ICAO Weather Vocabulary – Meteorological Phenomena, Hazards & METAR Terms

ICAO Weather Vocabulary

Meteorological hazards and adverse weather conditions represent one of the most critical communication domains in the ICAO Language Proficiency Assessment (Annex 3, Annex 1, and Doc 9835). During oral examinations, language raters evaluate whether pilots and air traffic controllers can accurately describe complex atmospheric phenomena, interpret tactical weather reports (METAR, TAF, SIGMET), negotiate airborne weather diversions, and articulate the aerodynamic hazards of in-flight icing, severe turbulence, and low-level wind shear.

To achieve ICAO Level 4 (Operational), Level 5 (Extended), or Level 6 (Expert), candidates must move beyond simple descriptions like "bad weather" or "heavy storm" to utilize precise aeronautical meteorological terms in plain English. This guide organizes essential weather vocabulary into five primary operational categories: Convective Activity & Thunderstorms, In-Flight & Ground Icing, Turbulence & Atmospheric Dynamics, Visibility & Obscurations, and Standard METAR/TAF Decoding.

ICAO Weather Vocabulary Overview

master technical terminology for cumulonimbus (CB) development, squall lines, microbursts, and gust fronts
understand distinct icing mechanisms: rime ice, clear ice, mixed ice, and Supercooled Large Droplets (SLD)
differentiate atmospheric dynamics: Clear Air Turbulence (CAT), mountain waves, and low-level wind shear
acquire precise vocabulary for visual restrictions: radiation fog, advection fog, haze, mist, and blowing snow
decode standard ICAO meteorological descriptors and abbreviations used in METARs, SPECI reports, and TAFs
practice model responses for ICAO Speaking Part 2 picture descriptions, Part 3 role-plays, and Part 4 discussions

Step 1: Convective Activity, Thunderstorms & Severe Weather

Convective weather systems present severe threats to structural integrity, aerodynamic stability, and engine performance. Use specific technical vocabulary when describing storm phenomena:

Meteorological Term	Operational Definition & Physical Characteristics	Example Sentence
Cumulonimbus (CB)	A vertically developed convective cloud with a fibrous or anvil-shaped top, capable of producing severe updrafts, downdrafts, hail, lightning, and icing.	"The crew requested a 20-mile tactical deviation to the north to skirt isolated cumulonimbus* cells along the arrival route."*
Squall Line	A non-frontal, narrow band of active, multi-cell severe thunderstorms that often develops ahead of a fast-moving cold front.	"A continuous squall line* blocked the oceanic entry gateway, forcing multiple flights into holding patterns."*
Microburst	A severe localized column of sinking air (downdraft) within a thunderstorm that produces intense, divergent wind shear near the surface.	"The aircraft encountered a microburst* on short final, triggering an immediate predictive wind shear escape maneuver."*
Gust Front	The leading boundary of cool, dense air descending from a thunderstorm that spreads rapidly across the surface, causing abrupt wind shifts.	"Surface winds shifted 90 degrees with gusts exceeding 45 knots as the gust front* crossed the aerodrome."*
Hail / Graupel	Precipitation in the form of balls or irregular lumps of ice (hail) or soft, snow-like pellets formed by rime accretion (graupel).	"Encountering severe hail* at FL280 caused structural pitting on the radome and cracked the outer windshield ply."*
Downburst	A strong ground-level wind system emanating from a convective downdraft, categorized as a microburst (≤ 4 km diameter) or macroburst (> 4 km diameter).	"The control tower warned arriving traffic of severe downburst* activity observed on Doppler radar."*

Step 2: In-Flight & Ground Icing Phenomena

Icing directly degrades lift, increases parasite drag, reduces stall margins, and affects engine air intakes. Differentiate between distinct ice accretion types:

Icing Type / Mechanism	Physical Formation & Characteristics	Aviation Hazard & Impact
Clear Ice (Glaze)	Formed by large, supercooled water droplets freezing slowly over the wing surface, creating a smooth, transparent, and dense coating.	Heavy, difficult to break with pneumatic de-icing boots; tends to spread aft of protected leading edges.
Rime Ice	Formed by small, supercooled water droplets freezing instantly upon contact, trapping air pockets to create a rough, milky-white, brittle layer.	Disrupts the laminar boundary layer quickly, leading to rapid drag rise, but is easily removed by pneumatic boots.
Mixed Ice	A combination of rime and clear ice formed when water droplets of varying sizes collide simultaneously with the airframe in stratified clouds.	Produces an irregular, rough surface that severely degrades airfoil camber and high-lift performance.
Supercooled Large Droplets (SLD)	Liquid water droplets existing at sub-freezing temperatures whose diameter exceeds 50 micrometers (freezing drizzle or freezing rain).	Extremely dangerous; flows past heated leading-edge boots and freezes on unprotected control surfaces, creating roll upset risks.
Carburetor / Induction Icing	Ice accumulation inside the fuel-air induction tract or throttle valve caused by vaporization cooling and pressure drop, even at ambient temperatures up to +20°C.	Causes progressive engine power loss or complete engine stoppage in piston aircraft if carburetor heat is not applied.
Tailplane Icing	Ice accretion on the leading edge of the horizontal stabilizer, which can trigger an uncommanded pitch-down tailplane stall when landing flaps are extended.	Requires immediate flap retraction and nose-up pitch recovery; opposite of traditional wing stall recovery.

Step 3: Turbulence & Atmospheric Dynamics

Turbulence descriptions in radiotelephony and oral exams must convey intensity, spatial location, and origin clearly:

Clear Air Turbulence (CAT): Severe, non-convective high-altitude turbulence occurring in cloudless skies, typically associated with strong horizontal/vertical wind shear near the jet stream core or tropopause breaks.
Low-Level Wind Shear (LLWS): A sudden change in wind direction and/or velocity within 2,000 feet of ground level, creating dangerous, rapid energy fluctuations during takeoff or final approach.
Mountain Wave (Orographic Waves): Strong atmospheric oscillations generated on the lee side of mountain ridges when stable, high-velocity winds blow perpendicular to the terrain, often marked by stationary lenticular clouds.
Rotor Waves (Rotor Clouds): Violent, turbulent low-level cylindrical vortices forming beneath the crests of mountain waves, capable of causing severe loss of aircraft control.
Wake Turbulence: Counter-rotating trailing wingtip vortices generated by heavy aircraft generating lift, requiring standardized time or distance separation for trailing departures and arrivals.
Thermal / Convective Turbulence: Irregular vertical air currents caused by solar heating of the Earth's surface, common over plowed fields, runways, and urban areas during daytime VFR operations.
ICAO Weather Scenarios
Aviation Vocabulary Pronunciation
Pilot Decision Making Discussion

Step 4: Visibility Reductions & Atmospheric Obscurations

Restricted visibility requires specific terminology to identify the meteorological suspension or particulate matter:

Phenomenon	ICAO Identifier	Physical Nature & Aviation Implications
Radiation Fog	`FG`	Formed on clear, calm nights when the ground cools rapidly by terrestrial radiation, cooling adjacent moist air to its dewpoint. Disperses with solar heating.
Advection Fog	`FG`	Formed when warm, moist air moves horizontally over a cold land or sea surface; can persist with moderate winds and covers extensive geographical areas.
Mist	`BR`	Microscopic water droplets suspended in the air reducing visibility to between 1,000 and 5,000 meters, with relative humidity ≥ 95%.
Haze	`HZ`	A suspension of extremely fine, dry particles (dust, pollution) in the atmosphere that produces a uniform veiled appearance; relative humidity < 95%.
Blowing Snow	`BLSN`	Snow lifted from the ground by surface winds to an eye level of 2 meters or more, drastically reducing horizontal runway visibility and masking surface markings.
Volcanic Ash	`VA`	Fine abrasive pulverized rock, glass, and mineral particles expelled during volcanic eruptions; causes engine flameout, turbine blade melting, and radome sandblasting.

Step 5: Decoding Standard METAR, SPECI & TAF Descriptors

Aviation language raters frequently ask candidates to verbally interpret raw or summarized METAR reports. Master these standardized ICAO prefixes and abbreviations:

Code	Meteorological Meaning	Code	Meteorological Meaning
`TS`	Thunderstorm	`RA`	Rain
`FZRA`	Freezing Rain	`SN`	Snow
`SHSN`	Snow Showers	`GR`	Hail (≥ 5 mm)
`GS`	Small Hail / Graupel (< 5 mm)	`DZ`	Drizzle
`SQ`	Squall	`CB`	Cumulonimbus
`TCU`	Towering Cumulus	`CAVOK`	Ceiling and Visibility OK
`BKN`	Broken (5–7 oktas)	`OVC`	Overcast (8 oktas)
`VV`	Vertical Visibility (obscured sky)	`WS`	Wind Shear

Step 6: Practical Oral Application & Model Exam Answers

Examine how technical weather vocabulary is structured into high-scoring spoken responses across different exam sections:

Drill 1: Weather Deviation Request (Part 3 Role-Play)

"Zurich Radar, Scandinavian 814, we have an active line of towering cumulonimbus clouds directly on our track ahead with severe radar attenuation. We request an immediate heading deviation thirty degrees right of track for twenty-five nautical miles, maintain flight level three fow-er ze-ro."

Drill 2: Picture Description of Winter De-Icing (Part 2 Picture)

"In the foreground of the photograph, a twin-engine commercial airliner is parked on an illuminated de-icing pad during active freezing precipitation. Two de-icing rigs equipped with extendable hydraulic booms are spraying Type I de-icing fluid across the wings and empennage to eliminate accumulated rime ice, followed by Type IV anti-icing fluid to protect against subsequent accretion during ground taxi holdover time."

Drill 3: Wind Shear Strategy Discussion (Part 4 Discussion)

"When an aircraft encounters low-level wind shear during final approach, the immediate danger is a rapid collapse of indicated airspeed and energy state. If a pilot encounters a severe downdraft or microburst, attempting to salvage the approach is dangerous. The standard response is an immediate go-around, applying maximum go-around thrust, rotating to the target pitch attitude, and maintaining configuration until safely clear of the shear boundary."