Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-017

Stratus

Stratus is a grey layer of cloud with a fairly even base, low over the ground, sometimes with drizzle, snow grains or ice prisms. When the sun shows through it, its outline is clear. Stratus that reaches the ground is fog.

Plate FB-SKY-017The generaRevision 1, 2026-09-24Status draftAlso called St, low overcast, stratus nebulosus, stratus fractus, scud, low cloud, lifted fog

A painted section of stratus, heights to scale from the ground to 4,000 feet (1.2 kilometres): a flat grey layer from 600 to 1,400 feet (180 to 430 metres) under a warm tinted band of inversion from 1,400 to 1,800 feet (430 to 550 metres); fine drizzle falling from its base on the left, over a figure standing on the ground; and a green hill rising to 1,250 feet (380 metres) whose top is lost in the cloud.
Fig. A Stratus in section. A flat grey layer a few hundred feet up, drizzle beneath it, a hilltop lost in it.MaximizeThe drawing, to scale

What it is

Stratus is a grey layer of cloud with a fairly even base, low over the ground, from which drizzle, snow grains or ice prisms may fall. When the sun can be seen through it, its outline is clear. It stands in the low étage, often only a few hundred feet up, Cloud classification FB-SKY-001. Stratus with its base on the ground is fog, Fog types FB-WAT-020.

The Atlas lists two species: nebulosus, a featureless grey layer, and fractus, ragged shreds. Three varieties, opacus, translucidus and undulatus, describe a layer that hides the sun, one that shows it and one with waves. One supplementary feature, praecipitatio, records drizzle or snow reaching the ground; the Atlas gives stratus no accessory clouds. Fig. B draws each.

What it is not

It is not nimbostratus, Nimbostratus FB-SKY-015: nimbostratus is deeper and darker and gives steady rain or snow, where stratus gives drizzle. It is not stratocumulus, Stratocumulus FB-SKY-016, which has rolls and lumps and shading. It is not fog, though the only difference is height: fog is stratus on the ground, and fog that lifts becomes stratus.

Lookalikes

Fog
The same cloud with its base on the ground.
Nimbostratus
Deeper and darker, with steady rain or snow.
Stratocumulus
Rolls or lumps with shading, not an even base.

The machine

Stratus forms where moist air near the ground is cooled to its dew point through a shallow layer. On a clear still night the ground cools and cools the air above it; by morning a layer of cloud can lie across a valley. Moist air moving over colder ground or colder water is cooled from below in the same way, which is how sea fog and coastal stratus form. A light wind mixes the cooled air through a few hundred feet, and the cloud forms as a layer rather than at the ground.

Fig. A draws what holds it there: an inversion above, a layer where the air warms with height and stops the moist air mixing upward. The cloud cannot grow up, so it stays thin and flat. The sun of the morning warms the ground under it, the air under the base warms and mixes, and the layer lifts, thins and breaks, often by midday.

Its droplets are small and many and the air in it rises slowly, so what falls is drizzle: drops too small to splash, drifting down. Where the ground rises into the layer, as the hill in Fig. A does, a hilltop is in cloud and a road over it is in fog.

Small paintings of stratus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the species nebulosus and fractus; the varieties opacus, translucidus and undulatus; and the supplementary feature praecipitatio; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The stratus inventory. Every name the Atlas lists for stratus, 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 stratus from the ground to 4,000 feet (1.2 kilometres): a flat layer of water droplets from 600 to 1,400 feet (180 to 430 metres), under an inversion from 1,400 to 1,800 feet (430 to 550 metres); drizzle falling from its base; and a hill rising to 1,250 feet (380 metres) whose top is in the cloud.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-017-AA FLAT LAYER OF WATER CLOSE TO THE GROUND UNDER AN INVERSION, DRIZZLE BENEATH, A HILLTOP IN THE CLOUD; HEIGHTS TYPICALFIG. A STRATUS IN SECTION0123400.51kft(km)LOWINVERSION: AIR WARMER ALOFTKEYWater dropletsInversionGround12345TITLEStratus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-017-ADRAWN AS linework on paperSOURCES WMO, AMS, Office of the Federal Coordinator for Meteorology
Fig. A, the drawing A section to height scale. A flat layer of water close to the ground under an inversion, drizzle beneath, a hilltop in the cloud; heights typical HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. The layer of water droplets
  2. Drizzle
  3. A hill with its top in the cloud
  4. The inversion above the layer
  5. The low étage
Inventory of stratus as the International Cloud Atlas lists it, each name with a small drafted sketch and one line on what it is: the species nebulosus, fractus; the varieties opacus, translucidus, undulatus; the supplementary features praecipitatio; the accessory clouds a hyphen.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-017-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR STRATUS; SKETCHES NOT TO SCALEFIG. B THE STRATUS INVENTORY1SPECIES2, AT MOST ONEstratus nebulosusA featureless veil or layer with no detailstratus fractusRagged shreds, their outlines changing fast2VARIETIES3, ANY THAT APPLYstratus opacusThick enough to hide the sun or moonstratus translucidusThin enough to show where the sun or moon isstratus undulatusWaves in the sheet or its elements3SUPPLEMENTARY FEATURES1, ANY THAT APPLYstratus praecipitatioRain, snow or hail reaching the ground4ACCESSORY CLOUDS-TITLEStratus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-017-BDRAWN AS linework on paperSOURCES WMO, AMS, Office of the Federal Coordinator for Meteorology
Fig. B, the drawing Its inventory in the Atlas. Every species, variety, feature and accessory cloud the International Cloud Atlas lists for stratus; 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 near the ground cooled to its dew point, by the ground cooling at night or by moist air moving over cold ground or water
  • Light wind, which mixes the cooled air through a shallow layer
  • An inversion above, which holds the moist air near the ground

Scales

time
hours; it often forms overnight and lifts or breaks by midday
horizontal
a valley to hundreds of miles of coast
vertical
a few hundred feet to about 1,600 ft (500 m) deep
orlanski
meso

Signatures

surface
a low ceiling in the sky report; drizzle and poor visibility
satellite
smooth and bright in the visible, close to the ground's temperature in the infrared
radar
no echo, or very light drizzle

The numbers

QuantityValue, and the kind of number it is
AbbreviationStStandard, International Cloud Atlas
ÉtageLow: from the ground to 6,600 ft (2 km)Standard, International Cloud Atlas
Made ofWater droplets; ice crystals when very coldStandard, International Cloud Atlas
What fallsDrizzle, snow grains or ice prisms, when anything fallsStandard, International Cloud Atlas
FogStratus with its base at the ground is fogStandard, Glossary of Meteorology
Species2: nebulosus, fractusStandard, International Cloud Atlas
Varieties3: opacus, translucidus, undulatusStandard, International Cloud Atlas
Supplementary features1: praecipitatioStandard, 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
CeilingThe height of the lowest layer reported broken or overcastStandard, Federal Meteorological Handbook No. 1

How the station sees it

An airport station measures stratus directly: the laser ceilometer reports its base, often only a few hundred feet, and its cover, usually broken or overcast. The lowest broken or overcast layer is the ceiling, and a low stratus ceiling is what closes airports to visual flight. The station reports the drizzle and the reduced visibility beneath it. It does not name the genus.

How it is warned

The cloud is not warned. When it lowers to the ground as fog thick enough to cut visibility sharply, the Weather Service can issue a Dense Fog Advisory for drivers. Freezing drizzle from stratus below 32 °F (0 °C) glazes roads with little warning from the sky; the Weather Service covers it in a Winter Weather Advisory. For aircraft, stratus ceilings are forecast hour by hour.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1, Surface Weather Observations and Reports.
  4. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-017, revision 1, 2026-09-24. The number is permanent; cite it.