Fog vs Mist: The Whole Difference Is 5/8 of a Mile

Fog and mist are the same thing: tiny water droplets hanging in the air at ground level. The official dividing line is how far you can see. In a US weather report, visibility under 5/8 mile (1 km) is fog, and anything from 5/8 mile out to 7 miles (11 km) is mist.

Mist is also far more common. At three coastal US airports, San Francisco among them, weather stations logged mist 2 to 11 times as often as fog in 2025.

We compared the two as of October 2026 using the World Meteorological Organization’s definitions, the FAA’s Aviation Weather Handbook, the National Weather Service glossary and a full year of hourly airport weather reports.

Visibility: where mist becomes fog

The World Meteorological Organization draws the line at 0.62 mile (1 km). Its International Cloud Atlas says “fog” is used when droplets cut horizontal visibility at the surface to less than 1 km, and “mist” when they do not. It adds that “in practice, mist is considered synonymous with ‘light fog’.”

The United States uses the same line in its own unit. The FAA’s Aviation Weather Handbook defines fog as droplets that reduce visibility “to less than 5/8 SM (1 km)”, and mist as reducing it to less than 7 statute miles but at least 5/8 mile.

Automated stations make the line even sharper. In the 2025 reports we counted, every fog hour read 1/2 mile or less and every mist hour 3/4 mile or more, because their reports step straight from 1/2 to 3/4 mile.

Public forecasts care about thicker fog. The UK Met Office says public forecasts generally call it fog below about 200 yards (180 m), and dense fog below about 110 yards (100 m). The National Weather Service says fog “is often hazardous when the visibility is reduced to 1/4 mile or less.”

Takeaway: for pilots and weather reports, fog means you can see less than about 5/8 mile; a road forecast may wait until it is much thicker.

How they form, and how much moisture each needs

Both form the same way. The FAA handbook says fog appears when the temperature and dewpoint “become identical (or nearly so)”, either by cooling the air or by adding moisture, and that it “seldom forms” when the spread between them is more than 4°F (2°C).

Mist is the slightly drier stage. The same handbook gives it a relative humidity of 95 to 99% and calls it “an intermediate between fog and haze,” while adding that “there is no distinct line between any of these categories.”

Haze is the one that is genuinely different: a suspension of particles too small to see, not water droplets. Automated stations split mist from haze by the same spread, reporting mist when temperature and dewpoint are within 4°F (about 2°C) and haze otherwise.

The fog itself comes in types named after what cooled or moistened the air: radiation, advection, upslope, frontal and steam fog. Our guide to the types of fog in aviation covers each one and where pilots meet it.

Takeaway: fog and mist are one process at two densities, while haze is dry particles and drizzle is water that falls.

How they look, and how long they last

Fog “forms a whitish veil that covers the landscape,” says the WMO, and the air in it “usually feels damp, moist or wet.” When lit, individual fog droplets are often visible to the naked eye, moving in a turbulent way.

Mist is thinner. The FAA describes “a thin grayish veil that covers the landscape,” similar to fog but less of a block to vision.

Mist also clears faster. The Met Office says it “typically dissipates more quickly than fog and can disappear rapidly with even slight winds.” BBC Weather adds that it can thicken overnight into fog once visibility drops below 0.62 mile (1,000 m).

Takeaway: fog is white and lingers, mist is gray and goes with the first breeze.

How a weather report codes them

In a METAR, the hourly airport weather report, fog is FG and mist is BR. The odd abbreviation comes from the French word for mist, brume.

Only fog gets qualifiers. The FAA handbook lists shallow (MIFG), patches (BCFG) and partial (PRFG) as descriptors coded only with fog, and states flatly: “Mist (BR) is not coded with any descriptor.” Freezing fog (FZFG) is reported when the temperature is below 32°F (0°C).

Forecasts follow the same rule. In a TAF, a forecast visibility from 5/8 to 6 miles is coded BR and anything under 5/8 mile FG, “FZBR is not a valid significant weather combination,” and mist is never forecast with visibility above 6 miles.

Abroad, the mist band is narrower. New Zealand’s Civil Aviation Authority, following the international code, uses BR only for visibility between 0.62 and 3.1 miles (1,000 to 5,000 m). A US station can report mist out to 7 miles.

Takeaway: FG and BR mean the same weather in every METAR, but a US report will call it mist at visibilities a European one would leave blank.

What each means for flying

Aviation weather maps color airports by flight category. The FAA handbook sets the visibility bands: LIFR below 1 mile, IFR from 1 to under 3 miles, MVFR from 3 to 5 miles and VFR above 5 miles.

Fog always lands in the worst band, because 5/8 mile is already under 1 mile. Mist can land in any of the four. Of the 2,618 mist hours at our three airports in 2025, 7.6% were LIFR on visibility alone, 30.9% IFR, 42.8% MVFR and 18.7% VFR.

That matters for visual pilots. Under 14 CFR 91.155, flying under visual flight rules below 10,000 feet in Class B, C, D or E airspace needs 3 miles of visibility, so roughly 4 in 10 of those mist hours reported visibility below that.

Fog shuts that door almost entirely. Even special VFR, the clearance that lets a pilot operate near a controlled airport below normal VFR minimums, needs at least 1 mile of ground visibility for an airplane to take off or land under 91.157.

At 1 mile or less, the METAR also starts reporting runway visual range where the airport has the equipment: the instrument-measured distance a pilot can see down the runway.

Takeaway: fog always means instrument conditions, while mist ranges from a legal VFR day to low IFR.

How often each one happens

We counted every routine hourly METAR for 2025 at three coastal airports. Arcata-Eureka in Northern California reported mist in 1,494 hours and fog in 324. Nantucket logged 952 hours of mist and 428 of fog, the most fog of the three.

When fog did form, it was usually thick. In 57% of the fog hours at the three airports, visibility was 1/4 mile or less, the level at which the National Weather Service issues a Dense Fog Advisory when it covers a wide area.

San Francisco International logged the fewest hours of either: 172 of mist and 16 of fog, or 10.8 mist hours for every hour of fog.

Takeaway: mist is the everyday condition and fog the rarer, thicker one, even on coasts famous for fog.

The difference in one sentence

Fog and mist are the same suspended water droplets, and a weather report calls it fog when you can see less than 5/8 mile (1 km) and mist when you can see farther, up to 7 miles in the US or about 3 miles abroad.