NCAA CPL Meteorology cheat sheet

ISA at mean sea level: pressure 1013.25 hPa, temperature 15 °C, density 1.225 kg/m³; temperature falls at 6.5 °C/km to the 11 km tropopause, where T ≈ −56.5 °C and p ≈ 226 hPa.

Tropopause height varies with latitude: about 8 km over the poles, 11 km in mid-latitudes, 16–18 km over the equator. The troposphere holds roughly 80 % of the atmospheric mass and almost all water vapour.

Subscale settings: QNH makes the altimeter read aerodrome elevation on the ground, QFE reads height above the aerodrome reference point, and QNE (1013.25 hPa) is used at and above the transition level.

1 hPa corresponds to about 27 ft near mean sea level and about 30 ft at higher levels; en-route level separation is therefore 1 000 ft = 34 hPa only in the lower atmosphere, never in the standard-pressure band.

Pressure altitude = elevation + 30 × (1013 − QNH) feet (approximation). It is the altitude in the ISA at which the actual pressure occurs.

Density altitude is pressure altitude corrected for temperature. It increases with high temperature, high humidity and low pressure, and reduces lift, thrust, propeller efficiency and climb gradient.

Flying from high pressure into low pressure, or from warm into cold air, without resetting the subscale leaves the aircraft lower than the altimeter indicates - the classic terrain-clearance trap.

Temperature altimeter correction rule of thumb: true altitude error ≈ 4 ft per 1 000 ft above the altimeter setting source per 1 °C of ISA deviation; cold-air corrections are applied on approach in mountainous or harmattan-cooled conditions.

Dry-air composition by volume: nitrogen ≈ 78 %, oxygen ≈ 21 %, argon ≈ 0.9 %, carbon dioxide ≈ 0.04 %; water vapour varies from near zero to about 4 % and is the gas that drives weather.

Lapse rates: dry adiabatic 9.8 °C/km ≈ 3 °C/1 000 ft; saturated adiabatic ≈ 5.4 °C/km ≈ 1.5 °C/1 000 ft but ranges 4–9 °C/km; average environmental lapse rate ≈ 6.5 °C/km ≈ 2 °C/1 000 ft.

Stability rules: ELR > DALR = absolutely unstable; DALR > ELR > SALR = conditionally unstable; ELR < SALR = absolutely stable; an isothermal layer or inversion is very stable and traps haze, dust and smoke.

Lifting condensation level ≈ 125 m (400 ft) per 1 °C of surface dewpoint depression (temperature minus dewpoint); this gives convective cloud base and the height where the SALR replaces the DALR.

Relative humidity = (vapour pressure ÷ saturation vapour pressure) × 100. It rises when air cools at constant pressure or when moisture is added, without any change in absolute moisture content.

Latent heat: vaporisation ≈ 2.5 MJ/kg, released on condensation and absorbed on evaporation; fusion ≈ 334 kJ/kg. This release is what makes saturated ascent warmer than unsaturated ascent.

CAPE is the positive area between the level of free convection and the equilibrium level; CIN is the negative area below the LFC. Large CAPE with small CIN favours deep convection.

Potential temperature θ is conserved in unsaturated adiabatic ascent; equivalent potential temperature θe is conserved in saturated ascent, so θe is the best single indicator of air-mass instability.

Diurnal cycle: surface heating steepens the low-level lapse rate and destabilises the air; nocturnal radiative cooling builds inversions, stabilises the boundary layer and produces low-level wind shear.

Cooling to saturation occurs by lifting (orographic, frontal, convergence, convection), by radiation, or by advection over a colder surface such as the monsoon air moving over a cool land or sea surface.

Cloud genera grouped by base height (tropical values are higher than mid-latitude): high - cirrus, cirrostratus, cirrocumulus above about 6 km (20 000 ft); middle - altostratus, altocumulus 2–6 km; low - stratus, stratocumulus, nimbostratus, cumulus below 2 km; cumulonimbus spans all levels.

Precipitation character: nimbostratus gives continuous steady rain or snow; cumulus and cumulonimbus give showery precipitation; stratus gives drizzle; cirrus gives no significant precipitation, though virga may be visible below.

Cumulonimbus: deep vertical development, glaciated anvil, and associated severe turbulence, strong icing, hail, lightning, microbursts and gust fronts; West African monsoon tops frequently reach 12–15 km.

Altocumulus lenticularis indicates standing mountain-wave flow and strong wind at cloud level; it is a marker of mechanical turbulence and downdrafts on the lee side of high ground.

Drizzle drops are smaller than 0.5 mm, rain drops 0.5 mm or larger, hail 5 mm or more (reported GR), small hail or snow pellets GS, ice pellets PL; freezing rain is rain that freezes on impact.

Fog is reported when visibility falls below 1 000 m; mist (BR) is used for 1 000–5 000 m. Haze (HZ), dust (DU) and smoke (FU) reduce visibility through suspended dry particles rather than water droplets.

Cloud formation mechanisms: orographic lifting, frontal lifting, low-level convergence, free convection, radiative cooling and advection over a cold surface; dissipation follows subsidence, mixing or heating of the cloud layer.

In West Africa the main rain-producing clouds are cumulonimbus and cumulus congestus in squall lines and mesoscale convective systems; stratiform nimbostratus is common during the mature monsoon and produces widespread low cloud and reduced visibility.

Geostrophic wind applies to straight, frictionless flow above the friction layer, where the pressure gradient force exactly balances Coriolis: Vg = (1/ρf)(Δp/Δn), wind parallel to the isobars.

Coriolis parameter f = 2Ω sin φ: about 1.46 × 10⁻⁴ s⁻¹ at the poles and 3.8 × 10⁻⁵ s⁻¹ at 15° latitude; it tends to zero at the equator, so geostrophic and gradient-wind arguments fail in the deep tropics.

Gradient wind: with the same pressure gradient, cyclonic (low-pressure) flow is subgeostrophic and anticyclonic (high-pressure) flow is supergeostrophic; anticyclonic curvature has a limiting radius beyond which balanced flow cannot exist.

Buys Ballot's law (Northern Hemisphere): with your back to the wind, low pressure is on your left; flow is anticlockwise around a low and clockwise around a high.

Friction layer (typically the lowest 500–1 000 m): wind speed decreases towards the surface and the wind backs (turns anticlockwise) with descent, crossing the isobars towards low pressure at 20°–45° over land.

Global circulation cells: Hadley (0°–30°), Ferrel (30°–60°) and Polar (60°–90°). The Northern Hemisphere trade winds are north-easterly; the subtropical high near 30°N (Azores–Sahara) drives the Harmattan towards West Africa.

Jet streams: subtropical jet near 200–250 hPa at about 30° latitude, polar-front jet near 300 hPa; clear-air turbulence is most frequent on the cold (poleward) side of the core and near sharp troughs.

Local circulations: sea breeze (day, onshore), land breeze (night), anabatic upslope and katabatic downslope winds, mountain–valley winds, and gust fronts from thunderstorm outflows.

Low-level wind shear is operationally significant when the headwind/tailwind component changes by 15 kt or more, or vertical velocity changes by about 500 ft/min; the critical zone is below 1 000 ft AGL on approach and departure.

Air-mass classification by source region: cA, cP, cT, mP, mT. West Africa is dominated by cT (Saharan Harmattan) and mT (maritime tropical monsoon from the Gulf of Guinea); polar air masses never reach the region.

Cold front: slope about 1:50 to 1:100, fast moving, narrow belt of cumuliform cloud and showery precipitation, sharp wind veer, temperature fall and pressure rise after passage, often with a pre-frontal squall line.

Warm front: slope about 1:100 to 1:200, broad stratiform sequence cirrus → cirrostratus → altostratus → nimbostratus, steady rain well ahead of the surface position, gradual temperature rise, risk of freezing rain and frontal fog.

Occluded front: the cold front overtakes the warm front and lifts the warm sector clear of the surface; cold and warm occlusions differ in the temperature profile above the surface but both give extensive cloud and precipitation.

Mid-latitude depression life cycle: incipient wave on the polar front, open wave with a warm sector, mature stage, occlusion, then decay; systems track with the 500 hPa steering flow and typically last several days.

Subsidence behind a cold front produces a ridge, clearing skies, good visibility and a cold, unstable showery air mass with cumulus and strong, gusty surface wind.

In the tropics, true fronts and polar-front depressions are absent because Coriolis is too weak; the equivalent boundaries are the ITD/ITCZ, gust fronts, outflow boundaries and monsoon surges.

Frontal fog and stratus form where saturated air is cooled by a cold surface ahead of a warm front or where rain evaporates into cooler air; they are the main low-visibility and low-cloud hazards of frontal passage.

The ITCZ is the equatorial convergence zone between the north-east and south-east trades. Over West Africa its surface position is the Inter-Tropical Discontinuity (ITD), separating moist south-westerly monsoon air from dry north-easterly Harmattan air.

ITD mean latitude: about 4–6 °N in January, reaching 15–20 °N in July–August; its northward march controls the onset of the rains, and the monsoon trough lies to its south.

West African easterly waves: wavelength 2 000–2 500 km, period 3–5 days, moving westward at 5–8 m/s along about 10–15 °N; they are the main seedling for Sahel squall lines and for Atlantic tropical cyclones.

Sahel squall lines: a leading gust front, a wide band of cumulonimbus, heavy rain, hail and strong straight-line winds; they propagate westward faster than the mean easterly flow and cross Nigeria in one to two days.

Tropical cyclone formation requires sea surface temperature of at least 26.5 °C to about 50 m depth, latitude of 5° or more, low vertical wind shear, upper-level divergence and a pre-existing disturbance; stages are disturbance, depression (≤33 kt), tropical storm (34–63 kt), hurricane (≥64 kt).

Upper-air features in the West African summer: African easterly jet near 700 hPa at about 15 °N (10–15 m/s), tropical easterly jet near 200 hPa at about 10 °N (30–40 m/s), and the Saharan air layer with very dry, dusty air above the monsoon flow.

Rainfall seasonality: coastal Nigeria has a bimodal regime peaking in June and September; the Sahel has a single August peak; onset is February–March on the coast and June–July in the far north, with withdrawal beginning in the north in September.

The "August break" is a two to four week lull in coastal rainfall in July–August caused by a southward displacement of the monsoon trough and cooler Gulf of Guinea water; it is a lull, not a dry season.

Tropical cyclones do not reach Nigeria, but their remnants and the modulation of the ITD by Atlantic sea-surface temperatures control monsoon surges and dry spells over the region.

Thunderstorm life cycle: cumulus stage (updraft only, no precipitation reaching the ground), mature stage (updraft and downdraft together, lightning, hail, heaviest rain, strongest gusts), dissipating stage (downdraft dominant, anvil spreading).

Microburst: a downdraft under a thunderstorm less than 4 km across and lasting minutes, producing a headwind gain followed by a severe tailwind loss; it is the classic cause of low-level wind shear accidents.

Icing types: clear (glaze) icing from large supercooled drops between 0 °C and −10 °C, spreading far back over the airframe - the most dangerous; rime icing from small drops between −10 °C and −20 °C, brittle and milky; mixed icing in between; freezing rain is the most severe.

Icing severity depends on liquid water content, droplet size, temperature and exposure time; supercooled large droplets are worst, and cumulonimbus between 0 °C and −20 °C is the most hazardous icing zone in the tropics.

Clear-air turbulence is most frequent near jet-stream cores on the cold side, in sharp upper troughs and in strong frontal zones; it occurs with no visual warning.

Turbulence categories: convective or thermal, mechanical (surface friction, buildings, mountains), frontal, clear-air, and wake turbulence; mountain waves downwind of the Jos and Adamawa highlands cause severe turbulence and downdrafts.

Fog formation mechanisms: radiation fog (clear, calm, long nights), advection fog (warm moist air over a cold surface), frontal fog, upslope fog and steaming fog; radiation fog is most common in the harmattan season over the northern interior.

Visibility-reducing phenomena and codes: BR (mist, 1 000–5 000 m), HZ (haze), DU (dust), SA (sand), SS (sandstorm), FU (smoke), FG (fog below 1 000 m); harmattan dust can drop visibility below 1 000 m at Kano, Sokoto and Maiduguri.

Hail over Nigeria occurs mainly in March–April in the north (Jos, Kano, Bauchi) with pre-monsoon thunderstorms; hailstones of 5 mm or more are reported as GR and are a serious airframe and engine hazard.

METAR/SPECI element order: identification and time (UTC), wind, visibility, RVR, present weather, cloud, temperature and dewpoint, QNH, trend or remarks. METAR is routine (half-hourly or hourly); SPECI is issued on significant change.

Wind group dddffKT: three digits of true direction, two digits of speed, G for gusts, VRB for variable or light wind, and dddVddd when direction varies by 60° or more with a mean speed above 3 kt. Units are knots unless a country uses m/s.

Cloud amount: FEW 1–2 oktas, SCT 3–4, BKN 5–7, OVC 8; height in hundreds of feet above aerodrome level; CB or TCU appended when present; NSC when no significant cloud; NCD for no cloud detected by automatic systems.

CAVOK means visibility 10 km or more, no cloud below 5 000 ft or below the highest minimum sector altitude, no cumulonimbus or towering cumulus, and no significant weather.

Weather codes: DZ drizzle, RA rain, SN snow, SH shower, TS thunderstorm, FZ freezing, GR hail ≥5 mm, GS small hail or snow pellets, BR mist, HZ haze, DU dust, SQ squall, FC funnel cloud, VA volcanic ash; "+" heavy and "−" light.

Trend forecasts attached to METAR/SPECI: NOSIG (no significant change expected), BECMG (gradual change within a stated period), TEMPO (temporary fluctuations of less than one hour and less than half the period); TAF uses BECMG, TEMPO and FM.

TAF: aerodrome forecast with a validity of 9–30 hours (24 or 30 hours at major international aerodromes), issued at fixed intervals, giving wind, visibility, weather, cloud and expected changes with times in UTC.

SIGMET warns of significant en-route hazards (thunderstorms, tropical cyclones, volcanic ash, severe turbulence, severe icing, low-level wind shear) and is valid up to 4 hours, extended to 6 hours for volcanic ash and tropical cyclones; AIRMET covers moderate icing or turbulence, widespread visibility below 5 km and mountain obscuration at lower levels.

GAMET is a low-level area forecast (typically below FL150) with 6-hour validity, issued every 6 hours in numbered sections covering wind, visibility, weather, cloud, icing, turbulence and freezing level; local special reports (SPECI, AIRMET, warnings) supplement it.

Mean sea level chart: isobars at 4 hPa intervals with highs, lows, troughs, ridges and fronts plotted; the surface wind crosses the isobars towards low pressure by 20°–45° over land owing to friction.

Station model: temperature and dewpoint in whole °C, pressure as the last three digits in tenths of a hectopascal with a 9 or 10 prefix (998 = 999.8 hPa, 012 = 1001.2 hPa), pressure tendency, past and present weather, cloud cover and cloud type.

Wind barb: half barb 5 kt, full barb 10 kt, pennant 50 kt; the shaft is drawn from the station circle towards the direction the wind blows from, and calm is a circle.

Upper-air constant-pressure charts at 850, 700, 500, 300 and 250 hPa: solid lines are geopotential height, dashed lines temperature, arrows are wind; humidity is plotted at 850 and 700 hPa.

Typical heights: 850 hPa ≈ 1 500 gpm, 700 hPa ≈ 3 000 gpm, 500 hPa ≈ 5 500–5 900 gpm, 300 hPa ≈ 9 100 gpm; 500 hPa contours are drawn at 40 or 60 gpm intervals.

Thickness (1000–500 hPa) is proportional to mean virtual temperature: 5 280 gpm indicates a cold air mass and 5 400–5 460 gpm a mild one; the 540 dam line is the classic mid-latitude rain/snow boundary.

Isobar and contour spacing indicates wind speed: closely packed contours mean a strong pressure gradient, hence strong geostrophic wind; wind blows nearly parallel to the contours with low pressure to the left in the Northern Hemisphere.

Significant weather (SIGWX) charts show cumulonimbus, icing, turbulence, jet streams, tropopause height, sand or dust storms and tropical cyclone tracks; mid-level charts cover roughly FL100–FL450 and low-level charts the layer below FL100.

Ascent is indicated on upper charts by positive vorticity advection, divergence aloft, and jet-streak exit regions; subsidence is indicated by confluence and negative vorticity advection behind troughs.

Two seasons dominate: the wet season (south-westerly monsoon) from roughly March to October in the south and June to September in the far north, and the dry season (north-easterly Harmattan) from November to March.

Mean annual rainfall decreases sharply northward: Calabar and Port Harcourt exceed 2 500 mm, Lagos about 1 500 mm, Jos about 1 300 mm, Kano about 900 mm, Sokoto and Maiduguri about 500–600 mm.

Harmattan (December–February): dry, dust-laden north-easterly trade wind from the Sahara, relative humidity often below 30 %, large diurnal temperature range, and dust haze reducing visibility to 1–5 km and occasionally below 1 000 m.

Temperature: the highest values (near 40 °C) occur in the north in February–April before monsoon onset; Lagos stays at 28–33 °C all year; the Jos plateau near 1 200 m runs 15–25 °C and carries the coolest aerodrome climate.

Thunderstorm frequency is highest over the Middle Belt and north (Jos, Kaduna, Bauchi with 80–100 thunderstorm days per year) and lower on the coast (about 40–60 days); most activity is April–September with afternoon and evening maxima.

Squall lines affect northern Nigeria in April–June and again in September, producing strong gust fronts, heavy rain, hail and dust, and are the principal cause of low-level wind shear, go-arounds and wind damage at Kano, Kaduna and Sokoto.

The "August break" lull in coastal rainfall does not remove the hazard: humidity stays high, low stratus and haze persist and convective activity continues inland.

Aerodrome references: Lagos DNMM about 41 m elevation, Abuja DNAA about 342 m, Kano DNKN about 476 m, Jos DNJO about 1 290 m; high terrain along the Jos–Bauchi plateau requires terrain clearance planning on north–south routes.

Tropical cyclones from the Atlantic do not reach Nigeria, but easterly-wave passages and ITD surges produce multi-day convective episodes that disrupt terminal operations and en-route flight levels.