Meteorology — PPL(A)

The troposphere is the layer you fly in — it extends to roughly 36,000 ft in mid-latitudes. Temperature decreases with altitude at the Environmental Lapse Rate (ELR). When ELR exceeds the Dry Adiabatic Lapse Rate (DALR), the atmosphere is unstable; convective cloud and turbulence result.

• ISA sea level: 15°C, 1013.25 hPa, density 1.225 kg/m³.
• ISA temperature lapse rate: 2°C per 1,000 ft (troposphere).
• DALR: 3°C per 1,000 ft (unsaturated air rising).
• SALR: approximately 1.5°C per 1,000 ft (saturated air rising — varies with temperature).
• Tropopause: approximately 36,000 ft at mid-latitudes (lower at poles, higher at equator).

In the northern hemisphere, wind circulates anticlockwise around a low (cyclone) and clockwise around a high (anticyclone). Buys Ballot's Law: stand with the wind on your back, low pressure is to your left. Surface friction backs the wind slightly toward the low.

• Low pressure: ascending air, cloud development, instability, poor weather, strong winds.
• High pressure: descending air, stable conditions, radiation fog risk, inversions.
• Isobars close together: strong pressure gradient, strong winds.
• Isobars far apart: weak gradient, light winds.
• Veering: wind changing clockwise (S → W). Backing: anti-clockwise (W → S).

A front is the boundary between two air masses of different temperature and humidity. Frontal systems are responsible for the majority of UK weather changes and in-flight hazards.

• Warm front: shallow angle (~1:150), approaches with Ci → Cs → As → Ns. Persistent rain ahead, low cloud after passage.
• Cold front: steeper angle (~1:50), approaches quickly. Towering Cb, heavy showers, squalls, rapid clearance after passage.
• Occluded front: cold front catches warm front. Complex weather; properties depend on warm or cold occlusion.
• Warm sector: between warm and cold fronts. Stratus, drizzle, poor visibility, stable.

• Cirrus (Ci): high-level ice, fibrous streaks — first sign of an approaching warm front.
• Cirrostratus (Cs): high-level sheet — halos around sun or moon.
• Altostratus (As): mid-level grey sheet — sun appears as through ground glass.
• Nimbostratus (Ns): thick, dark, continuous rain — instrument conditions.
• Stratocumulus (Sc): low lumpy layer — common UK cloud, generally benign.
• Stratus (St): low featureless layer — poor visibility, drizzle, common in warm sector.
• Cumulus (Cu): vertical development, associated with instability.
• Cumulonimbus (Cb): severe thunderstorm cloud — icing, turbulence, lightning, windshear. AVOID.

Airframe icing is a serious hazard. It adds weight, degrades aerofoil shape, blocks pitot/static sources, and can jam controls. Understanding when and where icing occurs is essential for exam and for safe VFR operation.

• Rime ice: rough, opaque — forms in cloud with small supercooled droplets. Freezing level to −20°C.
• Clear ice: smooth, dense — forms in large supercooled droplets or freezing rain. Most dangerous. Adds most weight.
• Hoar frost: crystalline surface frost — forms below 0°C on ground, not in flight.
• Most icing risk: just above the freezing level in cloud. Temperature −2°C to −20°C.
• Severe icing: Cb, frontal cloud, freezing rain (warm air over-running cold).

A METAR is a routine aerodrome weather observation. Decode left to right: type, ICAO, time, wind, visibility, weather, cloud, temperature/dewpoint, QNH, trends/remarks.

• METAR EGLL 121250Z — EGLL, 12th at 12:50 UTC.
• Wind: 24015KT — 240° true at 15 kt. Gusting: 24015G25KT.
• Visibility: 9999 — 10 km or more. 0600 — 600 m.
• Weather: -RA (light rain), +TSRA (heavy thunderstorm with rain), BR (mist), FG (fog), RASN (rain and snow).
• Cloud: FEW018 BKN030 — few at 1,800 ft, broken at 3,000 ft. OVC = overcast.
• Temperature/Dewpoint: 12/09 — +12°C, dewpoint +9°C.
• QNH: Q1013 — altimeter setting 1013 hPa.
• CAVOK: visibility ≥10 km, no cloud below 5,000 ft or MSA, no significant weather.

A TAF is an aerodrome forecast, typically 9 or 24 hours. Groups indicate changes: BECMG (gradual change), TEMPO (temporary, less than half the period), PROB30/40 (probability).

• BECMG 1416: gradual change between 14:00 and 16:00 UTC.
• TEMPO 1820: temporary condition between 18:00 and 20:00 (each occurrence < 60 min).
• PROB30 TEMPO: 30% probability of temporary condition — treat as possible, not probable.
• PROB40: 40% probability. PROB40 TEMPO is more likely and must be planned against.
• FM1400: from 14:00 UTC — replaces all previous conditions.