berkeley Hills and the ridges in Marin that overlook the bay all work well. Shoreline parks with low sun behind the water can make the wisps glow, which helps them stand out. An early ferry to Angel Island or across the bay sometimes passes through patches of steam fog close to the water.
Timing matters more than location. Steam fog is easiest to see from sunrise to mid-morning, before the sun warms the air and shrinks the gap between air and water temperatures. By around 9 or 10 a.m. on most days, the wisps fade away. After a cold, clear night in December or January, head to a bay viewpoint early for the best chance.
Dress warmly when you go, since the same conditions that make steam fog can leave frost on piers, boardwalks and docks, so watch your footing near the water. A thermos of something hot and a camera are the only other things you need.
