Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-034

Special clouds

Special clouds are clouds the International Cloud Atlas names for where they came from: over a fire, from a smokestack, a cooling tower or an aircraft's engines, over a waterfall or a forest. Each keeps the name of the genus it looks like, with a word for its origin added.

Plate FB-SKY-034Beyond the generaRevision 1, 2026-09-24Status draftAlso called homogenitus, homomutatus, flammagenitus, cataractagenitus, silvagenitus, contrail, pyrocumulus, pyrocumulonimbus, fire cloud, cooling tower cloud

A painted section over a prairie, heights to scale from the ground to 43,000 feet (13 kilometres): a wildfire with orange flames and a grey brown smoke column rising to a smoke stained cumulus flammagenitus from about 13,000 to 21,000 feet (4 to 6.4 kilometres); a small cooling tower with steam rising into a small cumulus homogenitus near 5,000 feet (1.5 kilometres); an aircraft near 35,000 feet (10.7 kilometres) trailing a contrail that widens behind it into a broad sheet; a waterfall between two rock faces with a spray cloud at its foot; and a dark forest with a band of mist rising from it.
Fig. A Clouds named for where they came from. A fire, a cooling tower, an aircraft, a waterfall and a forest, and the cloud each makes.MaximizeThe drawing, to scale

What it is

Most clouds are named for their shape, Cloud classification FB-SKY-001. A few are named for where they came from. The International Cloud Atlas calls them special clouds and keeps the name of the genus each one looks like, with a word for its origin added: flammagenitus, from a fire; homogenitus, from human activity; homomutatus, a cloud of human origin changed past recognition; cataractagenitus, from the spray of a waterfall; and silvagenitus, from a forest.

So the cumulus that towers over a large wildfire is cumulus flammagenitus, the pyrocumulus of fire crews; the cumulus over a power station's cooling tower is cumulus homogenitus; and a contrail is cirrus homogenitus, until the wind spreads it into a sheet no longer recognisable as a trail, when it becomes cirrus homomutatus. Fig. A draws each.

What it is not

It is not smoke. Smoke is dry particles; a fire cloud is the water cloud that forms on top of the smoke, where the rising air has cooled enough to condense. It is not fog, which forms over a wide area rather than from one source.

Lookalikes

Smoke
Dry particles, not water or ice; a fire cloud is the cloud that forms on top of the smoke
Cirrus
Natural high ice cloud; a spread contrail becomes cirrus homomutatus
Fog
Cloud at the ground, formed over a wide area rather than from one source

The machine

Every special cloud has a local source of heat or of water vapour. A fire heats the air and adds water from the burning fuel, and the plume rises until it reaches its condensation level; over a very large fire it can grow into cumulonimbus flammagenitus, a pyrocumulonimbus, with lightning of its own, Pyroconvection FB-EVT-141.

A contrail is water vapour from an aircraft's engines, frozen at once in air colder than about -40 °F (-40 °C). Fig. B follows one. In dry air its ice evaporates within a minute or so. In air saturated for ice it lasts, and the wind, changing with height, spreads it into a sheet that can cover a large part of the sky.

Six small painted skies in two rows and three columns. In dry air, a short white contrail behind an aircraft, a minute later only a faint stub behind it, and an hour later an empty sky. In air saturated for ice, a short contrail behind an aircraft, a minute later a longer and thicker trail, and an hour later a wide sheet of streaked ice cloud, with two arrows of wind changing with height beneath it; a note that a contrail forms where the air is cold enough, typically below about minus 40 degrees Fahrenheit (minus 40 degrees Celsius), and lasts only where the air is saturated for ice.
Fig. B A contrail in dry and saturated air. The same trail on leaving the engines, a minute later and an hour later, in dry air and in air saturated for ice.MaximizeThe drawing, to scale

The drawings

Each figure drawn as an engineering sheet, its parts numbered, to print at 11 by 17.

Section from the ground to 43,000 feet (13 kilometres): a cumulus flammagenitus of water droplets from about 13,000 to 21,000 feet (4 to 6.4 kilometres) above a wildfire and its smoke; a small cumulus homogenitus near 5,000 feet (1.5 kilometres) above a cooling tower; a contrail of ice near 35,000 feet (10.7 kilometres) behind an aircraft, spreading into a wide sheet farther back; spray cloud at the foot of a waterfall; and mist rising from a forest.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-034-AA FIRE CLOUD, A COOLING TOWER CLOUD, A CONTRAIL AND ITS SPREAD, WATERFALL SPRAY AND FOREST MIST; HEIGHTS TYPICALFIG. A CLOUDS NAMED FOR WHERE THEY CAME FROM010203040024681012kft(km)LOWMIDDLEHIGHAIRCRAFTKEYIce crystalsWater dropletsGround and rock1234567TITLESpecial clouds, clouds named for where they came fromVOL. III THE SKY · BEYOND THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-034-ADRAWN AS linework on paperSOURCES WMO, AMS, National Wildfire Coordinating Group
Fig. A, the drawing Clouds named for where they came from. A fire cloud, a cooling tower cloud, a contrail and its spread, waterfall spray and forest mist; heights typical HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. Cumulus flammagenitus, over a fire
  2. The smoke beneath it
  3. Cumulus homogenitus, over a cooling tower
  4. A contrail, cirrus homogenitus
  5. Spread by the wind: cirrus homomutatus
  6. Spray cloud: cataractagenitus
  7. Mist over a forest: silvagenitus
Time strip in two rows and three columns. In dry air, a contrail forms behind the aircraft, is a short stub a minute later as its ice evaporates, and is gone an hour later. In air saturated for ice, the contrail forms, lengthens and thickens a minute later, and an hour later has been spread by wind changing with height into a wide sheet of ice cloud, cirrus homomutatus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-034-BONE CONTRAIL, TWO SKIES, THREE MOMENTS: IT FORMS IN BOTH, AND LASTS ONLY IN AIR SATURATED FOR ICEFIG. B A CONTRAIL IN DRY AIR AND IN AIR SATURATED FOR ICEON LEAVING THE ENGINESA MINUTE LATERAN HOUR LATERDRY AIRAIR SATURATED FOR ICETHE ICE HAS EVAPORATED-SPREAD BY THE WIND INTO A SHEETWIND CHANGING WITH HEIGHTA CONTRAIL FORMS WHERE THE AIR IS COLD ENOUGH, TYPICALLY BELOW ABOUT -40 °F (-40 °C); IT LASTS ONLY WHERE THE AIR IS SATURATED FOR ICE.12345TITLESpecial clouds, a contrail in dry air and in air saturated for iceVOL. III THE SKY · BEYOND THE GENERATYPETIME STRIPSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-034-BDRAWN AS linework on paperSOURCES WMO, AMS, National Wildfire Coordinating Group
Fig. B, the drawing A contrail in dry air and in air saturated for ice. One contrail, two skies, three moments: it forms in both, and lasts only in air saturated for ice NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Water vapour from the engines freezing behind the aircraft
  2. In dry air, the ice evaporates within a minute or so
  3. In air saturated for ice, the trail persists and thickens
  4. Wind changing with height spreads it
  5. A sheet of cirrus homomutatus

Ingredients

  • A local source of heat or of water vapour: a fire, a power station, an engine, a waterfall, a wet forest
  • Air near saturation, so the extra heat or vapour carries it to condensation
  • For a contrail, air cold enough that the engines' water vapour freezes at once

Scales

time
a minute for a short contrail; hours for a spreading one or a fire cloud
horizontal
a few hundred feet over a tower; hundreds of miles for spread contrails
vertical
a few hundred feet for spray and forest mist; to 50,000 ft (15 km) for a fire cloud that becomes a thunderstorm
orlanski
micro to meso

Signatures

surface
smoke and a sudden cumulus over a fire; a line of cloud along a flight path
satellite
long thin lines of contrail, and spread sheets from them; a fire cloud's cold top over a hot spot
radar
smoke and ash echo from a large fire, and rain and hail from a fire cloud that has become a thunderstorm

The numbers

QuantityValue, and the kind of number it is
Names, by originflammagenitus (fire), homogenitus (human activity), homomutatus (a homogenitus cloud changed past recognition), cataractagenitus (waterfall), silvagenitus (forest)Standard, International Cloud Atlas
How a name is madeThe genus, then the origin: cumulus flammagenitus, cirrus homogenitusStandard, International Cloud Atlas
PyrocumulonimbusCumulonimbus flammagenitus: a fire cloud tall enough to make lightningStandard, International Cloud Atlas
Contrail formationTypically where the air is colder than about -40 °F (-40 °C)Typical, Glossary of Meteorology
Contrail persistenceOnly where the air is saturated with respect to iceTextbook, Glossary of Meteorology
Airport ceilometerCloud bases to 12,000 ft (3.7 km) overheadStandard, Automated Surface Observing System (ASOS) User's Guide

How the station sees it

An airport station reports special clouds as it reports any cloud, by cover and base height, without the name. Satellites see spreading contrails as long straight lines and sheets, and a fire cloud as a cold top over a hot spot.

How it is warned

The clouds are not warned; what makes them can be. A Red Flag Warning is issued for the weather that lets fires spread, Fire weather FB-EVT-139, and a fire cloud over a wildfire means the fire has taken over its own weather.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. National Wildfire Coordinating Group. NWCG Glossary of Wildland Fire (PMS 205).
  4. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-034, revision 1, 2026-09-24. The number is permanent; cite it.