Storm Station

The Field Book/Vol. IV, The Water/FB-WAT-020

Fog types

Fog is cloud at the ground: water drops, or ice crystals, hanging in the air thickly enough to cut the visibility below 5/8 of a mile (1 km). It forms when air near the ground is cooled to its dew point or given enough water to saturate, and its kinds are named by which of those happens and how.

Plate FB-WAT-020Phase changeRevision 1, 2026-09-25Status draftAlso called fog, radiation fog, advection fog, upslope fog, steam fog, sea smoke, precipitation fog, frontal fog, valley fog, ice fog, freezing fog, mist, dense fog

Six small sections of fog: radiation fog, the ground radiating heat to a clear night sky and cooling the air above it; advection fog, warm moist air blowing over cold water; upslope fog, moist air flowing uphill and saturating as it rises; steam fog, cold air moving over much warmer water with vapour rising from it; precipitation fog, rain from warm air aloft evaporating into cold air near the ground; and ice fog, vapour freezing into crystals in air below about minus 22 degrees Fahrenheit.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-WAT-020-ASIX KINDS OF FOG, EACH A SMALL SECTION OF HOW ITS AIR IS COOLED OR GIVEN WATER UNTIL IT SATURATES; SCHEMATICFIG. A SIX KINDS OF FOGRADIATION FOGTHE GROUND COOLS UNDER A CLEAR, CALM NIGHT SKYHEAT RADIATED TO SPACEADVECTION FOGMOIST AIR BLOWS OVER COLD WATER OR COLD GROUNDWARM, MOISTCOLD WATERUPSLOPE FOGMOIST AIR FLOWS UPHILL AND COOLS AS IT RISESWHERE IT SATURATESSTEAM FOGCOLD AIR MOVES OVER MUCH WARMER WATERCOLD AIRWARM WATERPRECIPITATION FOGRAIN FROM WARM AIR ALOFT EVAPORATES INTO COLD AIRWARM AIR ALOFTCOLD AIR, SATURATED FROM ABOVEICE FOGVAPOUR FREEZES AS CRYSTALS IN AIR BELOW ABOUT −22 °FFED BY EXHAUST AND CHIMNEYS123456TITLEFog types, six kinds of fogVOL. IV THE WATER · PHASE CHANGETYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 1 of 3DATE2026-09-25IDFB-WAT-020-ADRAWN AS linework on paperSOURCES AMS, Office of the Federal Coordinator for Meteorology, NWS
Fig. A Six kinds of fog. Six kinds of fog, each a small section of how its air is cooled or given water until it saturates; schematic NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. Radiation fog
  2. Advection fog
  3. Upslope fog
  4. Steam fog
  5. Precipitation fog
  6. Ice fog

What it is

Fog is cloud at the ground: drops of water, or ice crystals, dense enough to cut the visibility below 5/8 of a mile (1 km). It forms in one of two ways. The air near the ground is cooled to its dew point, or water evaporates into it until it saturates, Water vapour as a gas FB-ENG-011. Its kinds are named by how that happens, and Fig. A draws six.

What it is not

It is not mist: the same drops, thinner, with the visibility between 5/8 and 6 miles. It is not haze or smoke, which are dry particles in air that is not saturated. Fig. B sets the visibilities an observation uses.

Lookalikes

Mist
The same drops, thinner: the visibility 5/8 to 6 miles.
Haze and smoke
Dry particles, not water; the air is not saturated and the dew point is well below the temperature.
Low stratus
Fog lifted off the ground, or the same cloud where the ground is lower, Stratus FB-SKY-017.

The machine

Cooling from below

Radiation fog forms on clear, calm nights, when the ground radiates its heat to space and chills the air against it, Radiation budget FB-ENG-007. It settles in valleys and burns off from the edges after sunrise. Advection fog forms when warm, moist air blows over cold water or cold ground, as along the Pacific coast in summer.

Cooling by rising

Upslope fog forms when moist air flows uphill and cools as it rises. The equation below gives the climb: the spread between temperature and dew point closes about 4.4 °F every 1,000 ft. Fig. C draws the High Plains. Air at 50 °F with a dew point of 44 °F saturates after rising about 1,370 ft (417 m), and the plains rise more than 3,000 ft toward Denver.

Adding water

Steam fog, or sea smoke, forms when cold air moves over much warmer water, as over the Great Lakes in late fall. Precipitation fog forms when rain from warm air aloft evaporates into the cold air beneath it. Ice fog forms in the coldest air, below about −22 °F (−30 °C), and is fed by exhaust and chimneys.

Scale of visibility from 1/16 to 10 statute miles, logarithmic: fog, FG, below 5/8 of a mile (1 kilometre); mist, BR, from 5/8 to 6 miles; above that no obscuration reported; and the Dense Fog Advisory commonly at 1/4 mile (0.4 kilometres) or less.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-WAT-020-BTHE VISIBILITIES THAT NAME FOG AND MIST IN AN OBSERVATION AND THAT COMMONLY RAISE A DENSE FOG ADVISORY, ON A LOGARITHMIC SCALE OF MILESFIG. B HOW THICK IS FOG1(1.6)2(3.2)3(4.8)5(8.0)7(11.3)10(16.1)1/16(0.10)1/8(0.20)1/4(0.40)1/2(0.80)VISIBILITY, STATUTE MILES (KM), LOGARITHMICFOG, FGBELOW 5/8 MI (1 KM)MIST, BR5/8 TO 6 MI (1 TO 9.7 KM)NOT REPORTED ASAN OBSCURATIONDENSE FOG ADVISORY: COMMONLY1/4 MI (0.4 KM) OR LESS, SET BY EACH OFFICEFOG AND MIST AS AN OBSERVATION REPORTS THEM; THE ADVISORY AS THE WEATHER SERVICE ISSUES IT123TITLEFog types, how thick is fogVOL. IV THE WATER · PHASE CHANGETYPECLASSIFICATIONSCALEVISIBILITY TO SCALE, LOGARITHMICREVREV 1 DRAFT SHEET 2 of 3DATE2026-09-25IDFB-WAT-020-BDRAWN AS linework on paperSOURCES AMS, Office of the Federal Coordinator for Meteorology, NWS
Fig. B How thick is fog. The visibilities that name fog and mist in an observation and that commonly raise a Dense Fog Advisory, on a logarithmic scale of miles VISIBILITY TO SCALE, LOGARITHMICMaximizeThe sheet, SVG, 11 by 17
  1. Fog, below 5/8 mile
  2. Mist, 5/8 to 6 miles
  3. The Dense Fog Advisory, commonly 1/4 mile or less
Section from the Rockies east across the plains, looking north: the plains rising from 1,500 feet to Denver at 5,280 feet; moist air at 50 degrees Fahrenheit with a dew point of 44 degrees flowing west up the slope; the level where it saturates, about 1,370 feet above where it started; and fog on the ground with stratus above it wherever the ground stands higher than that level.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-WAT-020-CSECTION FROM THE ROCKIES TO THE PLAINS, LOOKING NORTH; MOIST AIR FLOWING WEST UP THE RISING GROUND AND SATURATING; HEIGHTS TO SCALE, DISTANCE NOTFIG. C UPSLOPE FOG ON THE HIGH PLAINS01,0002,0003,0004,0005,0006,0007,000FTTHE PLAINS, 1,500 FTDENVER, 5,280 FTTHE ROCKIESMOIST AIR FROM THE EAST, 50 °F, DEW POINT 44 °FSATURATED 1,370 FT (417 M) HIGHER, AT 2,870 FTFOG ON THE GROUND, STRATUS ABOVEHEIGHTS TO SCALE, DISTANCE NOT; THE FOG’S DEPTH DRAWN SCHEMATIC123TITLEFog types, upslope fog on the high plainsVOL. IV THE WATER · PHASE CHANGETYPESECTIONSCALEHEIGHT TO SCALE, DISTANCE NTSREVREV 1 DRAFT SHEET 3 of 3DATE2026-09-25IDFB-WAT-020-CDRAWN AS linework on paperSOURCES AMS, Office of the Federal Coordinator for Meteorology, NWS
Fig. C Upslope fog on the high plains. Section from the Rockies to the plains, looking north; moist air flowing west up the rising ground and saturating; heights to scale, distance not HEIGHT TO SCALE, DISTANCE NTSMaximizeThe sheet, SVG, 11 by 17
  1. Moist air flowing west, upslope
  2. The level where it saturates
  3. Fog on the ground, stratus above

Ingredients

  • Moist air near the ground, its dew point close to its temperature
  • A way to close the gap: cooling from below, cooling by rising, or water added by evaporation
  • Light wind, enough to spread the fog but not to mix in drier air from above

Scales

time
from an hour at dawn to days in a cold valley or on a cold coast
horizontal
a single valley to hundreds of miles (hundreds of km) of coast or plain
vertical
from a few feet (1 m) to about 1,000 ft (300 m) deep
orlanski
meso

Equations

How far rising air climbs before it saturates

Δz≈T−TdΓd−ΓTd\Delta z \approx \frac{T - T_d}{\Gamma_d - \Gamma_{T_d}}
Δz\Delta z
the rise, ft
T−TdT - T_d
the dew point spread at the start, °F
Γd\Gamma_d
the dry adiabatic rate, 5.4 °F per 1,000 ft (9.8 °C per km)
ΓTd\Gamma_{T_d}
the rate at which the rising air's dew point falls, about 1.0 °F per 1,000 ft (1.8 °C per km)

Assumes Air rising without mixing or condensing; the spread closes about 4.4 °F every 1,000 ft (8.0 °C per km).

Working form Air on the plains at 50 °F with a dew point of 44 °F, a spread of 6 °F, saturates after rising about 1,370 ft (417 m). The plains rise more than 3,000 ft (900 m) from central Kansas to Denver, so the air flowing west reaches saturation well before the mountains, and upslope fog and stratus fill the western plains.

Signatures

sounding
a surface inversion with the air saturated beneath it; the temperature and dew point lines together at the ground
surface
the visibility falling below 5/8 mile; the temperature and the dew point equal; light wind
satellite
a smooth; flat grey or white sheet in the valleys and along the coast at dawn; eroding from its edges as the sun climbs
radar

The numbers

QuantityValue, and the kind of number it is
FogA visible aggregate of minute water droplets suspended in the atmosphere near the surface, reducing the visibility below 5/8 mile (1 km)Standard, Glossary of Meteorology
Fog in an observationFG when the visibility is less than 5/8 statute mile; BR, mist, from 5/8 to 6 statute miles; FZFG, freezing fog, when the temperature is below freezingStandard, Federal Meteorological Handbook No. 1
Dense Fog AdvisoryCommonly for fog reducing the visibility to 1/4 mile (0.4 km) or less, set by each officeTypical, Directive 10-515
Ice fogFog of ice crystals, usually at temperatures below about −22 °F (−30 °C)Standard, Glossary of Meteorology
DepthRadiation fog commonly a few feet to several hundred feet (a few metres to about 100 m) deepTypical, Glossary of Meteorology

How the station sees it

An airport station measures the visibility with a forward scatter sensor and reports FG below 5/8 mile, with the temperature and the dew point equal. Road stations measure it along the highways.

How it is warned

A Dense Fog Advisory is issued for fog commonly thinning the visibility to 1/4 mile (0.4 km) or less, and a Freezing Fog Advisory when the drops freeze on roads. Drive slowly with low beams, and leave space.

VTECPhenomenonThe alerts that carry it
FGdense fogDense Fog Advisory
ZFfreezing fogFreezing Fog Advisory
MFmarine dense fog

Every alert in force now

See also

Sources

  1. American Meteorological Society. Glossary of Meteorology.
  2. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  3. National Weather Service. Directive 10-515, WFO Non-Precipitation Weather Products Specification.

Definition after the Glossary of Meteorology. Plate FB-WAT-020, revision 1, 2026-09-25. The number is permanent; cite it.