Clouds & precipitation

050 MeteorologyChapter 3

Cloud levels & types

Level	Base height	Composition
Low	SFC – 6500 ft	Water droplets — St, Sc, Ns
Medium	6500 – 23 000 ft	Mixed — As, Ac
High	16 500 – 45 000 ft	Ice — Ci, Cs, Cc

Key fact

10 genera: Ci, Cc, Cs, Ac, As, Ns, Sc, St, Cu, Cb. Stratiform = stable, layered, large horizontal extent. Cumuliform = unstable, vertical development. Ci — wispy; Cs — halo; Ac lenticularis — mountain waves; Cb — anvil, TS/hail.

Cloud coverage (METAR)

Code	Octas
SKC	0
FEW	1–2
SCT	3–4
BKN	5–7
OVC	8

Key fact

Cloud base: height of base above reference (usually airfield). Cloud ceiling: lowest BKN/OVC layer covering ≥50% sky below 20 000 ft.

Six cloud triggers

Key fact

1. Convection — unstable, daytime heating → Cu/Cb
2. Orographic — forced ascent over terrain
3. Turbulence — stable layer mixing (St/Sc)
4. Convergence — troughs, fronts
5. Frontal — warm = stable stratiform; cold = cumuliform
6. Thin-layer lifting — unstable sandwich → AC/CC lenticular

Formula — Cloud base (approximate)

Base (ft) ≈ (Temperature − Dew point) × 400

Use DALR below cloud base; SALR within/above cloud.

Turbulence clouds (St/Sc)

Key fact

Requires stable atmosphere (ELR below 1.8), wind ≥ 8 kt, dew point within turbulent layer. Mixing drives effective lapse toward DALR. Inversion above turbulence layer — smooth above cloud. Icing: light rime possible if cloud above freezing layer.

Bergeron & coalescence

Key fact

Bergeron–Findeisen (ice crystal): saturation vapour pressure over water exceeds ice → vapour sublimates onto ice crystals; mixed-phase clouds; produces rain or snow depending on temperature profile.

Key fact

Coalescence: droplet collision in clouds above 0°C without ice; mid-latitudes often drizzle only. Greater cloud depth → larger drops.

Precipitation types

Type	Characteristics	Typical cloud
Drizzle (DZ)	0.2–0.5 mm	St, Sc
Rain (RA)	0.5–5.5 mm	Ns, As, Cu, Cb
Freezing rain (FZRA)	Supercooled; clear ice risk	Warm front, Ns aloft
Snow (SN)	Branched crystals	Ns, As, thick clouds
Ice pellets (PL)	Frozen raindrops	Indicates FZRA aloft
Hail (GR)	Cb, continental mid-latitudes	Cb
Virga	Evaporates before surface	Any

Key fact

Continuous precipitation ≥60 min — stratiform (Ns, As). Showers — cumuliform, short. After rain shower: temperature drops, dew point rises (cold air from above, humidity increase).

Exam trap

Cirrostratus gives halo; altostratus does not. Ns blocks sun completely; continuous rain/snow. Cu humilis → mediocris → congestus → Cb with increasing instability and surface heating.