Fog forms when air near the ground becomes saturated and tiny water droplets remain suspended in it. Cooling air toward its dew point is one route; adding moisture or mixing different air masses can also help. Fog is essentially a cloud at the surface, so it reduces visibility by scattering light between you and distant objects.
Invisible vapor becomes visible droplets
Air contains water vapor that you cannot normally see. When conditions favor condensation, some of that vapor becomes small liquid droplets. A mass of those droplets makes the familiar pale layer that hides trees, hills or buildings. The droplets are small enough to remain suspended rather than immediately falling as ordinary rain.
Dew and fog share a connection with saturation, but appear in different places. Dew is liquid water forming on a surface. Fog consists of droplets suspended in air. A damp lawn does not automatically imply fog, and a foggy coastline does not require every nearby surface to have cooled overnight.
Radiation fog grows as the ground loses heat
After sunset, the ground can lose heat by emitting infrared radiation. The air close to it cools in turn. When enough moisture is present and the air reaches saturation, radiation fog can develop. Clear skies and light winds commonly favor this process, although the details vary with terrain and the weather pattern.
Valleys often collect cool, dense air draining from higher ground. That helps explain why a low field may disappear into fog while a nearby ridge remains clear. The term radiation here refers to ordinary heat transfer, not contamination or a special hazardous source.
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Advection fog forms when moist air crosses a colder surface
Advection means horizontal movement of air. Moist air moving over cooler water or land can cool to saturation and form fog. Coastal fog is a familiar setting for this process. The same general idea can apply when air moves over a cold snow-covered surface.
Unlike a shallow patch that forms overnight in a sheltered field, an advecting fog bank can keep arriving with the wind. Sunshine does not guarantee rapid clearing when the supply of cool, moist air continues. A photograph of fog at a bridge shows the visible conditions, but cannot by itself prove the exact temperature history of that particular bank.
Moisture and mixing create other kinds of fog
Evaporation from relatively warm water into cold air can create wisps of steam-like fog. The visible wisps are droplets, not invisible water vapor. Rain falling into cooler air can also add moisture through evaporation and help create fog near fronts.
Upslope movement provides another cooling route as air rises along terrain. These labels are useful explanations rather than completely sealed categories. More than one mechanism can contribute to a real fog event, and local observations are needed to work out which dominates.
Freezing fog is different from ice fog
Fog droplets can remain liquid below freezing. When those supercooled droplets encounter a suitable cold surface, they can freeze and deposit ice: this is freezing fog. Ice fog consists of small ice crystals suspended in very cold air. The similar names do not mean the particles are identical.
For a traveler, reduced visibility may be accompanied by slippery surfaces. Do not assume a foggy road is merely wet because no snow is falling. Pay attention to local forecasts and warnings, particularly around bridges, exposed paths and elevated areas.
Why fog clears, and how to observe it safely
Fog can thin when warming lowers relative humidity, when drier air mixes into the layer or when wind replaces the foggy air. It may appear to lift into low cloud instead of simply vanishing. The everyday phrase burns off describes an outcome, not literal burning.
From a safe stationary location, compare a nearby landmark with a more distant one and note how visibility changes over time. If driving is necessary, follow official local advice, slow down, leave extra space and use appropriate low-beam lighting rather than high beams that reflect from droplets. Avoid stopping in a travel lane to photograph the view.
Explore related questions
10 Types of Clouds and How to Identify Them
Landscape Photography: Build Depth and Handle Bright Skies
Sources and further reading
National Weather Service: how fog forms
National Weather Service: radiation fog
National Weather Service: fog types
Optional tools for learning more
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