Fog, mist and obstructions to visibility

Every fog is the same event — air cooled to its dewpoint, or moistened until its dewpoint reaches it. What differs is the mechanism, and the mechanism is what tells you whether it will burn off by ten or sit there all day.

Weather Information — FI.III.C · CA.I.C · PA.I.C

The one number to watch

Temperature and dewpoint converging is the whole story. A METAR reporting 17/17 is telling you the air is saturated — the only reason you can see anything at all is that the moisture has not condensed yet. A spread of 1 °C or less before dawn is a field that is going to be in fog.

Below are the US stations sitting closest to their dewpoint right now. Read the reports: some of them will already be in BR or FG, and the ones that are not are the interesting cases — ask yourself what is keeping them clear, and which of the mechanisms below is missing.

The seven mechanisms

Radiation fog

The ground radiates its heat away on a clear night, cools the shallow layer of moist air sitting on it below its dewpoint, and fog forms. It needs three things together: a moist surface layer under drier air, clear skies, and light wind. Calm is not ideal — a light wind of a few knots stirs the layer just enough to deepen the fog rather than laying dew. Shallow ground fog usually burns off quickly after sunrise; a deeper radiation fog generally clears before noon.

Valley and mountain fog

The same overnight cooling, but the terrain does the collecting: denser cold air drains off the high ground and pools in the valley, where it saturates. Most common in spring and autumn, and at its densest around sunrise — which is exactly when a lot of cross-countries are being planned to depart.

Advection fog

Moist air moves horizontally over a colder surface and is chilled below its dewpoint. Unlike radiation fog this one does not need a clear calm night and does not care what time it is. It is a coastal specialty, but it travels — advection fog can push a long way inland and stay for days as long as the flow keeps feeding it.

Upslope fog

Moist, stable air pushed up rising terrain cools adiabatically to its dewpoint. It wants wind, not calm — roughly 5 to 15 knots — because something has to keep driving the air uphill. Classic on the eastern slopes of the Rockies, and less frequent east of the Appalachians. It will not burn off with the sun; it stops when the upslope flow stops.

Frontal (precipitation-induced) fog

Warm, moist air is lifted over a front and rains into the colder air beneath. The falling precipitation evaporates into that cold air until it saturates, and fog forms from below. Because it follows the frontal zone, it can lie in a continuous band for hundreds of miles — the one fog you can meet en route rather than at the destination.

Steam fog

Very cold air moves over relatively warm water. Water evaporates into the cold air, immediately re-condenses, and rises in streamers that look like steam off a kettle. It is usually shallow — and because the air is being heated from below, expect convective turbulence flying through it. That is the part most people forget: steam fog is a turbulence cue, not just a visibility one.

Freezing fog

Fog whose droplets are supercooled liquid at or below freezing. They freeze on contact with anything they touch, which means the airframe as well as the runway. On a METAR it is FZFG, and it is a structural icing report as much as a visibility one.

Fog, mist or haze?

The distinction is a visibility threshold, and it is worth being exact because the METAR abbreviation changes with it.

BR is the abbreviation people stumble over in an oral. It is from the French brume, and it means mist — the intermediate state between fog and haze, not "brief" or "broken".

What this changes in a briefing

Knowing the mechanism turns the forecast into a prediction you can check. A destination forecast to go IFR in BR at dawn is a radiation-fog call, and the thing to verify is whether the sky is actually clearing overnight — if a deck moves in, the ground never radiates and the fog never forms. An advection fog forecast is a wind-direction call instead: watch for the flow backing off the water.

The same reasoning tells you what to expect on arrival. Radiation and valley fog lift with the sun. Upslope and advection fog do not, and will still be there at noon.

Keep going

• How to read a METAR — with a live report to practise on
• VFR, MVFR, IFR and LIFR
• How to read a TAF
• Cloud base calculator — the same spread, turned into a height
• Live visualizations

Sources

Formation mechanisms and the visibility thresholds are from the FAA Aviation Weather Handbook (FAA‑H‑8083‑28); reporting practice is in the AIM, chapter 7.