Thunderstorms & icing

050 MeteorologyChapter 6

Thunderstorm requirements

Key fact

Three ingredients: unstable atmosphere (ELR above SALR through great depth), high humidity, lifting trigger (convection, orographic, frontal, convergence, insolation on col/low). Air-mass TS: isolated, afternoon land, hardest to forecast. Frontal/squall line: most difficult to avoid; fastest moving; line ahead of cold front most destructive.

Lifecycle

Stage	Duration	Characteristics
Cumulus/development	15–20 min	Updrafts only; CU → TCU
Mature	20–30 min	Updrafts + downdrafts; precipitation starts; gust front ~25 NM ahead
Dissipating	30 min – 3 h	Downdrafts dominate; anvil visible

Key fact

TS moves with 700 hPa wind. Icing possible below −23°C in mature stage. Lightning most likely ±5000 ft around freezing level (−10°C to +10°C). Radar reflectivity ∝ droplet size/number → turbulence severity indicator.

Hazards

Key fact

Microburst: downdraft under 4 km width, ~50 kt typical (max ~100 kt), 4 km in 5 min; headwind→tailwind shear on approach. Gust front: cold outflow undercuts inflow. Hail ground to FL450 in CB. Tornado: CB-associated; 100–150 m diameter, 200+ kt funnel, moves 20–40 kt, spring/summer US mid-west.

Key fact

TS avoidance (AWR): 10 NM below FL200; 20 NM above FL200; visual 10 NM any level; overfly by ≥5000 ft if unavoidable. Avoid anvil — hail/lightning/turbulence. Cells: 5 NM GND–FL200, increasing with altitude to 20 NM above FL300.

Icing fundamentals

Key fact

Supercooled water droplets (SCWD): liquid below 0°C until contact. Airframe icing needs below 0°C and visible moisture (exceptions: freezing rain, hoar frost). Highest risk 0°C to −10°C; SCWD range 0°C to −40°C (mostly large droplets 0 to −10°C).

Icing types

Type	Droplet	Appearance	Severity
Rime	Small	Opaque, white, brittle	Light/mod
Clear/glaze	Large (FZRA)	Transparent, heavy	Mod/severe
Mixed	Both	Rough, opaque	Mod/severe
Hoar frost	Sublimation	Thin crystals, clear air	Lift reduction

Key fact

Freezing rain ahead of warm front — most hazardous clear ice, ~12 mm/min possible; book escape: turn back. Hoar frost: aircraft below 0°C enters warm moist air; surface temp below dew point below 0°C.

Icing by cloud type

Key fact

Cu/Cb: mod–sev 0°C to −23°C. Ns: clear 0 to −10°C, rime −7 to −13°C. St/Sc/As: rime 0 to −15°C, light–mod. Orographic lifting greatly increases icing risk. Descending into moist layer worse (cold airframe).

Key fact

Carburettor icing: RH over 30%, −5°C to +30°C (severe −20 to +15°C in cloud/precip). Engine intake/turbine icing to +10°C at high RPM in cloud/fog.

Exam trap

Moderate icing → change course/altitude desirable. Severe → change immediately or divert. Stratocumulus near 0°C surface with cold tops → severe icing aloft. Stall warning may not activate with ice — VS increases beyond calibrated warning.