Storm Station 247

The Field Book/Vol. III, The Sky/Cloud classification/FB-SKY-035

Orographic clouds

Orographic clouds are made by mountains: air forced up and over a ridge cools and condenses, then sinks and warms in the lee, and sets up waves downwind. The cap on the summit, the lenses at the wave crests and the rotor beneath them stand still while the wind blows through them.

Plate FB-SKY-035Beyond the generaRevision 1, 2026-09-24Status draftAlso called lenticular cloud, cap cloud, banner cloud, rotor cloud, mountain wave, lee wave, foehn wall, chinook arch, upslope cloud, altocumulus lenticularis

A painted section across a mountain ridge rising to 12,000 feet (3.7 kilometres), heights to scale from the ground to 29,500 feet (9 kilometres), the wind from the left: a cap cloud on the snowy summit, rain and snow on the windward slope, an arrow of the downslope wind in the lee, dashed streamlines rising over the ridge and waving downwind, smooth stacked lens clouds at the wave crests near 17,000 and 22,000 feet (5.2 and 6.7 kilometres), and a ragged rotor cloud turning beneath the first crest near 8,500 feet (2.6 kilometres).
Fig. A Air across a ridge. Wind over a mountain: the cap cloud, rain and snow on the windward side, lee waves, lens clouds and a rotor.MaximizeThe drawing, to scale

What it is

Orographic clouds are clouds made by mountains. When wind blows across a ridge, the air is forced up the windward slope, cools and may condense; it sinks and warms down the lee slope; and in stable air it springs back and forth in a train of waves downwind. Each part of that flow makes its own cloud, and because the flow stays fixed to the mountain, so do the clouds: they stand still while the wind blows through them.

Fig. A draws the whole pattern. On the summit sits a cap cloud. On the windward slope rain or snow falls. In the lee the air descends as a warm, dry wind. Downwind, lens clouds stand at the wave crests, altocumulus lenticularis in the Atlas's naming, Altocumulus FB-SKY-013, often stacked like plates; and beneath the first crest a ragged rotor cloud rolls about a horizontal axis. Fig. B draws each one small, with the banner cloud, the foehn wall and the upslope cloud of the plains.

What it is not

A lens cloud is not an ordinary altocumulus. Ordinary cloud drifts with the wind; a lens cloud forms at its upwind edge and evaporates at its downwind edge, so the air passes through it and the cloud stays put. Its smooth, sharp edges have made it the cloud most often reported as a flying object.

A rotor cloud is not a shelf cloud. A shelf cloud leads a thunderstorm and moves with it; a rotor stands in the lee of a ridge while the wind holds.

Lookalikes

Flying saucers
Lens clouds are smooth, sharp edged and still, and are reported as unidentified objects more than any other cloud
Ordinary altocumulus
Moves with the wind; a lens cloud stands still while the wind blows through it
Shelf cloud
Leads a thunderstorm and moves with it; a rotor stands in the lee of a ridge

The machine

Air lifted over a ridge cools as it rises, and condenses if it reaches its dew point: the cap cloud forms on the way up and evaporates on the way down, which is why it sits on the summit and goes no farther. Much of the water falls out on the windward slope as rain or snow.

The air then descends the lee slope. With its water rained out, it warms at the dry rate, about 5.4 °F for every 1,000 ft (9.8 °C for every km), all the way down, and arrives warmer and drier than it left. East of the Rocky Mountains this is the chinook, which can raise the temperature by tens of degrees in an hour and take snow away without melting it into puddles. A wall of cloud stands on the crest while it blows, and downwind a band of clear sky lies under the cloud deck: the chinook arch.

Where the air is stable, the descending air overshoots, rises again and oscillates, and the ridge sets up standing waves downwind, Lapse rates and stability FB-ENG-005. Where a crest lifts a moist layer to saturation, a lens cloud forms. The rotor beneath the first crest is an eddy turning under the wave, and the air in it is among the most turbulent a pilot can meet. Where the flow is strong enough, the waves can break and bring violent downslope winds to the foothills.

Six small paintings of clouds made by mountains, the wind from the left in each: a cap cloud sitting on a summit; a banner cloud streaming from the lee side of a lone peak; three smooth lens clouds stacked like plates; a ragged rotor cloud under a lens; a foehn wall on a crest with a deck of cloud downwind and clear sky beneath it; and upslope cloud lying along a long gentle slope.
Fig. B The mountain clouds as they are seen. Six clouds made by mountains, each in a small painting with its name and one line.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 29,500 feet (9 kilometres) across a ridge rising to 12,000 feet (3.7 kilometres), the wind from the left: the air rising over the ridge, with a cap cloud on the summit and rain or snow on the windward slope; a downslope wind in the lee, warmed and dried; downwind, streamlines in a train of waves, with smooth lens clouds stacked at the wave crests near 17,000 and 22,000 feet (5.2 and 6.7 kilometres) and a ragged rotor cloud turning beneath the first crest near 8,500 feet (2.6 kilometres).1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-035-AWIND ACROSS A RIDGE: THE CAP CLOUD, WINDWARD RAIN, THE WARM DOWNSLOPE WIND, THE LEE WAVES, THEIR LENSES AND THE ROTORFIG. A AIR ACROSS A RIDGE051015202502468kft(km)LOWMIDDLEHIGHWINDDOWNSLOPE WIND, WARMED AND DRIEDTURBULENTKEYWater dropletsRain or snow reaching the groundGround1234567TITLEOrographic clouds, air across a ridgeVOL. III THE SKY · BEYOND THE GENERATYPESECTIONSCALEHEIGHT TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 2DATE2026-09-24IDFB-SKY-035-ADRAWN AS linework on paperSOURCES WMO, AMS, Wallace
Fig. A, the drawing Air across a ridge. Wind across a ridge: the cap cloud, windward rain, the warm downslope wind, the lee waves, their lenses and the rotor HEIGHT TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17The painting
  1. The wind across the ridge
  2. The cap cloud on the summit
  3. Rain and snow on the windward slope
  4. The downslope wind, warmed and dried
  5. The lee waves
  6. Lens clouds at the crests
  7. The rotor cloud, turbulent
Six small elevations, each with its name and one line: a cap cloud sitting on a summit; a banner cloud streaming from the lee side of a lone peak; lens clouds stacked like plates; a rotor cloud rolling beneath a wave crest; a foehn wall on the crest with a chinook arch of clear sky beneath a cloud deck downwind; and upslope cloud formed as moist air is pushed up a long gentle slope.1122334455667788AABBCCDDEESTORM STATION 247THE FIELD BOOKPLATE FB-SKY-035-BSIX CLOUDS MADE BY MOUNTAINS, EACH DRAWN SMALL, THE WIND FROM THE LEFT IN EVERY SKETCHFIG. B THE MOUNTAIN CLOUDS AS THEY ARE SEEN1CAP CLOUDA cloud sitting on a summit, formed as the airrises over it and gone as it sinks; it stays putwhile the wind blows through it.2BANNER CLOUDA plume streaming from the lee side of a lonepeak, as from a flagpole, where the air rises inthe eddy behind it.3LENS CLOUDSAltocumulus lenticularis: smooth lenses at thecrests of the lee waves, often stacked likeplates, standing still for hours.4ROTOR CLOUDA ragged roll under the first wave crest, turningabout a horizontal axis; the air in it isviolently turbulent.CLEAR5FOEHN WALL AND CHINOOK ARCHA wall of cloud along the crest, and downwind aband of clear sky under a cloud deck: the archeast of the Rockies when the chinook blows.6UPSLOPE CLOUDStratus and fog formed as moist air is pushed up along, gentle slope, as over the high plains eastof the Rockies.EACH DRAWN SMALL AND NOT TO SCALE; WIND FROM THE LEFT IN EVERY SKETCH.TITLEOrographic clouds, the mountain clouds as they are seenVOL. III THE SKY · BEYOND THE GENERATYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 2 of 2DATE2026-09-24IDFB-SKY-035-BDRAWN AS linework on paperSOURCES WMO, AMS, Wallace
Fig. B, the drawing The mountain clouds as they are seen. Six clouds made by mountains, each drawn small, the wind from the left in every sketch NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17The painting
  1. Cap cloud
  2. Banner cloud
  3. Lens clouds
  4. Rotor cloud
  5. Foehn wall and chinook arch
  6. Upslope cloud

Ingredients

  • Wind blowing across a ridge, strong and steady in direction
  • Stable air, which springs back after it is lifted and so oscillates in waves downwind
  • Moist layers at the heights where the air is lifted, where the crests reach saturation

Scales

time
hours; the clouds stay while the wind across the ridge holds
horizontal
a few miles to tens of miles downwind of the ridge
vertical
from the ridge to the tropopause for the tallest waves
orlanski
meso beta to meso gamma

Signatures

surface
a sharp rise in temperature and fall in humidity with a strong downslope wind; gusts
satellite
bands of wave cloud parallel to the ridge, fixed in place, and a clear strip in its lee
radar
snow and rain on the windward slope; little in the lee

The numbers

QuantityValue, and the kind of number it is
Lens cloudsAltocumulus, cirrocumulus or stratocumulus lenticularis, at the crests of lee wavesStandard, International Cloud Atlas
Cap cloudFormed where the air rises over the summit and evaporated where it sinksTextbook, Wallace 2006
Downslope windAir descending the lee slope warms at about 5.4 °F per 1,000 ft (9.8 °C per km) once any cloud has evaporatedStandard, Glossary of Meteorology
RotorA horizontal eddy beneath the first wave crest; severe turbulence for aircraftTextbook, Glossary of Meteorology
ChinookThe warm dry downslope wind east of the Rocky MountainsStandard, Glossary of Meteorology
Airport ceilometerCloud bases to 12,000 ft (3.7 km) overheadStandard, Automated Surface Observing System (ASOS) User's Guide

How the station sees it

A station in the lee of a range sees the downslope wind as a sudden jump in temperature and fall in humidity, with strong, gusty wind. An airport's ceilometer reports cap and rotor cloud when they are overhead and below 12,000 ft (3.7 km). Satellites show wave cloud as bands parallel to the ridge, fixed in place, with a clear strip in the lee.

How it is warned

The clouds are not warned; the wind can be. Downslope windstorms are warned as High Wind Warnings, and the Weather Service issues them for the foothills when the waves are expected to break. The warm, dry, gusty air in the lee also raises fire danger, Fire weather FB-EVT-139. For aircraft, lens and rotor clouds are the visible sign of mountain wave turbulence.

See also

Sources

  1. World Meteorological Organization. International Cloud Atlas.
  2. American Meteorological Society. Glossary of Meteorology.
  3. Wallace, J. M. and P. V. Hobbs. Atmospheric Science, An Introductory Survey, 2nd ed. (2006).
  4. National Weather Service. Automated Surface Observing System (ASOS) User's Guide.

Definition after the International Cloud Atlas. Plate FB-SKY-035, revision 1, 2026-09-24. The number is permanent; cite it.