Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-010

Cirrus

Cirrus is high cloud made of ice, in white filaments, hooks or narrow bands with a fibrous look and often a silky sheen. Its streaks show the wind: the crystals fall into slower air below and trail behind the heads that made them.

Plate FB-SKY-010The generaRevision 1, 2026-09-24Status draftAlso called Ci, mares' tails, cirrus uncinus, cirrus fibratus, fall streaks, high cloud, cirrus homogenitus

A painted section of cirrus over a prairie, heights to scale from the ground to 46,000 feet (14 kilometres): five white hooked heads of ice crystals in a deep blue sky between about 29,500 and 35,500 feet (9 and 11 kilometres), each trailing a fall streak down and back to the left; the high étage bracketed from 16,400 to 43,000 feet (5 to 13 kilometres); and on the right, wind arrows that lengthen with height.
Fig. A Cirrus in section. Hooked heads of ice high in the sky, each trailing a fall streak into slower wind below.MaximizeThe drawing, to scale

What it is

Cirrus is the highest of the common clouds: white filaments, hooks and narrow bands of ice, fibrous in look and often with a silky sheen. It forms in the high étage, from 16,400 to 43,000 ft (5 to 13 km) in the middle latitudes, where the air is far below freezing and nearly all cloud is ice. The name is Latin for a curl of hair, one of the four roots of the classification, Cloud classification FB-SKY-001.

The Atlas lists five species of cirrus, by shape: fibratus, straight or gently curved filaments; uncinus, hooked like a comma, the cloud often called mares' tails; spissatus, patches dense enough to look grey toward the sun; castellanus, small turrets on a common base; and floccus, tufts with ragged bases. Four varieties describe how the filaments lie, and two features, mamma and fluctus, can appear with it. Fig. B draws each one.

What it is not

It is not cirrostratus, Cirrostratus FB-SKY-012, which is a veil over much of the sky and makes the halo. It is not cirrocumulus, Cirrocumulus FB-SKY-011, which is made of small grains, not filaments. A straight line of cloud along an aircraft's path is a contrail; when one lasts and spreads, the Atlas names it cirrus homogenitus.

Cirrus alone does not bring rain. Its crystals fall, but they evaporate in the drier air below long before they reach the ground.

Lookalikes

Cirrostratus
A veil over much of the sky, often with a halo. Cirrus stands in separate filaments or patches.
Cirrocumulus
Small grains or ripples, not filaments.
Contrails
A straight line along an aircraft's path. One that lasts and spreads becomes cirrus homogenitus.
Altostratus
A grey sheet, lower, thick enough to dim the sun.

The machine

Cirrus forms where moist air is lifted high enough for its vapour to freeze onto tiny particles as ice. The lifting may be a broad slow ascent ahead of a warm front, the outflow from the top of a thunderstorm, or a wave in the air over mountains. Once formed, the crystals grow and fall.

Fig. A draws what the falling does. The wind at cirrus height is usually stronger than the wind below it. The head, where the crystals form, moves with the fast air; the crystals falling from it enter slower air and are left behind. The trail they leave bends down and back into a hook or a streak: the fall streak. The shape of cirrus is a record of how the wind changes with height, drawn in ice.

Cirrus ahead of a warm front often thickens over some hours into cirrostratus and then altostratus, Altostratus FB-SKY-014, as the front comes nearer and the cloud lowers. A sky of cirrus spreading and thickening from one direction is the first sign of that sequence. Scattered cirrus that does not thicken usually means nothing more than moist air aloft.

The thick cirrus that spreads from the anvil of a thunderstorm is named for where it came from: cirrus spissatus cumulonimbogenitus, cirrus born of a cumulonimbus. It can drift hundreds of miles from the storm that made it.

Small paintings of cirrus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the species fibratus, uncinus, spissatus, castellanus and floccus; the varieties intortus, radiatus, vertebratus and duplicatus; and the supplementary features mamma and fluctus; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The cirrus inventory. Every name the Atlas lists for cirrus, each in a small painting.MaximizeThe drawing, to scale

The drawings

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

Section of cirrus from the ground to 46,000 feet (14 kilometres): five hooked heads of ice crystals between about 29,000 and 36,000 feet (9 and 11 kilometres), in the high étage, 16,400 to 43,000 feet (5 to 13 kilometres), each trailing a fall streak down and back as its crystals fall into slower wind; on the right, wind arrows that lengthen with height.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-010-ACIRRUS HEADS IN THE HIGH ÉTAGE, EACH TRAILING A FALL STREAK INTO SLOWER WIND BELOW; HEIGHTS TYPICAL OF THE MIDDLE LATITUDESFIG. A CIRRUS IN SECTION01020304002468101214kft(km)LOWMIDDLEHIGHWIND, STRONGERALOFTKEYIce crystals1234TITLECirrus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-010-ADRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. A, the drawing A section to height scale. Cirrus heads in the high étage, each trailing a fall streak into slower wind below; heights typical of the middle latitudes HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. A head of ice crystals, hooked at its top: uncinus
  2. The fall streak: crystals falling into slower wind
  3. The wind, stronger aloft
  4. The high étage
Inventory of cirrus as the International Cloud Atlas lists it, each name with a small drafted sketch and one line on what it is: the species fibratus, uncinus, spissatus, castellanus, floccus; the varieties intortus, radiatus, vertebratus, duplicatus; the supplementary features mamma, fluctus; the accessory clouds a hyphen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-010-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR CIRRUS; SKETCHES NOT TO SCALEFIG. B THE CIRRUS INVENTORY1SPECIES5, AT MOST ONEcirrus fibratusDetached filaments, straight or gently curved,with no hooks or tuftscirrus uncinusFilaments hooked at the top like a comma,without a rounded tuftcirrus spissatusPatches dense enough to look grey toward thesuncirrus castellanusTurrets rising from a common base, likebattlementscirrus floccusSmall tufts with ragged bases, often trailingfall streaks2VARIETIES4, ANY THAT APPLYcirrus intortusFilaments curved and tangled irregularlycirrus radiatusParallel bands that seem to meet at thehorizoncirrus vertebratusFilaments set like the ribs and spine of afishcirrus duplicatusTwo or more layers at slightly differentheights3SUPPLEMENTARY FEATURES2, ANY THAT APPLYcirrus mammaPouches hanging from the undersidecirrus fluctusShort lived curls or breaking waves on the top4ACCESSORY CLOUDS-TITLECirrus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-010-BDRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. B, the drawing Its inventory in the Atlas. Every species, variety, feature and accessory cloud the International Cloud Atlas lists for cirrus; sketches not to scale NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Species, at most one
  2. Varieties, any that apply
  3. Supplementary features, any that apply
  4. Accessory clouds, any that apply

Ingredients

  • Moist air lifted high enough to cool far below freezing, where ice crystals form and grow
  • Wind that changes speed or direction with height, which draws the falling crystals into streaks
  • Drier air below, in which the falling crystals evaporate before they reach the ground

Scales

time
minutes to hours for one streak; a spreading sheet can last a day
horizontal
a streak from under a mile to tens of miles long
vertical
a few hundred feet to about a mile (1.6 km) deep, in the high étage
orlanski
meso and synoptic

Signatures

surface
added to the sky report from sight at staffed stations
satellite
cold in the infrared, often faint or unseen in the visible
radar
no echo

The numbers

QuantityValue, and the kind of number it is
AbbreviationCiStandard, International Cloud Atlas
ÉtageHigh: 16,400 to 43,000 ft (5 to 13 km) in the middle latitudesStandard, International Cloud Atlas
Made ofIce crystalsStandard, International Cloud Atlas
Species5: fibratus, uncinus, spissatus, castellanus, floccusStandard, International Cloud Atlas
Varieties4: intortus, radiatus, vertebratus, duplicatusStandard, International Cloud Atlas
Supplementary features2: mamma, fluctusStandard, International Cloud Atlas
Accessory clouds-Standard, International Cloud Atlas
ContrailsA contrail that lasts and spreads is named cirrus homogenitus, and cirrus homomutatus once it no longer looks like a trailStandard, International Cloud Atlas
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 measures cloud overhead with a laser ceilometer, up to 12,000 ft (3.7 km). Cirrus stands higher, so the station's own sky report does not carry it. Observers at staffed stations add high cloud from sight. Satellites see it well: thin cirrus is cold in the infrared images and often faint in the visible ones, which is how a forecaster tells a high veil from low cloud.

How it is warned

Cirrus is not warned. The one cirrus tied to storms is the anvil blown off the top of a cumulonimbus, Cumulonimbus FB-SKY-019: a sheet of thick cirrus with a hard edge on the horizon, on a summer afternoon, can be the top of a thunderstorm still below the horizon. The warning, when it comes, is for the storm beneath it, Thunderstorm FB-EVT-001.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.
  4. Wallace, J. M. and P. V. Hobbs. Atmospheric Science, An Introductory Survey, 2nd ed. (2006).

Definition after the International Cloud Atlas. Plate FB-SKY-010, revision 1, 2026-09-24. The number is permanent; cite it.