You wake up, look outside, and the world has disappeared. Your street ends 50 feet away in a wall of grey. By 10 a.m., it's gone without a trace. Fog is one of the most dramatic and disorienting weather events a person can experience — and yet it forms through a mechanism so simple it can be explained in a single sentence: fog is what happens when air cools to the point where it can no longer hold all its moisture as invisible vapor, and that moisture condenses into tiny suspended droplets instead.
That basic principle plays out in a surprising number of different ways, producing several distinct types of fog that form for different reasons, in different places, at different times of day.
Fog is a cloud at ground level. The physics are identical — both are made of tiny liquid water droplets (or ice crystals at very cold temperatures) suspended in air. The only difference is altitude. Meteorologists define fog as reduced visibility below 1 kilometer (about 0.6 miles). When visibility is between 1 km and 2 km, it's classified as mist. Above that, it's haze — which is usually caused by dust or pollution rather than water droplets.
The key concept behind all fog formation is the dew point — the temperature at which air becomes saturated with water vapor. When air temperature drops to the dew point, the excess moisture has nowhere to go but out of the air, condensing onto tiny particles (dust, pollen, sea salt) as microscopic droplets. When enough of those droplets form near the ground, visibility drops and fog is born.
| Type | How It Forms | Common When |
|---|---|---|
| Radiation fog | Ground cools overnight, chills air above it | Clear, calm autumn nights |
| Advection fog | Warm moist air drifts over a cold surface | Coastal areas, spring/early summer |
| Valley fog | Cold dense air sinks into low terrain | Inland valleys, winter mornings |
| Upslope fog | Air rises, expands, and cools as it climbs | Hillsides and mountain slopes |
| Steam fog | Cold air flows over warm water | Lakes and rivers in autumn |
| Freezing fog | Fog droplets freeze on contact with surfaces | Below 32°F / 0°C |
This is the fog most people encounter most often. After sunset on a clear, calm night, the ground radiates its heat back into space rapidly — the same way a blacktop parking lot feels hot to the touch after a sunny day, then cools quickly once the sun goes down. As the ground cools, it chills the layer of air directly above it. If that layer cools to the dew point, fog forms.
The key ingredients are: clear skies (so heat can escape), calm winds (strong wind mixes air and prevents the surface layer from cooling enough), and moist air near the surface (a high dew point). This is why radiation fog is most common in autumn — nights are long and cooling is strong, but the air still holds summer moisture.
Radiation fog is typically shallow, forming in the lowest few hundred feet of the atmosphere. From above, it looks like a smooth white carpet covering the landscape. It almost always burns off within a few hours of sunrise, which is why "morning fog" is nearly synonymous with radiation fog.
If you've ever driven through San Francisco in June and found the city socked in with fog at noon, you've experienced advection fog — and you know it doesn't behave like morning fog at all. Advection fog forms when warm, moist air moves horizontally over a cold surface. As the warm air passes over the cold ground or water, its lowest layers are chilled to the dew point from below.
San Francisco's famous summer fog — nicknamed "Karl" by locals — is driven by this mechanism. Warm, moist Pacific air flows inland over the cold California Current, a band of upwelled deep ocean water running along the coast. The base of that air mass chills instantly, and fog rolls in through the Golden Gate. Because the ocean's cold surface temperature doesn't change during the day, the fog has no reason to burn off — the sun heats the fog droplets, but the surface beneath them stays cold, keeping the air saturated.
Advection fog can be thick, persistent, and extensive, sometimes blanketing hundreds of miles of coastline. It's also common in spring across the US East Coast and Gulf Coast when warm humid air from the Gulf of Mexico moves over cooler land.
Cold air is denser than warm air, which means it flows downhill like an invisible liquid and pools in valleys, low-lying areas, and basins. On a clear, calm night, radiation cooling happens on all the surrounding hillsides, and the cold air produced slides down into the valley floor. If that pooled cold air is moist enough, it hits the dew point and fills the valley with fog.
Valley fog can be remarkably thick and persistent, especially in long enclosed valleys in winter when the sun angle is low. California's Central Valley, the valleys of Tennessee and Kentucky, and the Po Valley in northern Italy are all famous for days-long fog events in winter where the valley floor is completely invisible while the surrounding hills bask in sunshine.
The visual effect from above is striking: a flat sea of white filling every low point in the terrain while ridgelines and hilltops stand above it like islands.
Steam fog is the wispy, rising fog you see floating above lakes, rivers, and ponds on cold autumn mornings. It works in reverse from advection fog: instead of warm air over cold water, it's cold air over warm water. The warm water evaporates moisture into the cold air directly above it. That moisture immediately saturates and condenses, creating tendrils of fog that rise from the surface and evaporate again as they mix with drier air higher up.
Steam fog is usually shallow and patchy rather than a thick blanket. It's most dramatic in early autumn when air temperatures drop suddenly but water temperatures are still warm from summer. You can also see it on heated pools and hot tubs on cold days — the same physics, just a more convenient setting.
This applies specifically to radiation fog. After sunrise, solar radiation heats the ground, which warms the air above it from below. As the air temperature rises above the dew point, the fog droplets evaporate — they convert back into invisible water vapor. The process starts at the top of the fog layer, where sunlight hits first, and works its way down. From the ground, it looks like the fog is "lifting" when it's actually evaporating in place.
The phrase "burns off" is vivid but slightly misleading — the sun isn't burning anything. It's simply warming the air enough that the moisture goes back into vapor form. On days with a very high dew point (very humid air), the fog may take longer to clear because the air temperature needs to climb further above the dew point before the droplets evaporate.
Advection fog, valley fog, and sea fog often don't burn off on this schedule — they persist as long as the underlying cold surface that's causing them remains cold.
Fog is visibility below 1 km (0.6 miles) due to suspended water droplets. Mist is the same water-droplet phenomenon but with visibility between 1 km and 2 km — thinner, softer, less disorienting. Haze is a reduction in visibility caused by dry particles — dust, smoke, or pollution — rather than water. Haze has a brownish or yellowish tint, especially when backlit; fog and mist are white or grey. You can also feel the difference: walking through fog leaves moisture on your skin and clothes, while haze does not.
Grand Banks off Newfoundland, Canada, is widely considered the foggiest place on Earth — the cold Labrador Current meets the warm Gulf Stream, creating near-permanent advection fog for much of the year. Ships in the area historically averaged over 200 foggy days per year. San Francisco averages about 108 foggy days annually. London's famous fog reputation, while historically earned, has improved dramatically since the 1952 Great Smog disaster prompted clean-air legislation — what Londoners call fog today is largely mist.
Inland, Fresno in California's Central Valley averages over 200 hours of dense fog per year during the "tule fog" season (November through February), making it one of the foggiest inland cities in the US and a significant driver of multi-vehicle highway accidents on Interstate 5 and Highway 99.
Yes, with reasonable accuracy, though fog remains one of the harder phenomena to predict precisely. Meteorologists look for the classic setup: clear skies, calm winds, high surface moisture, and a small temperature-dew point spread in the evening forecast. When those factors align, a fog advisory or dense fog advisory is issued. The timing and exact density are harder — a slight wind can mix the fog away, or a thin cloud layer overnight can prevent the ground cooling that would trigger it.
Modern high-resolution forecast models have improved significantly, but a dense fog advisory for your area is still worth taking seriously — particularly for driving.
→ Check today's conditions and dew point for your city on ClearCast