Map of morning fog You cannot see further.

1 · NEAR EDGEEverything is visible, including the reference post.

2 · MIDDLE OF THE VALLEYThe post is barely visible, the trees are blobs.

3 · FAR SLOPENothing. No view does not mean no slope.

Original drawing (diagram), not a photograph and not a measurement. Three depths of a single invented morning with the same reference post; no distance scale.

This view holds no answer to the question of where the fog came from. The same white wall can result from the ground cooling overnight, from an inflow of moist air or from warm water.

Fog in meteorology: visibility < 1000 m. (Definition carried over from V2, to be checked against the IOŚ-PIB source.)

See what might have happened

1 · InterpretationsThe same view, two or three possible stories

Choose a practice morning. Below, each hypothesis has its own cross-section and a list of the data it needs. The cross-section appears only as an aid for one hypothesis at a time.

Morning A · Basin after a still night

A practice scenario, not a field report. A shallow basin surrounded by slopes. From its edge the nearby trees are clear, the middle of the basin is milky, and the opposite slope cannot be seen.

Assumed in the scenario: A basin. No open water in the area. Haze described by eye (visibility was not measured).
Not known: Wind, cloud cover at night, temperatures on the floor and on the slope, humidity, thickness of the layer.
In play: The radiation and advection hypotheses. Evaporation from water is outside this morning: there is no water.

Morning B · Lake shore at dawn

A second practice scenario, also invented. The shore of a lake in a valley. Reeds are visible near the shore, pale bands hang over the surface, and the far shore cannot be seen.

Assumed in the scenario: A lake in a valley. Haze described by eye (visibility was not measured).
Not known: Temperature of the water and of the air above it, wind, cloud at night, humidity, extent of the fog beyond the water.
In play: All three hypotheses. The water is an assumption of the scenario, not a documented fact.

Radiation hypothesis

Under a clear sky, heat escapes upwards from the ground (dashed arrows).
Cooler, denser air flows down the slopes to the floor.
The fog layer has a fairly sharp upper ceiling, with clearer air above it.
Thermometer near the floor.
Thermometer on the slope, a few dozen metres higher.

Data needed: cloud cover at night, wind near the ground, floor–slope temperature difference, humidity. The view from a distance alone does not settle it, because the ceiling says nothing about temperatures.

Advection hypothesis

Warmer, moist air flows in (arrows in the warmth colour).
The cooler surface cools it from below.
The layer is thick, grows towards the far side and moves with the wind (black arrow), not only in a hollow.
Clouds need not get in the way, because the fog does not depend on heat radiating away.

Data needed: wind direction and steadiness, temperature of the surface and of the incoming air (the workbench below does not model this measurement), extent of the fog. The view alone does not settle it, because a thick white layer can also be radiation fog.

Evaporation hypothesis

The water (blue, with white arrows of heat) is warmer than the air just above it.
Cooler air flows in over the water (black arrow).
The moisture condenses in vertical wisps and a low layer just above the surface.
Outside the water zone (the area in the dashed frame) the air is clear.

Data needed: temperature of the water and of the air just above it, wind, extent of the fog. The water in this diagram is an assumption of scenario B, not a documented water body. The diagram shows no ice and does not suggest going out onto ice or over water.

Colours in the drawings: water (scenario assumption), heat, fog. The drawings are original diagrams without a scale, not maps or measurements. The numbers correspond to the lists alongside.

2 · What is missingChoose the measurement you do not have

For each missing measurement the page shows what would happen to the three hypotheses under every possible result, and which hypotheses it would really separate. There is no probability of fog here, and no certainty.A model, not a measurement. The rules and thresholds are a simplified practice model carried over from V2, not checked by a meteorologist; the results are states within this model, not a measurement. Details in the Limits section.

The same table as text (works without JavaScript). With JavaScript enabled, an interactive workbench with a summary of the hypotheses appears below. The water measurement applies only to morning B.

Wind near the ground

Is the air near the ground still or moving, and how strongly?

An anemometer, an estimate from trees and smoke, or station data (state the distance from the place where the fog is).

Calm or a light breeze (< 2 m/s)Radiation: consistent · Advection: weakened · Evaporation: consistent.Cold air can settle on the floor. A steady inflow from outside is unlikely.
Steady wind, 2–5 m/sRadiation: weakened · Advection: consistent · Evaporation: consistent.A light, steady wind mixes the layer near the ground, so radiation fog weakens, but it can carry moist air.
Strong wind (> 5 m/s)Radiation: ruled out · Advection: weakened · Evaporation: weakened.In the rules of the exercise, strong wind breaks up the cold layer near the ground. Poor visibility in such wind is sometimes low cloud or precipitation rather than fog; that is a different question.

Separates strongly: Radiation ↔ Advection, Radiation ↔ Evaporation. Separates weakly: Advection ↔ Evaporation.

Floor–slope temperature difference

Is it cooler near the floor than a few dozen metres higher up the slope?

Two thermometers read at the same time, in the shade. In fog, stay on safe, familiar paths; this is no reason to go into difficult terrain. The ±1°C limit is a conventional assumption of the exercise, not a meteorological rule; the three results cover the whole scale (cooler, almost no difference, warmer).

Floor cooler by more than 1°C (e.g. a few °C)Radiation: consistent · Advection: weakened · Evaporation: unaffected.A layer of cold near the ground fits night-time cooling. In the rules of the exercise, with advection the floor and the slope have similar temperatures.
Almost no difference (within ±1°C)Radiation: weakened · Advection: consistent · Evaporation: unaffected.The absence of a cold layer weakens radiation fog. This measurement does not concern evaporation from water.
Floor warmer than the slope by more than 1°CRadiation: weakened · Advection: weakened · Evaporation: unaffected.This is the reverse of a cold layer near the ground, so radiation fog weakens. In the rules of the exercise, advection assumes similar floor and slope temperatures, so this reading does not favour it either; it is worth checking that both thermometers were read in the shade and at the same time.

Separates strongly: none. Separates weakly: Radiation ↔ Advection, Radiation ↔ Evaporation, Advection ↔ Evaporation.

Cloud cover at night

Was the sky open to the radiation of heat?

An observation of the night sky or station data (state the distance and the difference in height).

Clear (0–2/8)Radiation: consistent · Advection: unaffected · Evaporation: unaffected.Heat escapes from the ground into space; this is the condition for radiation fog. This result does not concern advection or evaporation.
Partial (3–5/8)Radiation: weakened · Advection: unaffected · Evaporation: unaffected.Clouds hold back part of the cooling.
Overcast (6–8/8)Radiation: ruled out · Advection: unaffected · Evaporation: unaffected.Clouds give heat back to the ground and block the cooling. Advection and evaporation can operate under cloud.

Separates strongly: Radiation ↔ Advection, Radiation ↔ Evaporation. Does not separate: Advection ↔ Evaporation.

Water versus air (morning B only)

Is the water warmer than the air just above it?

Two temperatures from the same time. Do not go out onto ice or over water to take a measurement; a reliable source from a safe shore, labelled with its distance, is enough.

Water warmer than the airRadiation: unaffected · Advection: unaffected · Evaporation: consistent.Moisture evaporates from warm water and condenses into wisps in the cool air. This does not rule out other mechanisms at the same time.
Similar temperaturesRadiation: unaffected · Advection: unaffected · Evaporation: weakened.Without a clear contrast, vapour from the water weakens.
Water cooler than the airRadiation: unaffected · Advection: unaffected · Evaporation: ruled out.Evaporation from the water then does not drive the fog. This result says nothing about radiation or advection.

Separates strongly: Radiation ↔ Evaporation, Advection ↔ Evaporation. Does not separate: Radiation ↔ Advection.

Extent and shape of the fog

Where does the fog end: over the hollow, only over the water, or does it stretch wider?

An observation from a safe, familiar viewpoint, or several observations from different places.

Lies in the hollow, sharp upper ceiling, clear above itRadiation: consistent · Advection: weakened · Evaporation: weakened.A shallow layer lying in a hollow fits the settling of cold air.
Only just above the surface, in vertical wisps; clear a few dozen metres from the shoreRadiation: ruled out · Advection: weakened · Evaporation: consistent.Fog tied to the water rather than to the whole hollow points to the water as the source of moisture.
Extensive, thick layer, moving with the windRadiation: weakened · Advection: consistent · Evaporation: weakened.A thick, moving layer fits the transport of air masses better than local settling.

Morning B: separates strongly Radiation ↔ Advection, Radiation ↔ Evaporation; weakly Advection ↔ Evaporation. Morning A: weakly Radiation ↔ Advection.

Humidity: the difference T – Td

How close to saturation is the air where the fog is?

A hygrometer or station data. The threshold of ≤ 1°C is a conventional assumption of this exercise, not a universal rule. The ranges meet: up to and including 1°C, above 1 up to and including 4°C, above 4°C; every reading fits one result.

T – Td ≤ 1°CRadiation: consistent · Advection: consistent · Evaporation: consistent.Air close to saturation is needed by every hypothesis.
T – Td above 1 up to 4°CRadiation: weakened · Advection: weakened · Evaporation: weakened.It still needs clear cooling.
T – Td > 4°CRadiation: ruled out · Advection: ruled out · Evaporation: ruled out.With visible fog, such a reading more likely calls for checking the measurement (whether it comes from this place; the station may be far away). For vapour above warm water this simplification needs verification.

Separates no pair. It only checks whether the air is close to saturation at all.

3 · LimitsWhat this page does and what it does not

This is an exercise in reasoning, not a forecasting or diagnostic tool.

How to read the results

There is no probability of fog here, and no “certainty”. A result is the state of a hypothesis under simplified rules: consistent, weakened, ruled out, or open.
“Ruled out” means that, in the rules of the exercise, the result does not fit the hypothesis. In the field, mechanisms can overlap.
A missing measurement means “open”, not “zero”. The workbench considers the results you record; it does not fill in the rest.
A single remaining hypothesis is not proof. The mechanism of a fog cannot be established with certainty from a photograph or a few numbers.

What is a model and what is not

Both mornings are invented for the purposes of the exercise. They are not field reports and have no witnesses or measurements.
The rules (clouds block radiation fog, strong wind breaks up the cold layer, the threshold T – Td ≤ 1°C, the contrast between water and air) come from V2 and have not been checked by a meteorologist. Treat the threshold figures as examples.
The drawings are diagrams without a scale. They are not a map or a measurement.
A measurement from a station other than the place where the fog is, is a different measurement. A station often lies on a plateau or at an airport, so it may show different temperatures from the floor of a basin. Record its distance and difference in height.
The page does not point to viewpoints, does not assess the safety of driving or going out, and does not encourage going out onto ice or over water. Check the forecast and warnings before setting out; in fog, do not stop on the roadway.

What remains from V2 and what was removed

Kept: the dependencies between rules, the distinction between “not known” and zero, the note on how representative a station is, the texts on valleys and water.
Removed: the local register of observations and the comparison of two outings (saving in the browser is not needed here), export to the clipboard, six manual parameters with an automatic assessment of your own morning, SVG layer switches, patterns A–E.
Replaced by a simpler approach: two worked mornings instead of a parameter workbench, while patterns D (wind) and E (cloud) became measurement results in the workbench.
The photograph from V2 is not embedded; there is only a link in the sources.

< 1000 mThe visibility threshold below which haze is called fog. Carried over from V2, to be checked in the IOŚ-PIB study.

40–77 daysThe range of the average annual number of days with fog at Polish stations, 1973–2020, according to V2. This is climatic background, not a probability for a single morning. To be checked in the source.

4 · SourcesWhere these rules come from and what needs checking

Links carried over from V2. They were not reopened while this version was being built, so we do not declare them checked. They are not dependencies of the page, only links.

Met Office: What is fog?Types of fog, valleys, the effect of night-time conditions. To be checked: which rules of the exercise follow from this.
National Weather Service: FogCondensation, cooling of the air, supply of moisture. To be checked.
IOŚ-PIB: Mgła w Polsce w latach 1973–2020 [Fog in Poland, 1973–2020]Definition, historical data and variation. The source of the figures 1000 m and 40–77 days; to be checked.
Atlas klimatu Polski 1991–2020 [Climate Atlas of Poland 1991–2020]A map of the average annual number of days with fog. To be checked.
Wikimedia Commons: “Morning fog in Upper Silesia” (photograph from V2)A link only; the photograph is not embedded in this version. Licence according to V2: CC BY-SA 4.0, to be checked before any use. The photograph illustrates the phenomenon; it does not settle its mechanism.

Missing: a meteorological consultation to confirm the thresholds and which hypotheses really remain open given data from the field; a series of original photographs with reference points and the labelled distance of the station.