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Field Guide · Cloud Morphology

Shapes Written in Air

Unfamiliar cloud silhouettes act as natural wind-tunnel tracers, marking standing mountain waves, velocity shear boundaries, and rapid phase transitions.

Standing Waves and Shear Rollers

A still photograph of stacked, porcelain-smooth disks above Mount Rainier, Mount Fuji, or the Patagonian Andes creates the unmistakable impression of solid objects parked in calm air. Yet if an observer records a 60-second video of Altocumulus standing lenticularis, tiny edge wisps race leeward at gale speeds while the overall silhouette refuses to budge.

When stably stratified moist air crosses a mountain barrier, buoyancy restores displaced air parcels downward on the lee side, setting up stationary atmospheric gravity waves that propagate tens of kilometers downstream and high into the stratosphere. Each wave crest lifts air adiabatically below its dew point so droplets condense along the upwind edge; descending air in the trough warms adiabatically and evaporates those droplets along the downwind edge. Layered dry and moist strata sculpt multi-decked "stack of plates" formations.

Standing Lee-Wave Condensation Envelope

Lifting Condensation Level (Saturation Threshold) Lee Wave Crest = Cloud · Trough = Clear Air
Multi-layered lenticular wave cloud illuminated in warm sunset light stationary above an alpine mountain crest
Stacked Altocumulus lenticularis duplicatus. Smooth edges indicate laminar flow inside the cloud level, yet glider pilots know severe rotor turbulence often churns unseen in the boundary layer directly beneath the wave trough.
Fluctus (Kelvin–Helmholtz Instability) Unlike terrain-locked lenticulars, breaking curling waves (fluctus) require low Richardson numbers (<0.25): vertical wind shear across a sharp thermal inversion overcomes buoyancy stability, curling cloud tops into fleeting crests that dissipate within 90 to 180 seconds.
Circular cavum fallstreak hole punched cleanly through a thin supercooled altocumulus cloud deck with wispy ice virga trailing at center
Cavum (fallstreak hole) with central cirrus virga trail. Often reported as rocket plumes or optical anomalies, its geometry reflects pure Wegener–Bergeron–Findeisen ice growth.
Inverted Convection & Microphysics

Mammatus, Asperitas, Cavum & Morning Glory

  • Cumulonimbus Mamma (Mammatus Pouches)

    Pendant, udder-like lobes hang beneath thunderstorm anvils where ice-laden anvil air sinks into drier sub-cloud air. Sublimation of falling ice crystals cools pockets of air negatively buoyant relative to surroundings. Dramatic sunset grazing light exaggerates pouch relief; mammatus frequently appear on the decaying rear flank after peak storm intensity has already passed.

  • Cavum (Supercooled Glaciation Cascade)

    Thin altocumulus layers at −15 °C to −30 °C frequently consist of pure supercooled liquid droplets lacking ice nuclei. When a climbing or descending aircraft expands air behind propeller tips or wings, localized adiabatic cooling below −40 °C nucleates ice crystals instantaneously. Because saturation vapor pressure is lower over ice than water, surrounding droplets evaporate onto the growing crystals, carving an expanding circular or canal-shaped hole with snowy virga falling down the middle.

  • Volutus Roll Clouds (The Morning Glory)

    Detached horizontal tubes stretching hundreds of kilometers—most reliably observed over Australia's Gulf of Carpentaria near Burketown each September–October—form along colliding sea-breeze solitary atmospheric bores rather than thunderstorm downdrafts.

WMO Classification & Field Reality Check

Cloud Feature WMO Classification Typical Altitude Primary Physical Mechanism Common Photographic Misread
Lenticularis Species (Ac, Cc, Sc) 2,000 – 12,000 m Orographic standing gravity waves Mistaken for solid stationary structures or artificial craft
Mamma Supplementary feature 4,500 – 9,000 m (anvil) Sublimation cooling & negatively buoyant pockets Assumed to mean a tornado is imminent directly overhead
Asperitas Supplementary feature (2017) 1,200 – 3,500 m Ducted atmospheric gravity waves & wind shear Over-processed HDR photos make benign wave bases look apocalyptic
Arcus (Shelf vs. Volutus) Supplementary / Species 300 – 2,000 m Cold-pool outflow lifting warm moist inflow Horizontal scud motion mistaken for vertical tornadic rotation

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