Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of how fog forms. The models are simplified to show the idea, not to forecast real fog.
Drag things and slide the sliders.
Drag the hiker down the bank into the fog. How far can you see? When does it count as fog?
Fog means visibility under 1 km; above that, the same droplets are mist. Walk down and watch visibility fall.
Walk down into the fog. Can you still see far?
Cool the air. When do all the seats fill up, and when do droplets appear?
Lower the temperature of air holding a fixed amount of vapor. Relative humidity climbs to 100% at the dew point; below it, vapor condenses.
Make the air colder. When do the tiny drops come out?
The curve is the real saturation curve; the “bus” is a picture of it, not a model of molecules.
Drag the Moon (or the time slider) through the night. When does the air reach its dew point?
On a clear, calm night the air cools fastest after sunset. Fog forms when it meets the dew point.
Move the Moon. When does the fog come?
A simple cooling curve: real nights depend on clouds, wind and moisture.
The air is 15°C. Make the water colder or warmer. Which kind of fog do you get?
Warm air over colder water gives advection fog; cold air over warmer water gives steam fog.
Make the water cold, then warm. What happens?
Set the evening humidity gap, rain, sky and wind, then run the night. Can you make fog by dawn?
Line up a small temperature–dew point gap, late rain, clear skies and a light breeze.
A simplified rule-of-thumb model built from the forecasting conditions in our sources.
Drag the Sun up the sky. Then try a cloudy morning. What changes?
Sunlight warms the ground and air, so droplets evaporate. Under overcast skies warming is weak.
Move the Sun up. Where does the fog go?
Fog is a cloud that sits on the ground. When you walk in fog, you are walking inside a cloud!
The air has water in it, but you can't see it. It floats around as a gas.
Warm air has lots of room for this hidden water. Cold air has less room. When air gets cold enough, the room fills up.
The extra water turns into tiny drops. Each drop grows on a speck of dust. Lots and lots of tiny drops make a cloud. If the cloud is on the ground, it's fog.
When the sun comes up, it warms the ground and the air. Warm air has room again, so the tiny drops disappear.
Where does all that water go next? See how the water cycle works.
Fog is a cloud that touches the ground.115 It forms when the air near the ground cools down to its dew point: the temperature at which the air can't hold any more water vapor.913 Once that happens, the extra vapor condenses into countless tiny liquid droplets, each one growing around a speck of dust, salt, smoke or pollen.810 Those floating droplets scatter light, and you see a grey-white wall of fog.
So fog needs two things: moisture in the air, and a way to cool that air (or add even more moisture) until it reaches saturation.9 Every type of fog is just a different way of making that happen.
You've seen this happen on a smaller scale: the drops on the outside of a glass of ice water on a summer day, or glasses that "fog up" when you walk from an air-conditioned room into humid air. It's the same condensation.10
The dew point is the temperature air has to be cooled to for its relative humidity to reach 100%. Cool it any further and some vapor must come out as liquid, as fog, dew or rain.7 The higher the dew point, the more moisture is in the air. Forecasters watch the gap between the air temperature and the dew point: when the two get very close on a calm night, fog is likely.13
air temperature falls to dew point → fog
| Type | How the air gets saturated | Where and when |
|---|---|---|
| Radiation fog (ground fog) | The ground loses heat to space on a clear, calm night and chills the air just above it.2 | Very common in fall and winter; thickest in valleys and near water, around dawn.39 |
| Valley fog | A kind of radiation fog: cold, heavy air drains down slopes and pools in the valley.2 | Mountain and river valleys.9 |
| Advection fog (including sea fog) | Warm, moist air moves over a colder surface, like cold ocean water or snow.4 | The US West Coast year-round; can form in wind and under cloud, and last for days.45 |
| Steam fog (evaporation fog) | Cold air moves over warmer water; vapor evaporates into it and quickly condenses.2 | Lakes and rivers on chilly autumn mornings; "Arctic sea smoke".14 |
| Upslope fog | Wind pushes moist air up a slope, and the air cools as it rises.13 | Hills and mountainsides; even gentle slopes.13 |
| Frontal fog | Warm raindrops evaporate into a layer of cool air near the ground.2 | Near weather fronts.14 |
| Freezing fog | Fog droplets stay liquid below freezing, then freeze on contact with surfaces.6 | Cold winter mornings; coats roads, trees and cars with ice.6 |
After sunset, air on the ridgetops and upper slopes cools and becomes dense and heavy, so it drains downhill and collects on the valley floor. There it keeps cooling until it's saturated.2 That's why you can stand on a sunny hilltop and look down at a valley full of fog. In California's Central Valley, a thick radiation fog called tule fog forms on many nights from late October through February.3
Along the Pacific coast, the cold California Current flows south, and the sea is much colder than the warm air along the coast.15 When warm, moist air drifts over that cold water, its lowest layer cools to the dew point and a bank of sea fog forms.5 Wind then pushes it inland, which is why the towers of the Golden Gate Bridge can vanish into fog.15 The foggiest place on Earth works the same way: the Grand Banks off Newfoundland, where the warm Gulf Stream meets the cold Labrador Current, averages over 200 fog days a year.13
Radiation fog is fussy. It only forms when conditions are "just right".13
Because the air is usually coldest just before sunrise, radiation fog is most common in the early morning.9 Even with all of this, fog is hard to forecast: probably no two fogs are controlled by exactly the same mix of factors.14
Nothing is burning. After sunrise, the Sun heats the ground, and the ground warms the air above it. Warmer air can hold more vapor, so the fog droplets evaporate back into invisible gas.9 Valley fog can vanish quickly once the Sun is up.2 But if the sky is overcast, the ground doesn't heat up much, and fog can hang around for most of the day.9 Advection fog doesn't depend on the Sun in the same way, so it can last for days.4
| Fog | Mist | Cloud | |
|---|---|---|---|
| Made of | Tiny water droplets (or ice crystals) | Tiny water droplets | Tiny water droplets or ice crystals |
| Where | At ground level | At ground level | From high in the sky down to the ground |
| How far you can see | Less than 1 km (0.62 mi) | More than 1 km | Depends where you are |
The official definitions come from visibility: fog cuts it below 1 kilometer, while mist is thinner.1314 The UK Met Office calls it thick fog below 180 meters, and says transport is badly disrupted below 50 meters.12 Clouds can form as high as 12 miles up, and fog is simply the kind that forms at the bottom.1
It's dangerous for travel. Dense fog causes many road accidents and flight delays every year.16 In the US, the National Weather Service issues a Dense Fog Advisory when visibility often drops to a quarter of a mile or less.6 Fog droplets scatter headlight beams back toward the driver, which is why high beams make things worse.9 The NWS advice: slow down, use low-beam headlights (and fog lights if you have them), leave extra space, and if you can barely see, pull off somewhere safe with your hazard lights on.6
It can trap pollution. In December 1952, fog mixed with coal smoke sat over London for four days and killed thousands of people. It led to new clean-air laws in the UK.1315
It can also be a gift. California's coastal redwood forests collect fog droplets for water.9 In dry places near the coast, people put up large mesh "fog nets". Droplets collect on the mesh and drip into a tank. One screen can gather more than a hundred gallons of water in a day, and the village of Bellavista, Peru, uses them for its water.15
Fog is one step in a much bigger journey: the same water keeps moving between the sea, the sky and the land. See how the water cycle works.
Air always holds some invisible water vapor, and cold air can hold less of it than warm air. When air near the ground cools to its dew point, the extra vapor turns into countless tiny water droplets on specks of dust, salt or smoke. Those floating droplets are fog: a cloud touching the ground.
Yes. Fog forms the same way as a cloud, by water vapor condensing into tiny droplets. The difference is only where it happens: fog is a cloud at ground level.
Fog usually forms when the relative humidity near the ground reaches about 100%, meaning the air has cooled to its dew point. But 100% humidity doesn't always bring fog, and in very polluted air fog can form at around 95%.
Most foggy mornings are radiation fog. On a clear, calm night the ground loses heat to space, the air above it cools to its dew point, and fog forms. It's often thickest around dawn, when the air is coldest, and in valleys where cold air pools.
Both are tiny water droplets in the air. It's called fog when you can't see farther than 1 kilometer (about 0.6 miles), and mist when you can see farther than that.
After sunrise, the Sun warms the ground and the ground warms the air. Warmer air can hold more water vapor, so the fog droplets evaporate. People say the fog 'burns off', but nothing actually burns. Under a cloudy sky the air warms slowly, and fog can last all day.
Clean fog is just tiny drops of water. But fog can trap pollution close to the ground, and in the past, fog mixed with coal smoke was deadly: London's Great Fog of 1952 killed thousands of people. The biggest everyday danger of fog is poor visibility on roads.