Fog: The Five Types and When They Burn Off

Fog is a cloud sitting on the ground. The physics is identical to the stratus deck at 800 ft; the only difference is that the base has reached the surface, and now the visibility it steals is the visibility you land in. Every type of fog comes down to the same event: air cooled to its dew point, or moistened up to its temperature, until water vapor condenses into droplets you can no longer see through.

What separates the five classic types is how the air and the dew point meet. That mechanism decides where the fog lives and when it forms. It also decides the part that catches pilots: whether the morning sun will clear it on schedule or leave it sitting past your planned departure. Get the mechanism right and the forecast stops being a guess. This post is one hazard inside the larger picture of the weather that hurts GA pilots; fog earns its own treatment because it is the one that most often turns a legal-looking morning into a scrubbed one.

Fog or mist? The visibility line

Before the types, the vocabulary, because a METAR draws a hard line here. Fog is coded FG and reported when surface visibility drops below 5/8 statute mile. Above that, from 5/8 up to 6 statute miles, the same suspended droplets are coded BR, mist. Same water, same gray, different token, and the token matters: FG in an observation means visibility is already below the threshold where most VFR flying stays comfortable, while BR is the warning shot.

The code carries qualifiers worth knowing. MIFG is shallow fog, lying below eye level so the tower still reports decent visibility while the runway lights vanish. BCFG is patches. FZFG is freezing fog, droplets that freeze on contact and glaze the airframe and the runway at once. PRFG is partial fog, covering a chunk of the field but not all of it. When any of these show up, the plain-English reading of the observation matters more than the raw string. Paste it into our METAR decoder if a group stops you, and see how fog reads in a METAR versus a TAF for how the same conditions get observed and forecast.

The five types

Radiation fog

The classic. On a clear, calm night the ground radiates its heat to space, cools, and chills the shallow layer of air resting on it. If that air is moist enough and the temperature drops to the dew point, fog forms, almost exclusively at night or near daybreak, when cooling has run longest. The recipe is clear sky, light or no wind, and a small temperature–dew point spread.

Radiation fog is a low-ground creature. It pools in valleys and river bottoms because the cold, dense foggy air drains downhill and collects there, which is why one airport can be socked in while a field 15 NM away on higher terrain is clear. It also thickens toward sunrise, so the fog is often at its worst right when the day’s first departures are rolling. The saving grace: it usually burns off within a few hours of sunrise as the sun warms the ground, the ground warms the air, and the droplets evaporate. Usually. The burn-off is the trap, and it gets its own section below.

Advection fog

Advection fog forms when warm, moist air moves horizontally over a colder surface. The moving air loses heat to the ground or water beneath it and cools to its dew point. It needs wind, unlike radiation fog: a light breeze, roughly up to 15 knots, keeps the moist air feeding across the cold surface and actually deepens the fog rather than clearing it. Above that, the wind tends to lift the layer into low stratus.

This is the coastal fog of the California and New England shorelines, and the sea-fog that rolls inland on an onshore flow. The critical difference from radiation fog: advection fog does not burn off with morning sun alone. The heat source that formed it is elsewhere, and it will sit over the cold surface, day or night, until the wind shifts, the air mass changes, or the surface warms enough to break the balance. A coastal field that clears at noon does so because the flow changed, not because the clock struck a burn-off hour. Plan around the wind and the air mass, not the sunrise.

Upslope fog

Upslope fog forms when wind pushes moist air up rising terrain. As the air climbs it cools adiabatically, and if it reaches its dew point on the way up, fog forms against the slope. The Great Plains rising toward the Rockies is the textbook setting: an easterly upslope flow can bury fields along the Front Range while the plains behind them stay clear.

Like advection fog, upslope fog needs wind to sustain it, and like advection fog it will not obey a burn-off schedule; it persists as long as the upslope flow feeds it. Kill the wind or reverse it to downslope and the fog dissipates, because downslope air warms and dries as it descends.

Steam fog

Steam fog forms when cold, dry air moves over much warmer water. Moisture evaporates off the warm surface into the cold air, saturates it, and condenses into fog that appears to rise off the water like steam off a mug. You see it over lakes and rivers on a cold autumn morning and over open water when an arctic air mass arrives, where it is called sea smoke.

Steam fog is usually shallow and thin, more a curiosity than a stopper, but it carries two teeth for pilots. The evaporation and condensation drive low-level turbulence in the layer, so the air over that “picturesque” river fog is working. And when the water droplets freeze, likely given the cold air that made it, you have freezing fog and its icing.

Precipitation (frontal) fog

The fifth type moistens the air from above instead of cooling it from below. Warm rain falling through the cooler air beneath a warm front evaporates as it falls, raising the dew point of that lower air until it saturates and fogs. Because it lives along the warm front’s shallow frontal surface, it is also called frontal fog, and it can cover a wide area for a long time, as long as the rain keeps falling and the front keeps stalling.

This is the fog that makes a rainy, low-ceiling frontal passage worse than the ceiling alone suggested. You are already flying reduced visibility in precipitation; frontal fog drops it further and holds it there. It will not burn off on any daytime schedule, because the front, not the sun, controls it.

Reading the setup before it forms

Fog does not usually ambush an attentive pilot. The two cues are in the numbers you already pull.

The first is the temperature–dew point spread. As the spread closes toward zero, the air is approaching saturation; when it reaches zero, the air is saturated and fog or low stratus is on the table. Watch the trend across the last few hourly METARs, not just the latest one. A spread stepping from 4°C to 2°C to 1°C over three hours on a clear, calm evening is radiation fog announcing itself hours before it forms. This is the same trend-reading habit that serves you on any night flight, where the ground cools fast after sunset and the spread can close in a hurry once the sun is gone.

The second is wind. Light or calm favors radiation fog; a steady breeze off cold water or up a slope favors advection or upslope fog; the wind direction tells you which coast, which slope, which air mass is feeding it. Pair the wind with the spread and you can usually name the fog you are about to get, which tells you how it will behave.

The burn-off trap

Two mistakes recur, and both come from treating fog as a single phenomenon with a single clock.

The first is assuming the burn-off happens on schedule. Only radiation fog reliably clears with morning sun, and even it depends on how thick the layer got overnight, how much moisture is in it, and whether high cloud moves in and robs the sun of the heating it needs. A deep radiation fog on a humid morning can hold past noon. Pinning a departure to “it’ll burn off by 9” and launching a passenger into a marginal field on that promise is how a comfortable plan becomes a hold, a divert, or worse. Watch the actual observations clear before you commit, not the hour you expected them to.

The second is assuming any fog will burn off. Advection, upslope, and frontal fog are governed by wind and air mass, not by the sun, and a pilot who learned “fog burns off in the morning” from a radiation-fog childhood will wait all day at a coastal field for a burn-off that was never coming. The right question is never “when does fog lift?” It is “what type is this, and what does this type need in order to clear?”

And the mirror-image error: departing early to beat fog that has not formed yet. On a clear, calm, high-humidity evening with the spread closing, the field that is severe-clear at dusk can be zero-zero by 0600Z. Getting airborne ahead of it is fine; planning to return to it three hours later on the assumption it is still clear is not. Check the destination and the departure field for the whole window you will be flying, and give the temperature–dew point spread the same weight you give the ceiling.

Fog is the most predictable hazard on the board and the one most often underestimated, precisely because it looks benign until the moment it isn’t. Name the type, watch the spread and the wind, and let the observations, not the schedule, tell you when it’s gone.

Common questions

What is the difference between FG and BR in a METAR?

Visibility. The same suspended droplets are coded FG (fog) below 5/8 statute mile and BR (mist) from 5/8 up to 6 statute miles. Qualifiers narrow it further: MIFG is shallow fog lying below eye level, BCFG is patches, PRFG covers part of the field, and FZFG is freezing fog that glazes the airframe and the runway at once.

Does all fog burn off in the morning?

No, and assuming it does is the classic trap. Only radiation fog reliably clears with morning sun, and even then a deep layer on a humid morning can hold past noon. Advection, upslope and frontal fog are driven by wind and air mass rather than heating, so a coastal field clears when the flow shifts, not at a burn-off hour.

How do I see fog coming before it forms?

Watch the temperature and dew point spread across the last few hourly METARs rather than just the latest one, and read it alongside the wind. A spread stepping from 4 °C to 2 °C to 1 °C on a clear, calm evening is radiation fog announcing itself hours ahead. Light or calm wind favors radiation fog; a steady breeze off cold water or up a slope favors advection or upslope fog.