After the module's violent chapters, three quieter hazard families that catch crews precisely because they are quiet: the inversion that ambushes both ends of a flight, the stratosphere whose smooth cruise comes with fine print, and the mountains — which do not invent a single new hazard, but amplify every one this module has taught.
The inversion at both ends of the flight
The syllabus asks for a comparison, so here it is. A strong inversion alone at takeoff or approach is mostly a nuisance of two kinds: the stable air below the lid traps haze and fog, degrading visibility, and crossing the lid brings a sudden temperature jump that changes aircraft performance: climbing into abruptly warmer, thinner air saps thrust and climb gradient, while a cold pool under the lid makes the altimeter flatter the true altitude, the atmosphere module's cold-air trap. The air itself, though, is calm. The same inversion combined with marked wind shear (calm decoupled air below, a strong wind or low-level jet sliding over the lid, the wind-shear lesson's night trap) is a different animal entirely: a sudden airspeed loss or gain moments after takeoff or on short final. The comparison the exam wants:
inversion alone — a performance and visibility problem;
inversion plus shear — an acute control and energy hazard.
Why airliners love the stratosphere
The atmosphere module put the tropopause near FL 360 in mid-latitudes; cruise above it and the flight moves into the weatherless attic. The advantages the syllabus wants summarised:
flight above the weather — no convective cloud, no storms;
air so dry that airframe icing all but disappears;
generally smooth conditions;
steady, predictable temperatures;
high true airspeeds in the thin air.
Together they are the reason the long-haul cruise lives there whenever the aircraft can reach it.
The lower stratosphere's terms and conditions
The attic has its own rules, and the syllabus lists them by variable:
Wind — the wind maximum sits at or just below the tropopause, in the jets; above it the wind generally decreases with height in the lower stratosphere;
Temperature — near-constant with height (about −56.5 °C in mid-latitudes), warming only slowly far above; but over the tropics the tropopause runs near −80 °C, cold enough to matter for fuel temperature and performance;
Air density — very low: high TAS for a given IAS, reduced engine thrust, and a narrowing margin between the low-speed and high-speed limits as the aircraft approaches its aerodynamic ceiling;
Turbulence — mostly smooth, with two exceptions: CAT near the tropopause breaks and jets, and mountain waves that propagate up into the stratosphere.
Mountains and fronts
A mountain range treats a crossing front the way a groyne treats a wave: the front is retarded and deformed: the cold air dams against the windward side, and the frontal cloud and precipitation intensify and are prolonged there — while the lee side receives a delayed, drier, often foehn-flavoured passage, the front sometimes accelerating again as it spills downslope. For planning, the rule of thumb: a front plus a range means worse and longer weather on the windward side than the chart's timing suggests.
The mountain's vertical machinery
The wind module built the machinery; here it is worn as a hazard. Mountains mean strong updraughts on the windward slopes and downdraughts on the lee that, near ridge height, can exceed an aircraft's climb capability; rotors turning beneath the wave crests, mountain waves running far downstream and far above the ridge; and the small print — gap and venturi accelerations, valley wind systems, and severe low-level shear and turbulence wherever the flow meets the rock. The sketch the exam wants is the wind module's own: