Radiation fog forms on clear, calm nights when the ground loses heat quickly and chills the air just above it to its dew point. It is very different from the marine layer fog that San Francisco is famous for. Knowing the difference explains where fog shows up in the Bay Area, when it burns off and why it can be so much denser than coastal fog. Radiation fog belongs to low valleys, river bottoms and calm inland areas. In California, it is most extreme in the Central Valley, where it becomes tule fog. In the Bay Area, it forms regularly in the Livermore Valley, Napa Valley, Petaluma and other flat areas sheltered from the wind.
How Radiation Fog Forms
The name comes from radiative cooling, the way the ground loses heat by sending infrared energy out toward space. The sun warms the ground during the day, and that heat escapes again at night. On clear nights, no clouds trap the heat and send it back down, so the ground cools quickly and the air touching it cools too. If that air reaches its dew point, its water vapor condenses into tiny droplets hanging just above the surface, and that is radiation fog.
Radiation fog usually needs four conditions to line up.
- Clear skies. Clouds act like a blanket that holds heat in, so the ground cannot cool enough for fog to form.
- Light or calm winds. Wind mixes the air and keeps the cool, moist layer near the ground from settling. A very light breeze can help spread fog once it forms, but stronger wind breaks it up.
- Plenty of moisture near the ground. Wet soil, recent rain, irrigation or a nearby body of water raise the dew point, so less cooling is needed before fog forms. That is why the first clear, calm night after a storm is often the foggiest. Rain itself rarely brings dense fog here, as our guide to precipitation fog explains.
- Long nights. Fog needs time to form, and long winter nights allow much more cooling than short summer ones. That is why radiation fog is mostly a late fall and winter fog.
Radiation Fog vs. Advection Fog: Key Differences
Bay Area residents see both kinds of fog regularly, and they behave very differently. San Francisco's summer fog is advection fog, which forms over the cold Pacific and is carried inland by the wind. Radiation fog forms in place over land on calm nights. Most of their differences follow from that one contrast.
Advection fog depends on the ocean and on the pressure difference that drives marine air inland. It arrives on the wind, often in the afternoon, and pulls back when that pressure difference weakens. Radiation fog forms after sunset, thickens through the night and is densest around dawn. Advection fog can linger as long as the marine layer keeps feeding it, but radiation fog usually burns off once the sun warms the ground. The bottom of the fog lifts first, visibility improves at street level and the fog clears from the ground up. On most days it is gone by 10 AM to noon.
The exception is tule fog in the Central Valley, an extreme form of radiation fog. Under strong temperature inversions, a layer of warm air above traps the cold, foggy air near the ground. The weak winter sun cannot break through, so the fog can last for days. That is why tule fog is a serious driving hazard, while most Bay Area radiation fog is a morning nuisance rather than a multi-day one.
Where Radiation Fog Forms in the Bay Area
Radiation fog forms in particular kinds of places, because it needs calm air, plenty of moisture and terrain where cold air can pool. Cold air is denser than warm air, so it drains downhill at night and collects in valley bottoms and low spots. The most fog-prone places are exactly where that cold air gathers, on valley floors, along creeks and in flat, low basins.
Napa Valley and the Sonoma Valley get radiation fog regularly in late fall and winter. It forms on the valley floor overnight and usually burns off to clear, mild afternoons. Petaluma and the flat, low parts of Marin get radiation fog more often than the hillside neighborhoods above them.
The Livermore Valley is one of the Bay Area's most fog-prone places in winter. It is one of the hottest, driest parts of the region in summer, yet winter mornings there can be foggy while coastal San Francisco is clear. The valley's flat floor and calm winter nights let cold air pool and fog form. The fog usually burns off quickly after sunrise, leaving clear afternoons.
How Long Does Radiation Fog Last?
Radiation fog usually clears within a few hours of sunrise. The sun warms the ground, the ground warms the air above it and the fog lifts and evaporates. Dense fog that forms at 2 AM is usually gone by 9 or 10 AM. In the Bay Area's inland valleys, that creates a classic winter day that is cold and foggy at dawn and sunny by mid-morning. Afternoons can reach the 50s or 60s even in December.
How fast it clears depends on how thick the fog is, the cloud cover above it and the temperature. A thin, shallow layer may clear before sunrise, while dense fog in a deep valley may last until noon. If high clouds move in overnight and slow the cooling, the fog may never get thick enough to matter. Checking the overnight cloud forecast and the expected low temperature gives you a good sense of whether radiation fog is likely.
Deep Dive: How Fast Does the Ground Lose Heat on a Clear Night?
Radiation fog depends on a fine balance between cooling and stirring. The ground has to lose heat fast enough to chill the air above it to its dew point. The wind has to be light enough that the cold air stays near the ground, but not so calm that the moisture just settles out as dew.
How Much Heat Does the Ground Lose to a Clear Sky?
Everything gives off infrared energy, and warmer objects give off more. By our calculation, ground at 45°F radiates about 350 watts per square meter, roughly the output of six bright incandescent bulbs on every square meter. A clear, dry winter sky sends back only about three quarters of that. The ground therefore loses roughly 80 watts per square meter all night long.
A low cloud deck changes that balance completely. The cloud base is nearly as warm as the ground, so it sends back almost as much energy as the ground gives off. The net loss falls close to zero, and the ground barely cools. That is why a single overnight cloud deck can prevent radiation fog even when everything else is right.
Once fog forms and grows thick, the cooling moves to the top of the fog. The fog top now faces the clear sky, while the ground below is shielded. Cooling at the top then makes the fog layer deeper through the rest of the night.
Why Does a Light Wind Decide Between Fog and Dew?
British researchers led by W. T. Roach studied radiation fog in the 1970s at Cardington, England. Their observations showed that fog formed only after the wind dropped to very low speeds and turbulence nearly stopped.
The UK Met Office's LANFEX campaign, reported by Jeremy Price and colleagues in 2018, measured this balance in detail across four neighboring valleys. Given enough clear-sky cooling, they found three regimes. With more stirring, heat mixed down from warmer air above kept the ground from cooling to its dew point. With less stirring, the ground reached its dew point and dew formed, but the air above stayed clear. Only when turbulence fell lower still did the air itself saturate and fog form.
Dew and fog compete for the same moisture. Water that condenses on grass is water that cannot form fog droplets in the air. That is one reason the foggiest nights in Napa or Livermore often follow a rainy day, since wet ground keeps supplying moisture to the air above it.
Radiation Fog and Bay Area Wine Country
Wine country gets two kinds of fog, and growers care about both. During the growing season and harvest, most of the fog that cools Napa and Sonoma vineyards is marine fog. It pushes in from San Pablo Bay and through the Petaluma Gap on summer and early fall nights. That cool air slows ripening and helps the grapes keep their acidity. Radiation fog becomes more common later in the fall and winter, after most of the harvest is in. Some growers worry that warmer nights could change these patterns and push harvests earlier.
On a late fall morning, Napa Valley's floor can lie under white fog at 7 AM with golden light on the hills by 10. That is radiation fog at its most beautiful. Together with the summer marine fog, it is one of the few places in California where fog is valued rather than just tolerated.
