Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-015

Nimbostratus

Nimbostratus is a thick, dark grey layer that hides the sun everywhere, with rain or snow falling from it steadily rather than in showers. It is counted as a middle cloud but usually reaches down into the low étage and up into the high, and its base is often lost in the rain.

Plate FB-SKY-015The generaRevision 1, 2026-09-24Status draftAlso called Ns, rain cloud, steady rain, pannus, scud, middle cloud

A painted section of nimbostratus over wet country, heights to scale from the ground to 20,000 feet (6 kilometres): a deep grey layer from a ragged base near 2,500 feet (0.8 kilometres) to a top near 15,000 feet (4.6 kilometres), paler above with flecks of snow, darker below the freezing level, marked at 5,000 feet (1.5 kilometres); rain falling steadily from the base to the ground across its whole width; and three ragged dark shreds of pannus beneath the base.
Fig. A Nimbostratus in section. A deep dark layer, snow in its upper part, rain falling steadily from it to the ground.MaximizeThe drawing, to scale

What it is

Nimbostratus is a thick, dark grey cloud layer, often dim and wet looking, from which rain or snow falls steadily. It is thick enough to hide the sun everywhere. Its base is often hard to see, lost in the falling rain and in ragged low cloud beneath it. The Atlas counts it as a middle cloud, but it usually reaches down into the low étage and up into the high, Cloud classification FB-SKY-001.

It is the plainest of the genera in the Atlas: no species and no varieties. Two supplementary features, praecipitatio, rain or snow reaching the ground, and virga, and one accessory cloud, pannus, the ragged shreds beneath it, are all it carries. Fig. B draws each.

What it is not

It is not altostratus, Altostratus FB-SKY-014, which is thinner and higher and shows the sun in places. It is not stratus, Stratus FB-SKY-017, which is low and flat and gives drizzle, not rain. It is not cumulonimbus, Cumulonimbus FB-SKY-019, which is heaped and gives showers that start and stop, with lightning and thunder. Rain from nimbostratus is steady: it can fall for hours at nearly the same rate.

Lookalikes

Altostratus
Thinner and higher; the sun shows through it in places.
Stratus
Low and flat, with drizzle rather than rain or snow.
Cumulonimbus
Heaped, with showers, lightning and thunder.

The machine

Nimbostratus forms where a deep layer of moist air is lifted broadly and slowly for hours, over a warm front or around a low, until the cloud reaches from near the ground to 10,000 ft (3 km) or far higher. The lift is gentle and wide, so the cloud is a layer; it is sustained, so the layer keeps making rain and snow.

Fig. A draws what happens inside it. In its upper part, where the air is below freezing, ice crystals grow at the expense of supercooled droplets and clump into snowflakes. They fall through the freezing level, 32 °F (0 °C), and melt into raindrops below it. When the freezing level is at or near the ground, the snow reaches the ground as snow; when a warm layer sits over a cold one near the ground, it can arrive as sleet or freezing rain. On radar the melting layer often shows as a bright band, because a wet snowflake reflects the beam strongly.

The falling rain moistens the air under the base, and shreds of cloud form in it: pannus, or scud, drawn under the base in Fig. A. They drift faster than the layer above and are often mistaken for the cloud's own base.

Small paintings of nimbostratus, one for every name the International Cloud Atlas lists for it, each labelled with its full name and its rank: the supplementary features praecipitatio and virga, and the accessory cloud pannus; with the count of each rank beneath, a hyphen where the Atlas gives none.
Fig. B The nimbostratus inventory. Every name the Atlas lists for nimbostratus, 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 nimbostratus from the ground to 20,000 feet (6 kilometres): a deep layer from a ragged base near 2,500 feet (0.8 kilometres) to a top near 15,000 feet (4.6 kilometres), ice crystals and snow in its upper part above the freezing level, drawn at 5,000 feet (1.5 kilometres), water droplets below; rain falling steadily from the base to the ground; and ragged pannus under the base.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-015-AA DEEP LAYER FROM LOW TO MIDDLE ÉTAGE, SNOW MELTING TO RAIN AT THE FREEZING LEVEL, RAIN TO THE GROUND; HEIGHTS TYPICALFIG. A NIMBOSTRATUS IN SECTION051015012345kft(km)LOWMIDDLEHIGHFREEZING LEVEL, 32 °F (0 °C)KEYIce crystals and snowWater dropletsRain or snow reaching the groundSnow123456TITLENimbostratus, a section to height scaleVOL. III THE SKY · THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-015-ADRAWN AS linework on paperSOURCES WMO, AMS, NWS
Fig. A, the drawing A section to height scale. A deep layer from low to middle étage, snow melting to rain at the freezing level, rain to the ground; heights typical HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. Ice crystals and snow
  2. The freezing level, 32 °F (0 °C), where snow melts to rain
  3. Rain or snow falling steadily to the ground
  4. Pannus: ragged cloud beneath
  5. The base, lost in the falling rain
  6. The middle étage
Inventory of nimbostratus 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 a hyphen; the supplementary features praecipitatio, virga; the accessory clouds pannus.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-015-BEVERY SPECIES, VARIETY, FEATURE AND ACCESSORY CLOUD THE INTERNATIONAL CLOUD ATLAS LISTS FOR NIMBOSTRATUS; SKETCHES NOT TO SCALEFIG. B THE NIMBOSTRATUS INVENTORY1SPECIES-2VARIETIES-3SUPPLEMENTARY FEATURES2, ANY THAT APPLYnimbostratus praecipitatioRain, snow or hail reaching the groundnimbostratus virgaFall streaks that evaporate before they reachthe ground4ACCESSORY CLOUDS1, ANY THAT APPLYnimbostratus pannusRagged shreds beneath the cloud: scudTITLENimbostratus, its inventory in the AtlasVOL. III THE SKY · THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-015-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 nimbostratus; 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 deep layer of moist air lifted broadly and slowly for hours, as over a warm front or around a low
  • Ice crystals growing in the upper part, which fall as snow
  • A melting level, below which the snow turns to rain when the air is warm enough

Scales

time
hours to a day or more of steady rain or snow
horizontal
hundreds of miles, the heart of a storm's cloud shield
vertical
from near the ground to 10,000 ft (3 km) or far higher
orlanski
synoptic

Signatures

surface
a low ceiling, steady rain or snow, and visibility cut by it
satellite
a wide, deep, cold cloud shield
radar
a broad even echo, often with a bright band where snow melts to rain

The numbers

QuantityValue, and the kind of number it is
AbbreviationNsStandard, International Cloud Atlas
ÉtageCounted as middle; usually extends into the low and the highStandard, International Cloud Atlas
Made ofWater droplets and raindrops below, ice crystals and snowflakes aboveStandard, International Cloud Atlas
What fallsContinuous rain, snow or ice pellets, not showersStandard, International Cloud Atlas
The sunHidden everywhereStandard, International Cloud Atlas
Species-Standard, International Cloud Atlas
Varieties-Standard, International Cloud Atlas
Supplementary features2: praecipitatio, virgaStandard, 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 reports nimbostratus as a low overcast, its ceiling often under a few thousand feet and falling as the rain goes on; the station does not name the genus. It reports the steady rain or snow, its rate and the visibility cut by it. On radar the cloud's rain shows as a wide, even echo, often hundreds of miles across, with the bright band where snow melts.

How it is warned

The cloud is not warned; what falls from it is. Steady rain for many hours over the same ground can flood rivers and streets, Flash flood FB-EVT-100; steady snow can build to a winter storm; and a warm layer aloft can turn it to freezing rain. Each has its own watch and warning from the Weather Service, and each comes from this cloud.

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-015, revision 1, 2026-09-24. The number is permanent; cite it.