BR vs FG: Mist or Fog, and the 5/8 Mile Line Between Them
BR vs FG looks like a judgement call about how thick the air is. It is not. It is a single number, and once you know the number you can predict which code a station will use before it sends the report.
Both codes describe the same thing physically: water droplets held in the air near the ground. What separates them is how far you can see through them.
The BR vs FG threshold
The line is five eighths of a statute mile.
At five eighths of a mile or more, and below seven miles, the code is BR, and
BR means mist. Below five eighths of a mile, the code is FG, and FG means
fog. There is no overlap and no observer discretion.
Five eighths is an odd number until you see where it comes from. It is 1000 metres, converted. The threshold was set in metric and the US inherited it, so it arrives in statute miles as an awkward fraction.
Above seven miles, neither code appears. The air may still hold moisture, but the station has nothing to report.
Why the difference matters on approach
Mist and fog sit either side of most approach minimums, which is exactly why the codes are split there.
Five eighths of a mile is 3300 feet of runway visual range, near enough. A
standard Category I ILS wants around 1800 to 2400 feet RVR. So a field reporting
BR is usually workable and a field reporting FG is usually not, without
lower minimums and the equipment to use them.
That makes the two codes a fast first filter. Read FG in a terminal forecast
and start thinking about the alternate. Read BR and check the trend instead.
The fog variants worth knowing
Fog rarely arrives as plain FG. Four qualifiers change what it means for you.
MIFG is shallow fog, under six feet thick. You can see over the top of it from
the flight deck and see nothing at all on the runway. It is the one that fools
people, because the tower reports good visibility while the touchdown zone is
buried.
BCFG is fog patches. Part of the field is in it and part is clear, so
conditions can change between the approach and the rollout.
PRFG is partial fog, where a substantial part of the aerodrome is covered but
not all of it. FZFG is freezing fog, which adds airframe and runway ice to the
visibility problem.
What the station is actually measuring
Neither code is a guess. Both come from a visibility sensor, and knowing how it works tells you when to doubt it.
An automated station measures visibility at one point, usually beside the runway. It fires a beam across a short gap and counts how much light scatters back off the droplets. More scatter means less visibility.
That gives an honest number for the air in front of the sensor. It says nothing
about the air half a mile away. Fog is patchy by nature, so a station can report
BR while the far end of the field sits in FG.
A human observer scans the whole horizon instead and reports the prevailing value, meaning the figure covering at least half the circle. That is why an augmented report can differ from a purely automated one at the same field.
When the sky is obscured you will also see a vertical visibility group such as
VV002. That is the ceiling replaced, not the surface visibility, and the two
are measured separately.
How mist becomes fog overnight
The transition is driven by the gap between temperature and dew point. Watch that gap in the METAR and you can see it coming.
As the ground cools after sunset the air above it cools too. The dew point stays roughly flat. When the two numbers meet, the air cannot hold its moisture and it condenses.
A field reporting 10/09 at midnight with light wind and clear sky is telling
you a story. One degree of spread, falling temperature, nothing to mix the air.
That field will very likely be BR by 0400 and FG by dawn.
Verdict on BR vs FG
Treat BR vs FG as a visibility readout rather than a weather type. The droplets are the same. The number is what changed.
Mist at five eighths of a mile and fog at four eighths are one small step apart in the air and a large step apart in what you can legally do. Watch the temperature and dew point converge, and you will see which one is coming.
Frequently asked questions
What is the difference between BR and FG in a METAR?
Visibility. BR is mist, reported when visibility is five eighths of a statute mile or more but under seven miles. FG is fog, reported when visibility drops below five eighths of a mile. Both are water droplets in the air.
Why is mist coded BR and not MI?
BR comes from brume, the French word for mist. ICAO codes draw on French in several places. MI was already taken by shallow, as in MIFG, so the French abbreviation avoided a collision.
Can a METAR report both BR and FG?
Not for the same air. It can pair FG with a qualifier, such as BCFG for fog patches or MIFG for shallow fog, where fog sits in part of the field and the prevailing visibility is better.
Does BR always turn into FG?
No, but it often does around dawn. Mist thickening into fog is one of the most predictable transitions in aviation weather, driven by the temperature falling to meet the dew point overnight.
What is FZFG?
Freezing fog. The droplets are below zero and freeze on contact, so they build ice on the airframe and on the runway. It is fog with an icing hazard attached, and it is reported when the temperature is below freezing.