Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-014

Altostratus

Altostratus is a greyish or bluish sheet in the middle étage over all or much of the sky, thin enough in places to show the sun as through ground glass. It makes no halo, and it is often the cloud that thickens into nimbostratus as a warm front comes nearer.

Plate FB-SKY-014The generaRevision 1, 2026-09-24Status draftAlso called As, ground glass sky, altostratus translucidus, altostratus opacus, middle cloud

A painted section of altostratus over a prairie, heights to scale from the ground to 26,000 feet (8 kilometres): a grey sheet from 10,000 to 18,000 feet (3 to 5.5 kilometres), paler and fibrous in its upper part of ice crystals, darker in its lower part of water droplets, with virga trailing beneath its right side; the sun above the sheet and a line from it to an observer on the ground; in a round window, the sun as seen from the ground, a pale blurred disc on grey with no halo.
Fig. A Altostratus in section. A grey sheet over the sky, and the sun seen through it as a pale disc, as through ground glass.MaximizeThe drawing, to scale

What it is

Altostratus is a greyish or bluish sheet or layer covering all or much of the sky, fibrous or even, in the middle étage, 6,600 to 23,000 ft (2 to 7 km) in the middle latitudes, Cloud classification FB-SKY-001. In its thinner parts the sun shows as through ground glass: a pale, blurred disc that casts no sharp shadow. It makes no halo.

The Atlas gives altostratus no species; a sheet has no shape to sort by. It lists five varieties: translucidus and opacus for a sheet that shows or hides the sun, duplicatus for two layers, and undulatus and radiatus for waves and bands. Three supplementary features, virga, praecipitatio and mamma, and one accessory cloud, pannus, can come with it. Fig. B draws each one.

What it is not

It is not cirrostratus, Cirrostratus FB-SKY-012, which is thinner and higher: shadows stay sharp under it and it often makes a halo. Altostratus never makes a halo, because it is too thick for light to pass cleanly through its crystals.

It is not nimbostratus, Nimbostratus FB-SKY-015, which is darker, hides the sun everywhere and has rain or snow falling from it steadily. The line between the two is crossed when a sheet thickens, and one often becomes the other within hours. It is not stratus, which is low, and even, with drizzle rather than rain.

Lookalikes

Cirrostratus
Thin enough for shadows, and makes a halo.
Nimbostratus
Darker, hides the sun everywhere, and rain or snow falls steadily.
Stratus
Low and uniform, with drizzle rather than rain.

The machine

Altostratus forms where a deep layer of moist air is lifted broadly and slowly over a wide area, most often over the sloping surface of a warm front or ahead of a low. The lift is gentle and wide, so the cloud is a sheet; it is deep, so the sheet spans heights where the air is below freezing and heights where it may not be.

Fig. A draws the sheet in two parts: ice crystals in its upper part, water droplets in its lower. Snow and rain begin to fall from it, and at first they evaporate in the drier air below as virga, drawn beneath one side. As the lifting continues, the air under the sheet moistens, the base lowers, and what falls starts to reach the ground. The sheet is then turning into nimbostratus.

The usual order of a warm front's sky, read from its first cloud, is cirrus, then cirrostratus, then altostratus, then nimbostratus, over half a day or so. Altostratus is the stage at which the sun goes watery.

Small paintings of altostratus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the varieties translucidus, opacus, duplicatus, undulatus and radiatus; the supplementary features virga, praecipitatio and mamma; and the accessory cloud pannus; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The altostratus inventory. Every name the Atlas lists for altostratus, 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 altostratus from the ground to 26,000 feet (8 kilometres): a grey sheet from 10,000 to 18,000 feet (3 to 5.5 kilometres), in the middle étage, ice crystals in its upper part and water droplets in its lower part, virga trailing beneath one side; and an observer seeing the sun above the sheet as a pale blurred disc with no halo.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-014-AA GREY SHEET IN THE MIDDLE ÉTAGE, ICE ABOVE AND DROPLETS BELOW, VIRGA BENEATH; HEIGHTS TYPICAL OF THE MIDDLE LATITUDESFIG. A ALTOSTRATUS IN SECTION051015202502468kft(km)LOWMIDDLEHIGHTHE SUN, AS THROUGH GROUND GLASS; NO HALOKEYIce crystalsWater droplets12345TITLEAltostratus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-014-ADRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. A, the drawing A section to height scale. A grey sheet in the middle étage, ice above and droplets below, virga beneath; heights typical of the middle latitudes HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. Ice crystals in the upper part
  2. Water droplets in the lower part
  3. Virga
  4. The sun as through ground glass; no halo
  5. The middle étage
Inventory of altostratus as the International Cloud Atlas lists it, each name with a small drafted sketch and one line on what it is: the species a hyphen; the varieties translucidus, opacus, duplicatus, undulatus, radiatus; the supplementary features virga, praecipitatio, mamma; the accessory clouds pannus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-014-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR ALTOSTRATUS; SKETCHES NOT TO SCALEFIG. B THE ALTOSTRATUS INVENTORY1SPECIES-2VARIETIES5, ANY THAT APPLYaltostratus translucidusThin enough to show where the sun or moon isaltostratus opacusThick enough to hide the sun or moonaltostratus duplicatusTwo or more layers at slightly differentheightsaltostratus undulatusWaves in the sheet or its elementsaltostratus radiatusParallel bands that seem to meet at thehorizon3SUPPLEMENTARY FEATURES3, ANY THAT APPLYaltostratus virgaFall streaks that evaporate before they reachthe groundaltostratus praecipitatioRain, snow or hail reaching the groundaltostratus mammaPouches hanging from the underside4ACCESSORY CLOUDS1, ANY THAT APPLYaltostratus pannusRagged shreds beneath the cloud: scudTITLEAltostratus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-014-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 altostratus; 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 a deep layer of moist air, as over a warm front
  • Ice crystals in the upper part of the sheet and water droplets in the lower part
  • Rain or snow falling from it, often evaporating before it reaches the ground

Scales

time
hours; it lowers and thickens over half a day ahead of a front
horizontal
hundreds of miles
vertical
3,300 to 10,000 ft (1 to 3 km) deep, or more
orlanski
synoptic

Signatures

surface
an overcast in the sky report when it is below 12,000 ft; light rain or snow may begin
satellite
a wide cold sheet, often the front edge of a storm's cloud shield
radar
light echo aloft; virga shows as echo that does not reach the ground

The numbers

QuantityValue, and the kind of number it is
AbbreviationAsStandard, International Cloud Atlas
ÉtageMiddle: 6,600 to 23,000 ft (2 to 7 km) in the middle latitudesStandard, International Cloud Atlas
Made ofIce crystals above and water droplets below, with raindrops or snowflakesStandard, International Cloud Atlas
The sun through itSeen dimly, as through ground glass; no haloStandard, International Cloud Atlas
Species-Standard, International Cloud Atlas
Varieties5: translucidus, opacus, duplicatus, undulatus, radiatusStandard, International Cloud Atlas
Supplementary features3: virga, praecipitatio, mammaStandard, International Cloud Atlas
Accessory clouds1: pannusStandard, 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). Altostratus below that height appears in the sky report as an overcast or broken layer at its base height; the station does not name the genus. Light rain or snow may begin in the report as the sheet thickens. On radar, snow falling from the sheet shows as echo aloft that does not reach the ground until the air below moistens.

How it is warned

Altostratus is not warned. It is the cloud of the hours before steady rain or snow, and the warnings that follow it, for heavy snow, freezing rain or flooding, are for what falls from the nimbostratus it becomes, Snow FB-WAT-012 and Rain FB-WAT-010.

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-014, revision 1, 2026-09-24. The number is permanent; cite it.