PPL Meteorology cheat sheet

ISA sea-level values: temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³; lapse rate 1.98 °C/1000 ft (6.5 °C/km) up to 11 km (36 090 ft), then isothermal to 20 km.

QNH is altimeter setting referenced to mean sea level; QFE is station pressure at aerodrome elevation; QNE is the standard setting 1013.25 hPa used above the transition altitude.

Altimeter over-reads by about 4 ft per 1 °C of ISA deviation per 1000 ft above the station; flying from high to low pressure or high to low temperature without resetting makes the true altitude lower than indicated.

Density altitude rises roughly 120 ft per 1 °C above ISA at a given pressure altitude; high density altitude lengthens takeoff roll, reduces climb rate and lowers engine/rotor performance.

Standard atmosphere pressure at altitude: 5000 ft ≈ 843 hPa, 10 000 ft ≈ 697 hPa, 18 000 ft ≈ 506 hPa, 34 000 ft ≈ 250 hPa; pressure halves approximately every 18 000 ft.

Relative humidity = (actual vapour pressure / saturation vapour pressure) × 100 %; dew point is the temperature to which air must be cooled at constant pressure to reach saturation.

Lapse rate names: DALR 3 °C/1000 ft (dry air), SALR about 1.5 °C/1000 ft (saturated), ELR the actual environmental gradient; instability when ELR > SALR, stability when ELR < SALR.

Cloud base from surface temperature and dew point spread: base (ft) ≈ (T − Td in °C) × 400, valid for convective cumulus over flat terrain.

Coriolis force is zero at the equator and maximum at the poles; in the Northern Hemisphere it deflects moving air to the right, in the Southern Hemisphere to the left.

Geostrophic wind flows parallel to straight isobars with Coriolis balancing the pressure-gradient force; surface friction turns wind about 30° (land) to 45° (sea) across isobars toward low pressure.

Buys Ballot law (Northern Hemisphere): with your back to the wind, low pressure lies on your left; in the Southern Hemisphere, on your right.

Wind speed scales with pressure gradient: closely spaced isobars mean strong wind; gradient wind around a low is reduced by cyclonic curvature, increased around a high.

Föhn/warm downslope wind: air cools at DALR on the windward side, loses moisture, then warms at SALR on the leeward side, giving a warmer, drier wind at the surface.

Anabatic (upslope) winds blow by day on sun-facing slopes; katabatic (downslope) winds flow at night by radiative cooling, strongest in stable air and in valleys.

Sea breeze blows from sea to land by day as land heats faster; land breeze reverses at night; both extend typically 20–50 km inland and reach 10–20 kt.

Jet streams form near the tropopause at the polar-front and subtropical troughs; the polar-front jet may exceed 200 kt and produces CAT and wind shear at its edges.

Cloud families by base: high (cirrus, cirrostratus, cirrocumulus, above 16 500 ft), middle (altostratus, altocumulus, 6500–16 500 ft), low (stratus, stratocumulus, 0–6500 ft), plus vertically developed cumulus and cumulonimbus.

Stable air produces layer clouds (stratus, stratocumulus, altostratus, nimbostratus) with steady precipitation; unstable air produces cumuliform clouds with showery precipitation and turbulence.

Fog types: radiation fog forms in clear, calm nights with high humidity; advection fog forms when warm moist air moves over a cold surface and needs wind; upslope fog forms as air is forced up terrain.

Visibility thresholds: fog when visibility < 1000 m; mist when 1000 m to 5 km; haze is dry particulate obscuration with high humidity below saturation.

Cumulonimbus hazards: microbursts, hail, icing above the freezing level, lightning, severe turbulence and wind shear; avoid by at least 20 km from the cell and do not fly beneath the anvil.

Icing types: clear (glaze) ice forms in large supercooled drops at 0 to −10 °C and is heaviest; rime ice forms in small droplets at −10 to −20 °C; mixed ice combines both.

Carburettor icing risk is highest at ambient temperatures between −7 °C and +21 °C with relative humidity above 50 %; it can occur in clear air and at cruise power settings.

Airframe ice certification: flight into known icing requires approved protection; the freezing level and 0 °C isotherm must be checked against planned levels and temperatures aloft.

Air masses: mT (maritime tropical, warm moist, unstable), cT (continental tropical, hot dry), mP (maritime polar, cool moist), cP (continental polar, cold dry); source regions are oceans and continents.

Cold front: steep slope (1:50 to 1:100), fast movement, narrow band of heavy showery precipitation, squall lines, rapid wind veer and temperature fall after passage.

Warm front: shallow slope (1:100 to 1:200), slow movement, wide band of steady precipitation ahead, gradual temperature rise, wind veer, and poor visibility in warm sector.

Occluded front forms where a cold front overtakes a warm front; the surface position of the warm front lies aloft, giving complex cloud sequences and continuous precipitation.

Frontal passage signs: pressure trough at the surface front, wind veer (Northern Hemisphere), temperature change, dew point change, and cloud/precipitation pattern shift.

Troughs of low pressure give poor weather and convergence; ridges give subsidence, stable air, clear skies and smooth flying; cols lie between two highs and two lows.

Tropical revolving storms (hurricanes, typhoons, cyclones) form over oceans with sea temperature above about 27 °C, between 5° and 20° latitude, and do not form at the equator.

Squall line: a line of active thunderstorms ahead of or along a cold front, producing strong gusty winds, heavy rain, hail and severe turbulence; avoid horizontally by at least 20 nm.

METAR is a routine aerodrome observation, issued typically hourly or half-hourly; SPECI is a special report issued on significant change; both use ICAO coded groups.

METAR wind group: 27015KT means 270° true at 15 kt; 27015G25KT gusting; VRB03KT variable; 27015KT 240V300 gives variable direction range.

METAR visibility is in metres (9999 = 10 km or more); RVR is given as R28/0400 with trend; weather groups use +/− for intensity, e.g. +TSRA, −SHRA, BCFG, FZRA.

Cloud groups: FEW (1–2 oktas), SCT (3–4), BKN (5–7), OVC (8); VV002 means vertical visibility 200 ft in obscured sky; NSC/NCD means no significant cloud.

TAF is a forecast for a aerodrome, valid typically 24–30 h; TEMPO means temporary fluctuations (usually less than 1 h each, total less than half the period); BECMG means gradual change over up to 2 h.

TAF change groups: FM (from, instantaneous change), TEMPO, BECMG, PROB30/PROB40 (probability of condition); valid period is given as DDHH/DDHH in UTC.

Significant weather charts: SIGWX and significant weather low-level charts show CAT, icing, CB, turbulence, jet streams, tropopause height and fronts; GAMET is a regional area forecast for low-level flights.

Wind and temperature aloft forecasts are issued for standard levels (e.g. FL050, FL100, FL180, FL300, FL340) in UTC, giving direction, speed and temperature for use in flight planning.

Wind shear is a change of wind speed and/or direction over a short distance; low-level shear on approach is critical below 1000 ft AGL and may cause sudden airspeed loss or gain.

Microburst: a downdraft under a cumulonimbus reaching the surface in under 5 minutes, spreading out less than 4 km, with headwind then tailwind shear; escape by go-around and avoid the cell.

Wake turbulence vortices sink below and drift with the wind; stay above and upwind of a heavier aircraft's flight path and wait about 2–3 minutes for landing behind a heavy.

Thunderstorm life cycle: cumulus (updrafts only), mature (updrafts and downdrafts, rain, lightning, hail), dissipating (downdrafts dominate); the mature stage holds the greatest hazard.

Lightning: avoid flight within 5 nm of a thunderstorm radar echo at any level; do not launch or land when a thunderstorm is within about 5 nm of the aerodrome.

Mountain waves form in strong stable cross-barrier flow, producing lenticular clouds, rotor turbulence and severe downdrafts on the lee side; expect strong vertical shear and possible icing.

Volcanic ash and sandstorms damage engines and pitot/static systems; ash is detectable on weather radar poorly, so PIREPs and SIGMETs are the primary warning.

SIGMET is issued for phenomena hazardous to all aircraft: severe turbulence, severe icing, volcanic ash, tropical cyclones, severe squall lines and radioactive release; AIRMET covers lighter hazards for low-level flights.

ISA deviation is the difference between actual and ISA temperature at a level; temperature above ISA means the aircraft is higher than the altimeter indicates when flying a constant indicated altitude.

True altitude correction: approximately 4 ft per 1 °C ISA deviation per 1000 ft above the reporting station; cold temperatures give true altitude lower than indicated.

Pressure altitude = indicated altitude with altimeter set to 1013.25 hPa; density altitude = pressure altitude corrected for non-standard temperature.

Transition altitude/level: below the transition altitude use QNH (local setting); above the transition level use standard 1013.25 hPa; the transition layer lies between them.

Cruising level semicircular rule depends on magnetic track and, in some regions, on QNH; verify the local AIP for the correct table and any regional exceptions.

Takeoff and landing performance must be computed from the reported QNH/QFE, temperature and wind, not from ISA assumptions, because density altitude changes the true airspeed for a given IAS.

Freezing level and 0 °C isotherm forecasts are used to plan flight levels and icing avoidance; temperatures aloft forecasts give the temperature at each standard level.

Wind at altitude is used to compute drift and ground speed; the wind triangle: heading = track − wind correction angle, and ground speed = TAS ± wind component along track.