Fog formation & clearance

Why does radiation fog favour clear nights and sheltered low ground?

Follow the surface's energy loss and the depth through which the cooling spreads.

Build the layer

Radiation fog develops when the ground loses enough energy by terrestrial radiation to cool the adjacent moist air towards saturation. Clear skies, long nights, a moist surface layer and weak wind favour it. Light turbulence can spread the cooling through a shallow layer; perfectly calm air may initially favour surface dew and very shallow patches rather than a uniformly deep fog. Cold-air drainage can concentrate the cooled air in valleys and basins.

The layer is commonly associated with a surface inversion and can range from wisps only a few metres deep to a substantial layer hundreds of metres deep after prolonged favourable conditions. It is often patchy at first and locally deepest in sheltered low terrain. A ridge or higher observation point can remain clear above it while a nearby runway is obscured. Cloud cover arriving aloft may reduce further radiative cooling, but existing fog can persist if the layer remains saturated.

Remove the cold saturated layer

After sunrise, surface heating and mixing can raise the air temperature and evaporate droplets. Dry-air entrainment or a change to warmer, drier advection can also help. Stronger mixing sometimes lifts fog into low stratus before it dissipates, so improved surface visibility does not imply an adequate ceiling. Weak winter sunshine, persistent inversion, snow cover or deep fog can delay clearing well beyond sunrise or allow fog to last all day.

Why can fog persist through a windy afternoon?

Identify the cooling encountered by air moving horizontally.

The surface cools the arriving air

Advection fog forms when moist air moves over a sufficiently colder surface and is cooled from below towards its dew point. It can occur over cold sea water, snow-covered land or cold ground. Over sea, warm moist air crossing a cold current or upwelling region can build a broad fog bank. An onshore wind can then carry marine fog across a coastal aerodrome, while warm advection over a cold continental surface can produce an inland event.

Unlike the simplest radiation-fog pattern, the cooling source does not require night-time radiative loss. Fog can form or persist by day, and moderate wind can maintain the supply of warm moist air over the cold surface. It is often extensive and may be deeper and more persistent than an early shallow radiation-fog patch. An increase in wind does not automatically clear it; the result depends on whether mixing dries the layer or the wind continues feeding its cooling pathway.

Change the supply or the contrast

Dissipation is favoured when the air becomes drier, the surface warms enough to reduce cooling, the wind changes the source or trajectory, or mixing introduces sufficient unsaturated air. Some marine fog merely moves offshore or lifts into stratus rather than disappearing. Local sunshine at one station is weak evidence of clearing if the upstream sea remains cold and the onshore moisture supply continues.

Why does warm water appear to steam in a cold outbreak?

Do not confuse this warm-water pathway with warm air being cooled over cold water.

Mixing makes the vapour visible

Sea smoke, or steam fog, develops when cold air moves over much warmer water. Evaporation adds vapour to the near-surface air, and mixing with the cold air above can produce a saturated mixture and visible droplets. The vapour itself remains invisible; the apparent steam is condensed water. Open water in a very cold outbreak or a lake surrounded by cold land provides a common setting.

The large temperature contrast drives heat and moisture exchange, often giving shallow, irregular rising wisps with convective overturning near the water. A stable layer above can limit the depth or trap the moisture. The same physical process can occur over other warm wet surfaces. Stronger wind may enhance evaporation and transport, so its effect cannot be reduced to 'wind always removes fog'.

Remove the mixing conditions

The fog weakens if the cold air warms enough to remain unsaturated after mixing, the water–air contrast or evaporation supply decreases, or a flow change removes the favourable cold-air feed. Dispersion into a drier environment can also dissipate the droplets. Diagnose which side is warming or cooling: sea smoke forms from cold air over warm water, while classical marine advection fog involves warmer moist air over colder water.

How can frontal rain create fog ahead of the surface front?

Follow the falling water through the cooler layer below it.

Precipitation and evaporation

Relatively warm rain falling from a frontal cloud layer into colder unsaturated air can evaporate. The evaporation adds water vapour and absorbs latent energy, both helping the lower air approach saturation. A widespread fog or very low cloud layer can develop, commonly ahead of a warm front over a cold surface air mass. Moist advection and the pre-existing cold ground can reinforce the process.

Frontal fog can coexist with continuous rain, low stratus and poor visibility over a broad area rather than being limited to a calm clear night. It may persist while rain and the cold lower air continue supplying the necessary moisture and cooling. A forecast explanation should identify the source of precipitation and the cold layer; simply saying 'there is a front' does not describe why fog forms at the surface.

Clearing depends on the new air mass

Fog can dissipate when precipitation and the evaporation supply stop, a warmer or drier air mass replaces the saturated cold layer, or enough mixing and warming remove saturation. Frontal passage does not guarantee instant clearance: the new air mass may itself be moist and low cloud can remain. Compare the expected post-front temperature, dew point, wind and precipitation rather than assigning a universal clearing time to the front symbol.

Why can a valley look clear while the ridge is in fog?

Use the terrain-relative position of the cloud.

A cloud touching rising ground

Orographic or upslope fog occurs when moist air is forced up sloping terrain, expands, cools and reaches saturation in contact with the surface. The same cloud can be reported as stratus from a lower valley and be fog to an observer on the hillside. Its formation can continue while upslope wind and a suitable moisture supply persist, including during daylight and with appreciable wind.

The fog tends to obscure windward slopes, passes and ridges, with its extent governed by the incoming air's condensation level, flow, stability and terrain. A valley aerodrome's reported cloud base and visibility can therefore be unrepresentative of a mountain route. The visible valley floor does not establish a clear pass. Local lift and mixing can also produce rapid fluctuations along irregular terrain.

Weaken lift or make the air unsaturated

A drier air mass, weaker or reversed upslope flow, or sufficient warming can raise the condensation level and reduce terrain contact. Downward flow can warm by compression and evaporate cloud on the lee side. Sometimes the fog lifts from one slope but remains as a cloud layer above it, leaving other terrain obscured. Assess the route and terrain heights, not just whether one observation point has become clearer.

Which forecast change would actually clear this fog?

Test a causal explanation against the upstream surface and air mass.

Two apparently similar low-visibility mornings

Airport A has a clear overnight sky, moist ground, a weak wind and a shallow surface inversion. Airport B has a persistent onshore flow crossing cold sea water beneath moist warmer air. Both can have fog near dawn, but the maintenance mechanisms differ. At A, surface heating and dry-air mixing may erode the cold layer. At B, the onshore supply can replace fog as quickly as local heating removes droplets.

A credible improvement forecast names a change capable of removing saturation through the relevant depth: increased net heating, dry advection, a warmer surface, a changed wind trajectory or weaker upslope lift. It also distinguishes improved horizontal visibility from a lifted but still low stratus ceiling. A forecast of 'fog lifting' needs interpretation: lifting the base above the ground may leave operationally significant low cloud.

Use the formation type as a reasoning aid rather than assuming every event belongs to exactly one mechanism. Radiation, advection, precipitation and terrain can cooperate. Updated observations and the layer profile test whether the expected change has started; a textbook clearing tendency alone does not establish that current conditions meet a particular operation's requirements.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.