Ch 11 Quiz: Mountain Waves & Turbulence — CAT & Orographic Effects

Mountain wave conditions: stable layer at mountain-top height, wind speed ≥ 15 kt, wind direction within 30° of perpendicular to ridge. Stable air oscillates downwind.

Any wind blows over a mountain

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Stable air flows over a mountain ridge with wind speed ≥ ~15 kt and wind direction within 30° of perpendicular to the ridge

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Unstable air rises over terrain

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Temperature inversion exists at sea level

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Rotor: turbulent cylinder of rolling air beneath the standing wave crests on the lee side. Contains severe to extreme turbulence and can cause structural damage.

Above the mountain top

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Below the wave crests on the lee side, roughly at mountain-top height — region of extreme turbulence and possible reversed flow

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Upwind of the mountain

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Above the tropopause

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Lenticular (lens-shaped) clouds form at wave crests when air is moist enough to condense there. Appear stationary — continuously forming on windward side and dissipating on lee side.

Mountain tops

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The crests of standing waves — stationary because air flows through them

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The rotor zone

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Below the tropopause always

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Banner cloud: plume of cloud streaming downwind from a sharp mountain peak (e.g. Matterhorn). Caused by reduced pressure (lee eddy) drawing moist air upward on the downwind side to condense.

Is a lenticular cloud

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Is a cloud streaming downwind from a peak, formed by turbulent eddying and orographic lifting on the lee side

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Indicates no turbulence

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Is the same as a rotor cloud

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ICAO turbulence: LIGHT, MODERATE, SEVERE (large abrupt changes, aircraft momentarily out of control, structural damage possible), EXTREME (violent beyond control, structural damage likely).

Aircraft momentarily out of control with possibility of structural damage

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Large abrupt changes in altitude/attitude; aircraft momentarily out of control; occupants thrown violently against straps — structural damage possible

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Light meal service disrupted

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Minor altitude excursions

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CAT: turbulence in cloudless regions, mainly near jet stream cores (cyclonic shear side), upper-level fronts, and mountain waves. No visual warning — detected by PIREPs.

In the boundary layer

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Near jet stream cores and upper-level fronts — no cloud visible

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Inside Cb clouds

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Only in winter

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In mountain wave, altimeter gives primary altitude reference. However, strong wave can cause rapid altitude changes requiring immediate power/attitude response. GPS altitude is a useful cross-check. VSI may lag significantly.

Altimeter only (pressure altitude)

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Careful use of power and attitude — the altimeter may lag; rely on GPS altitude as a cross-check

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GPS only

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VSI only

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METAR does not include turbulence codes. Turbulence is reported by pilots via PIREP: UA /OV …/TM …/FL …/TP …/TB (LGT/MOD/SEV/EXTM). Passed to other pilots via ATIS, ACARS, and datalink.

Only for moderate CAT

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PIREPs using the UA report format — METAR itself does not include turbulence codes

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Automatically by METAR sensors

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In every METAR

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Mechanical turbulence: wind-flow disruption by surface roughness creating eddies in the boundary layer. Intensity increases with wind speed and surface roughness. Hazardous during approach/landing in gusty crosswinds near buildings.

Temperature inversions

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Surface roughness, obstacles, and buildings disrupting smooth airflow — common in the boundary layer on windy days

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Mountain waves only

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Frontal activity

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Cb turbulence: most extreme encountered. Adjacent updrafts (50-100+ kt) and downdrafts create violent differential loads on an airframe. NEVER enter a Cb.

Wind shear only

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Violent up and down drafts — updrafts may exceed 100 kt adjacent to downdrafts of similar magnitude

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Temperature inversion

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Jet stream proximity

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Standing (stationary) wave: wave pattern anchored to mountain topography. Air moves through the wave pattern but crests/troughs remain fixed in space as long as flow conditions persist.

They do not move vertically

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They remain stationary relative to the mountain while air flows through them — unlike progressive waves

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They only form in summer

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They are always low-level

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Turbulence penetration: reduce to VRA/VB which provides best structural margin for gusts while maintaining controllability. Maintain level attitude — don’t chase altitude. Report to ATC.

Always fly through it at high speed

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Plan to avoid or request a deviation; if unavoidable, fly at turbulence penetration speed (VB/VRA), ensure seat belts are fastened

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Increase speed beyond VNE

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Descend to FL100 always

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Mountain wave wavelength: typically 10-20 km (range 5-35 km), determined by the Scorer parameter. Aircraft often encounter multiple crests sequentially.

1-2 km

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5-35 km (typically 10-20 km), depending on stability and wind speed

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100 km

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0.5 km

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PIREP turbulence: NEG, LGHT, MOD (moderate – harder to walk, loose objects move), SEV (severe – unable to walk, possible injury), EXTRM (aircraft damaged). MOD requires passenger seat belt use.

Mild bumps, no attitude change

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Moderate turbulence: abrupt changes in altitude/attitude, slight difficulty controlling aircraft — unsecured objects displaced, meals disrupted

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Aircraft out of control

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Only in cloud

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Mountain wave strongest when: high-speed, stable flow crosses the ridge. Thermal convection by day can disrupt stable waves. Activity often peaks in winter. Nocturnal cooling re-establishes stability.

No variation

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Typically strongest in mid to late afternoon when convective heating reduces stability, weakest at night when stable cooling re-establishes

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Strongest at midnight

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Same all day

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Turbulence measurement: operationally via PIREP intensity scale. Instrumentally: EDR — a measure independent of aircraft weight/speed. EDR is increasingly used for automated aircraft turbulence reporting (ACARS).

Barometric pressure change

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Gust velocity, vertical gust, g-load, or pilot assessment — EDR (Eddy Dissipation Rate) increasingly used for automated aircraft reporting

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Temperature change

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Wind direction change

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CAT altitude: predominantly upper troposphere/lower stratosphere, FL270-FL430. The cruise altitude band for transport-category aircraft. Lower CAT also occurs near mountain wave systems.

FL100

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FL270-FL430 — predominantly near jet stream cores and upper frontal zones

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Below FL150

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In the tropics only

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Richardson Number (Ri): ratio of buoyancy to shear. Ri < 0.25 = K-H instability → CAT. Ri < 1.0 = potentially unstable. Used in NWP models to diagnose CAT probability.

Rainfall intensity

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Atmospheric stability and wind shear turbulence potential — Ri < 0.25 indicates Kelvin-Helmholtz instability and possible CAT generation

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Cyclone intensity

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Fog formation

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K-H instability: when vertical wind shear overcomes buoyancy stabilisation (Ri < 0.25), the interface becomes unstable. Gravity waves break like ocean waves, producing billow clouds (Ac/As undulatus) and CAT.

Surface wave instability

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Shear instability at the interface between two layers with different wind speeds — produces breaking gravity waves (billows) that cause CAT

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Thermal convection

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Ocean-atmosphere coupling

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Mountain wave forecasting: radiosonde wind profile, stability profile, NWP model output. SIGMET (MTW) issued for severe mountain wave. PIREPs provide real-time confirmation.

METAR only

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Rawinsonde data (wind and stability profiles above the mountains), NWP models, and PIREPs — MWOs issue SIGMETs for severe mountain wave

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Surface wind only

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Radar only

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Billow clouds: wavy, parallel bands at a single level. Breaking waves indicate turbulence. Smooth undulatus = mild shear. Distinct breaking billow pattern → MOD turbulence.

Radiation fog onset

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Kelvin-Helmholtz instability — alternating crests and troughs of cloud in a shear layer, indicating possible moderate turbulence at that level

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No aviation hazard

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Tropical convection

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Mountain crossing: cross perpendicular to ridge at maximum altitude; increase altitude if possible to reach smooth wave zone above rotor; avoid flying at mountain-top level (rotor zone).

Fly through the known rotor zone quickly

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Cross the mountain range at right angles and at the highest safe altitude, avoiding the rotor zone below mountain-top height

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Fly parallel to the ridge

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Descend to low level to avoid waves

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SIGMET MTW (Mountain Wave): issued when severe or extreme turbulence due to mountain wave is forecast. SEV MTW = severe mountain wave. Pilots must deviate or accept the risk with full awareness.

Strong medium-turbulence wind

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Severe Mountain Wave — a SIGMET issued when severe turbulence is expected due to mountain wave activity in the defined area

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Surface mountain wind

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Slight mountain weather

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Trapped waves: energy confined to lower atmosphere, associated with strong turbulence and cloud (lenticular, rotor). Non-trapped (vertically propagating) waves: energy propagates upward to stratosphere/mesosphere.

No operational difference

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Trapped waves have energy confined to the lower troposphere (most significant for aviation); non-trapped waves can reach the stratosphere

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Non-trapped always more severe

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Trapped only at night

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Jet stream CAT avoidance: equatorward (anticyclonic shear) side of jet is smoother. If PIREP reports MOD/SEV CAT at FL350, request FL370 or FL330 and offset equatorward.

Fly faster

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Offset flight track equatorward (anticyclonic shear side — smoother) or request a flight level change to avoid the shear zone

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Descend to FL100

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Fly at Mach 1+

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CAT beyond cloud boundaries: the turbulent shear zone extends both within and beyond associated cloud features (Ci/Cc at frontal zones, lenticular cloud near wave crests). Exiting cloud may still be within the CAT zone.

Humidity changes

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In CAT zones, the turbulence field extends beyond cloud boundaries — exiting cloud does not guarantee smooth air

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Temperature drops

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Pressure increases

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Wave window: a clearance from ATC to fly through controlled airspace to access smooth wave lift above the rotor, or the altitude band where wave lift is accessible with controlled conditions.

A gap in regulations

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A period or altitude band in which mountain wave conditions are favourable for gliding — used to access strong wave lift above rotor zone

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A weather forecast product

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An ATC clearance

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Structural risk: at speeds above VB in severe turbulence, gust loads may exceed the aircraft’s design limit load factor. Transport aircraft typical limit: +2.5g to -1g for gust-load calculations.

Light turbulence

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Severe or extreme turbulence — especially if encountered at a speed above VB (turbulence penetration speed)

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Moderate turbulence

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Light chop

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Foehn pause: temporary cessation of the Fohn wind when cold air pooled in the valley pushes back against the warm descending Fohn air. Rapid temperature, humidity, and wind changes in alpine valley airports (Innsbruck, Kathmandu).

End of Fohn wind

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A temporary interruption of the Fohn wind as the cold pool temporarily re-invades the valley — warm-cold boundary oscillates

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Start of katabatic wind

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Sea breeze interruption

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DALR: 3C/1000 ft. At 3000 ft: 20 – (3×3) = 20 – 9 = 11C. Unsaturated (dry) rising parcel. If colder than environment at 3000 ft, the parcel is stable. If warmer, it is unstable and will continue to rise.

11C

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20 − (3 × 3) = 11C

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17C

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14C

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Ch 11 Quiz: Mountain Waves & Turbulence — CAT & Orographic Effects

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