Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-036

Noctilucent and nacreous clouds

Noctilucent and nacreous clouds form far above the weather: noctilucent near 51 mi (82 km) at the edge of space in summer, nacreous near 9 to 16 mi (15 to 25 km) in the winter stratosphere. Both are seen in twilight, lit by a sun already below the observer's horizon.

Plate FB-SKY-036Beyond the generaRevision 1, 2026-09-24Status draftAlso called noctilucent cloud, night shining cloud, polar mesospheric cloud, nacreous cloud, mother of pearl cloud, polar stratospheric cloud, upper atmospheric clouds

A painted twilight section from the ground to 62 miles (100 kilometres), in miles with kilometres: a deep blue sky warming to orange toward the horizon on the right, a small thunderstorm below the tropopause at 7 miles (11 kilometres), pearly iridescent lenses of nacreous cloud between 9 and 16 miles (15 and 25 kilometres), silvery blue rippled bands of noctilucent cloud near 51 miles (82 kilometres) below the mesopause at 53 miles (85 kilometres), the stratopause at 29 miles (47 kilometres), and an observer on the dark ground with the sun below the horizon, its light still reaching both clouds.
Fig. A The two clouds above the weather. Nacreous and noctilucent cloud, lit after sunset, on a height scale in miles.MaximizeThe drawing, to scale

What it is

Two kinds of cloud form far above the weather, and the Atlas names them apart from the ten genera as upper atmospheric clouds, Cloud classification FB-SKY-001.

Noctilucent cloud forms near 51 mi (82 km), just below the mesopause, the coldest level in the atmosphere, Atmosphere in section FB-ENG-001. It is thin ice, seen only in summer, at latitudes of about 50° to 70°, as silvery blue bands and ripples glowing in a dark sky long after sunset. Its name means night shining. It was first described in 1885, two years after the eruption of Krakatoa, by observers who were watching that eruption's vivid twilights, and it has been reported more often, and farther from the poles, in the decades since.

Nacreous cloud forms in the stratosphere, near 9 to 16 mi (15 to 25 km), in the polar winter. It is seen shortly before sunrise and after sunset, in vivid pearly colours that barely move, and is also called mother of pearl cloud. Fig. B sets the two side by side.

What it is not

Neither is cirrus. Cirrus is lit only while the sun is up where the observer stands; noctilucent cloud is still bright when every lower cloud has turned dark. Nacreous colours are brighter and steadier than the iridescence on the edge of an ordinary altocumulus, Corona, iridescence and glory FB-SKY-052. And neither is aurora, which is light made in the upper air itself and seen in full night, Aurora FB-SKY-070.

Lookalikes

Cirrus
Lit only while the sun is up where the observer is; noctilucent cloud glows after the lower clouds have gone dark
Iridescent altocumulus
Also colourful near the sun, but moving, and lower; nacreous colours are brighter and almost still
Aurora
Light made in the upper air itself, not sunlight on cloud, and seen in full night

The machine

Both need extreme cold. The summer mesopause over the high latitudes is the coldest place in the atmosphere, near -210 °F (-135 °C), because air rising there in summer expands and cools. At that cold the trace of water vapour present freezes onto specks of meteor dust. Nacreous cloud needs the stratosphere below about -108 °F (-78 °C), which it reaches only in the long polar night; its particles are ice, or droplets of nitric acid and water.

Fig. A shows why both are seen in twilight. With the sun a few degrees below the observer's horizon, the ground and the lower air are in the earth's shadow, but the air far overhead is still in sunlight. For noctilucent cloud the best time is when the sun is roughly 6° to 16° below the horizon: dark enough below for the cloud to show, high enough above for it to be lit.

The nacreous cloud's small, even particles bend sunlight into its colours. Its cousins, the polar stratospheric clouds of nitric acid, are where the chemistry that thins the ozone layer each polar spring takes place.

Two painted twilight skies side by side. On the left, noctilucent cloud: silvery blue rippled bands across a deep blue summer twilight above a dark line of trees. On the right, nacreous cloud: pearly lenses banded pink, green, blue and gold in an orange winter twilight over snow. Beneath each, its height, what it is made of, its season and where, and when and how it is seen.
Fig. B Noctilucent and nacreous, side by side. Each in its season, as it is seen, with what it is made of and when.MaximizeThe drawing, to scale

The drawings

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

Section from the ground to 62 miles (100 kilometres), in miles with kilometres, the layers where the U.S. Standard Atmosphere puts them: a thunderstorm below the tropopause at 7 miles (11 kilometres); nacreous cloud as lenses of ice between 9 and 16 miles (15 and 25 kilometres) in the stratosphere; noctilucent cloud as thin rippled bands near 51 miles (82 kilometres), below the mesopause at 53 miles (85 kilometres); and an observer on the ground in the earth's shadow, the sun below the horizon, its light still reaching both clouds.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-036-ATHE COLUMN TO 62 MI (100 KM): THE WEATHER, THE NACREOUS LAYER, THE NOCTILUCENT LAYER, AND THE TWILIGHT THAT LIGHTS THEMFIG. A THE TWO CLOUDS ABOVE THE WEATHER0102030405060020406080100mi(km)TROPOPAUSE, 7 mi (11 km)STRATOPAUSE, 29 mi (47 km)MESOPAUSE, 53 mi (85 km)TROPOSPHERE: ALL THE WEATHERSTRATOSPHEREMESOSPHERETHERMOSPHERE9 TO 16 mi (15 TO 25 km)ABOUT 51 mi (82 km)IN THE EARTH'S SHADOWKEYIce crystalsWater dropletsAir already in shadow123456TITLENoctilucent and nacreous clouds, the two clouds above the weatherVOL. III THE SKY · BEYOND THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-036-ADRAWN AS linework on paperSOURCES AMS, WMO, NOAA, NASA and the U.S. Air Force
Fig. A, the drawing The two clouds above the weather. The column to 62 mi (100 km): the weather, the nacreous layer, the noctilucent layer, and the twilight that lights them HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. The weather, in the troposphere
  2. The tropopause
  3. Nacreous cloud, in the stratosphere
  4. Noctilucent cloud, near the mesopause
  5. The sun, below the observer's horizon
  6. The observer, in twilight
Table comparing the two clouds: height, what each is made of, the cold each needs, season, where it is seen, when, how it looks, and its other names. Noctilucent: about 51 miles (82 kilometres), ice on meteor dust, near minus 210 degrees Fahrenheit (minus 135 degrees Celsius), summer, latitudes 50 to 70 degrees, deep twilight, silvery blue bands. Nacreous: 9 to 16 miles (15 to 25 kilometres), ice or nitric acid droplets, below minus 108 degrees Fahrenheit (minus 78 degrees Celsius), winter, near the poles, around sunrise and sunset, vivid pearly colours.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-036-BTHE TWO CLOUDS COMPARED, ROW BY ROW; FIGURES TYPICALFIG. B NOCTILUCENT AND NACREOUS, SIDE BY SIDENOCTILUCENTNACREOUS12HEIGHTAbout 51 mi (82 km), near the mesopause9 to 16 mi (15 to 25 km), in the stratosphereMADE OFIce on specks of meteor dustIce, or droplets of nitric acid and waterNEEDSThe coldest air on Earth, near -210 °F (-135 °C)Stratospheric air below about -108 °F (-78 °C)SEASONSummer, late May to mid August in the northWinterWHERELatitudes of about 50° to 70°; now and then farther southNear the poles; in the United States, mostly AlaskaWHEN SEENDeep twilight, the sun 6° to 16° below the horizonShortly before sunrise and after sunsetHOW IT LOOKSSilvery blue bands and ripples, glowing on a dark skyVivid, pearly iridescent colours that barely moveALSO CALLEDPolar mesospheric cloudMother of pearl cloud; polar stratospheric cloudFIGURES TYPICAL; THE HEIGHTS OF THE LAYERS AS THE U.S. STANDARD ATMOSPHERE SETS THEM.TITLENoctilucent and nacreous clouds, noctilucent and nacreous, side by sideVOL. III THE SKY · BEYOND THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-036-BDRAWN AS linework on paperSOURCES AMS, WMO, NOAA, NASA and the U.S. Air Force
Fig. B, the drawing Noctilucent and nacreous, side by side. The two clouds compared, row by row; figures typical NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Noctilucent cloud
  2. Nacreous cloud

Ingredients

  • Extreme cold: the summer polar mesopause is the coldest place in the atmosphere; nacreous cloud needs the stratosphere colder than about -108 °F (-78 °C)
  • A trace of water vapour, and particles to freeze on: meteor dust high up
  • Twilight, with the sun below the observer's horizon but still shining on the cloud

Scales

time
an hour or two of twilight; the clouds themselves can last for days
horizontal
hundreds of miles of banded sheet
vertical
a thin layer, under a mile (1.6 km) deep for noctilucent cloud
orlanski
synoptic

Signatures

satellite
polar mesospheric cloud seen from orbit as a bright ring around the summer pole; polar stratospheric cloud from space borne lidar in the polar winter
surface
none: both are far above every instrument at a weather station

The numbers

QuantityValue, and the kind of number it is
Noctilucent heightAbout 51 mi (82 km), near the mesopauseTypical, Glossary of Meteorology
MesopauseAbout 53 mi (85 km) in the standard atmosphereStandard, U.S. Standard Atmosphere
Noctilucent seasonSummer; late May to mid August in the northern hemisphere, at latitudes of about 50° to 70°Typical, Glossary of Meteorology
Noctilucent temperatureNear -210 °F (-135 °C)Typical, Wallace 2006
Nacreous height9 to 16 mi (15 to 25 km), in the stratosphereTypical, Glossary of Meteorology
Nacreous temperatureBelow about -108 °F (-78 °C)Typical, Wallace 2006
SeenIn twilight, when the sun is roughly 6° to 16° below the horizon for noctilucent cloudTypical, Glossary of Meteorology

How the station sees it

No weather station sees either: both are far above every instrument on the ground, and above the reach of any airport's ceilometer. They are known from observers on the ground in twilight, and from satellites and lidar, which see noctilucent cloud as a bright ring around the summer pole.

    How it is warned

    Neither is warned or affects the weather below. In the United States, noctilucent cloud is seen most often from the northern states in June and July, and has been seen farther south in recent summers; nacreous cloud is seen mostly from Alaska in winter. Both are worth looking for in the northern sky in deep twilight.

    See also

    Sources

    1. American Meteorological Society. Glossary of Meteorology.
    2. World Meteorological Organization. International Cloud Atlas.
    3. NOAA, NASA and the U.S. Air Force. U.S. Standard Atmosphere, 1976.
    4. Wallace, J. M. and P. V. Hobbs. Atmospheric Science, An Introductory Survey, 2nd ed. (2006).

    Definition after the Glossary of Meteorology. Plate FB-SKY-036, revision 1, 2026-09-24. The number is permanent; cite it.