Common exam traps

In no particular order, things students reliably get wrong:

• Oxygen percentage with altitude. It's 21% throughout the troposphere. Hypoxia is about partial pressure (Dalton), not concentration.
• Tropopause height. Higher at equator, lower at poles; higher in summer, lower in winter. Often phrased to catch you out with "increases" vs "decreases".
• Lapse rate distinctions. ELR is what's actually out there. DALR is what a dry rising parcel does. SALR is what a saturated rising parcel does. Mix them up and the stability question is wrong.
• Stability conditions. Stable when ELR < SALR. Unstable when ELR > DALR. Conditionally unstable in between. Easy to flip the inequalities under exam pressure.
• Wind backing vs veering. Backing is anticlockwise (270 → 240). Veering is clockwise (270 → 300). Surface wind backs relative to gradient wind (NH) due to friction. Cold front passage veers the wind (NH).
• Buys Ballot's Law hemisphere. Back to wind, low pressure on left in NH; on right in SH.
• Cloud sequence at a warm front. Ci → Cs → As → Ns. The lowest comes last, as the front approaches.
• Carb icing temperature range. Up to +25°C in humid conditions. Many students assume only below freezing.
• Most dangerous airframe icing range. 0°C to about −20°C. Not the coldest temperatures.
• High-to-low altimeter trap. Flying into lower pressure (or lower temperature) without resetting → you are lower than indicated. Mnemonic: "high to low, look out below."
• Strong wind across a ridge. Bernoulli effect → altimeter over-reads → you are lower than indicated.
• METAR cloud bases. Reported in hundreds of feet AGL above the aerodrome, not AMSL. SCT025 is 2500 ft above the aerodrome.
• Mountain wave conditions. Stable air, wind ≥20 kt at right angles (within ≈30°) to the ridge, increasing with height. All three required.
• Microbursts on approach. First effect is often an increased headwind (gain in airspeed) before the downdraft and tailwind hit you. Don't get fooled into reducing power early.