Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-012

Cirrostratus

Cirrostratus is a thin whitish veil of ice over much or all of the sky, smooth or fibrous, through which the sun still casts shadows. It is the cloud of the halo: a ring of light 22 degrees from the sun or moon, made by the veil's crystals bending the light.

Plate FB-SKY-012The generaRevision 1, 2026-09-24Status draftAlso called Cs, halo cloud, cirrostratus nebulosus, cirrostratus fibratus, 22 degree halo, high cloud

A painted section of cirrostratus over a prairie, heights to scale from the ground to 46,000 feet (14 kilometres): a thin milky veil of ice crystals from about 24,000 to 29,000 feet (7.3 to 8.8 kilometres); an observer on the ground looking toward the sun, 50 degrees above the horizon, with two lines 22 degrees either side of it meeting the veil where the halo is seen; in a round window, the sun as seen from the ground inside a ring of light 22 degrees from it, reddish on its inner edge.
Fig. A Cirrostratus in section. A veil of ice over the sky, the halo seen in it, and the ring as seen from the ground.MaximizeThe drawing, to scale

What it is

Cirrostratus is a thin whitish veil of ice over much or all of the sky, smooth or finely fibrous. It stands in the high étage, from 16,400 to 43,000 ft (5 to 13 km) in the middle latitudes, Cloud classification FB-SKY-001. It is thin enough that the sun still casts shadows through it, and sometimes so thin that the only sign of it is a milky look to the blue and a halo round the sun or moon.

The Atlas lists two species: fibratus, a veil with fine fibres in it, and nebulosus, a veil with no detail at all. Two varieties, duplicatus and undulatus, describe a veil in two layers and one with waves. The Atlas gives cirrostratus no supplementary features and no accessory clouds. Fig. B draws each name.

What it is not

It is not altostratus, Altostratus FB-SKY-014, which is thicker, grey and lower, and through which the sun shows as through ground glass with no halo. The halo is the quickest test: a halo means ice crystals thin enough to see through, and that is cirrostratus.

It is not cirrus, Cirrus FB-SKY-010, which stands in separate filaments and patches with clear sky between. It is not haze, which is dry particles low in the air and makes no halo.

Lookalikes

Altostratus
Thicker and grey. The sun shows as through ground glass, and there is no halo.
Cirrus
Separate filaments and patches with blue sky between.
Haze
Dry particles in the air, low, with no halo.

The machine

Cirrostratus forms where a broad layer of moist air high in the troposphere is lifted slowly and evenly, as happens over the sloping surface of a warm front hundreds of miles ahead of where the front meets the ground. The lift is gentle and wide, so the cloud is a sheet rather than heaps, and the air is far below freezing, so the sheet is ice.

The halo in Fig. A comes from the crystals' shape. Many of the ice crystals in cirrostratus are six sided columns and plates. Light entering one side and leaving by another is bent, and for that pair of faces the least it can be bent is about 22 degrees. Light piles up at that least angle, so a ring of light appears 22 degrees from the sun or moon, reddish on its inside edge. An outstretched hand, thumb to little finger, spans about that much of the sky. Other shapes and angles of crystal make the rarer arcs and spots of the halo family, Halo family FB-SKY-051.

A veil of cirrostratus that thickens and lowers over some hours, until the halo fades and the sun turns watery, is altostratus arriving. That is the sequence ahead of a warm front, and rain or snow often follows within a day, Nimbostratus FB-SKY-015. A veil that stays thin can pass with no weather at all.

Small paintings of cirrostratus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the species fibratus and nebulosus, and the varieties duplicatus and undulatus; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The cirrostratus inventory. Every name the Atlas lists for cirrostratus, 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 cirrostratus from the ground to 46,000 feet (14 kilometres): an even veil of ice crystals from about 24,000 to 29,000 feet (7.3 to 8.8 kilometres), in the high étage; an observer at the ground looking toward the sun, 50 degrees above the horizon, and two lines 22 degrees either side of it, where the halo is seen on the veil.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-012-AAN EVEN VEIL OF ICE IN THE HIGH ÉTAGE, AND THE 22 DEGREE HALO IT MAKES; ANGLES DRAWN TRUE, HEIGHTS TYPICALFIG. A CIRROSTRATUS IN SECTION01020304002468101214kft(km)LOWMIDDLEHIGHTO THE SUN22°22°THE HALO: A RING OF LIGHT 22° FROM THE SUN, SEEN WHERE THE VEIL ISKEYIce crystals1234TITLECirrostratus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-012-ADRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. A, the drawing A section to height scale. An even veil of ice in the high étage, and the 22 degree halo it makes; angles drawn true, heights typical HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. The veil of ice crystals
  2. The halo: light bent 22° from the sun
  3. The observer
  4. The high étage
Inventory of cirrostratus as the International Cloud Atlas lists it, each name with a small drafted sketch and one line on what it is: the species fibratus, nebulosus; the varieties duplicatus, undulatus; the supplementary features a hyphen; the accessory clouds a hyphen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-012-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR CIRROSTRATUS; SKETCHES NOT TO SCALEFIG. B THE CIRROSTRATUS INVENTORY1SPECIES2, AT MOST ONEcirrostratus fibratusDetached filaments, straight or gently curved,with no hooks or tuftscirrostratus nebulosusA featureless veil or layer with no detail2VARIETIES2, ANY THAT APPLYcirrostratus duplicatusTwo or more layers at slightly differentheightscirrostratus undulatusWaves in the sheet or its elements3SUPPLEMENTARY FEATURES-4ACCESSORY CLOUDS-TITLECirrostratus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-012-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 cirrostratus; 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

  • A broad, slow lifting of moist air high in the troposphere, as ahead of a warm front
  • Temperatures far below freezing, so the cloud is ice
  • Six sided ice crystals, which bend light passing through them by 22 degrees

Scales

time
hours; it often spreads and thickens over half a day
horizontal
hundreds of miles, often the leading edge of a warm front's cloud
vertical
a few hundred feet to a mile or more (1.6 km) deep, in the high étage
orlanski
synoptic

Signatures

surface
the sun or moon ringed by a halo; shadows still cast
satellite
a thin cold sheet in the infrared
radar
no echo

The numbers

QuantityValue, and the kind of number it is
AbbreviationCsStandard, 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
The haloA ring of light 22° in radius around the sun or moon, from light refracted through six sided ice crystalsStandard, Glossary of Meteorology
Species2: fibratus, nebulosusStandard, International Cloud Atlas
Varieties2: duplicatus, undulatusStandard, International Cloud Atlas
Supplementary features-Standard, International Cloud Atlas
Accessory clouds-Standard, 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 to 12,000 ft (3.7 km). Cirrostratus stands higher, so the station's own sky report does not carry it; staffed stations add it from sight. Satellites show it as a wide, thin, cold sheet in the infrared images, often the leading edge of a storm's cloud.

How it is warned

Cirrostratus is not warned. Its halo is the oldest weather sign there is, a ring round the moon before rain, and it is right often enough to have lasted: the veil is frequently the first cloud of an approaching front. It is a sign to read the forecast, not a forecast in itself.

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.

Definition after the International Cloud Atlas. Plate FB-SKY-012, revision 1, 2026-09-24. The number is permanent; cite it.