Clouds and Precipitation
What to know
Know the ten genera and, more importantly, their aviation significance. Stratiform cloud (stratus, altostratus, nimbostratus) implies stable air, steady precipitation, and layered obscuration; cumuliform cloud (cumulus, cumulonimbus) implies instability, turbulence, icing, and showery precipitation. Cirriform cloud signals high-level moisture and often an approaching warm front. Recognise the difference between drizzle, rain, snow, hail, and ice pellets, and how each forms (ice pellets imply a warm layer aloft over a subfreezing layer below).
Cloud cover reporting uses oktas: FEW (1–2), SCT (3–4), BKN (5–7, ceiling), OVC (8). Ceiling is the lowest BKN or OVC layer. Know how precipitation type and intensity appear in METAR and what freezing rain implies about the vertical temperature profile.
Traps
Questions may treat any cloud as a ceiling, or confuse BKN with SCT thresholds. Another trap is assuming cumulonimbus is required for icing-stratiform cloud near 0 °C produces significant airframe icing too. Do not confuse hail with ice pellets.
Worked example
A METAR reports BKN008 OVC015 with moderate rain. The ceiling is 800 ft; the overcast above is not the ceiling. Expect low-level moisture, poor visibility, and possible icing near freezing levels.