NCAA CPL Meteorology flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

ISA sea-level values

Temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³, with a 6.5 °C/km lapse rate up to the 11 km tropopause where temperature is about −56.5 °C.

QNH versus QFE

QNH set on the subscale makes the altimeter read aerodrome elevation on the ground; QFE makes it read zero. QNE is the standard 1013.25 hPa used at and above the transition level.

High-to-low altimeter error

Flying from higher pressure or warmer air into lower pressure or colder air without resetting the subscale makes the aircraft lower than the altimeter shows, a serious terrain-clearance hazard.

Density altitude

Pressure altitude corrected for non-standard temperature. It rises with high temperature, high humidity and low pressure, reducing engine power, thrust and lift, so take-off and climb gradients suffer.

Dry adiabatic lapse rate

Unsaturated air cooling by expansion at 3 °C per 1 000 ft (9.8 °C/km). It is steeper than the saturated rate, so parcel stability depends on whether the air is saturated.

Conditional instability

The state where the environmental lapse rate lies between the saturated and dry adiabatic rates: stable for unsaturated air, but unstable once the lifted parcel becomes saturated and releases latent heat.

Lifting condensation level

The height at which a rising parcel reaches saturation. Estimate it as 400 ft (125 m) for each 1 °C of surface dewpoint depression, giving convective cloud-base height.

Cumulonimbus aviation hazards

A deep glaciated convective cloud with an anvil, producing severe turbulence, strong icing, hail, lightning and microbursts; over West Africa during the monsoon its top frequently exceeds 12 km.

Nimbostratus versus cumulonimbus

Nimbostratus is a thick layered cloud giving continuous steady rain with poor visibility but no lightning; cumulonimbus gives showery rain with lightning, hail and gusty surface winds.

Precipitation type by drop size

Drizzle drops are smaller than 0.5 mm and fall from stratus; rain drops are 0.5 mm or larger. Hail is ice of 5 mm or more, reported as GR.

Geostrophic wind balance

Straight, frictionless flow above the friction layer where the horizontal pressure gradient force exactly balances Coriolis; the wind blows parallel to the isobars and is stronger where isobars are closer.

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 falls to zero at the equator, so geostrophic balance cannot be used in the deep tropics.

Gradient wind

Wind in curved frictionless flow: cyclonic flow is subgeostrophic because the centrifugal force acts outward with the pressure gradient, while anticyclonic flow is supergeostrophic and has a limiting curvature.

Sea breeze

A daytime onshore circulation caused by faster heating of land; denser sea air flows inland, often at 15–20 kt, and its leading edge can trigger thunderstorms over coastal Nigeria.

Cold front characteristics

A steep boundary (slope 1:50 to 1:100) moving fast, with a narrow band of cumuliform cloud and showers, a sharp wind veer, a temperature drop and a pressure rise after passage.

Warm front characteristics

A shallow boundary (slope 1:100 to 1:200) producing a wide stratiform sequence from cirrus through altostratus to nimbostratus, with steady rain falling well ahead of the surface position.

Occluded front

Formed when the faster cold front overtakes the warm front and lifts the warm sector completely off the surface; the surface chart then shows a single front with mixed cloud and precipitation.

Inter-Tropical Discontinuity

The surface position of the ITCZ over West Africa, separating moist south-westerly monsoon air from dry north-easterly harmattan air; it lies near 4–6 °N in January and 15–20 °N in July–August.

African easterly waves

Westward-propagating troughs along roughly 10–15 °N with 2 000–2 500 km wavelength and a 3–5 day period; they trigger Sahel squall lines and seed Atlantic tropical cyclones.

Tropical cyclone formation requirements

Sea surface temperature of at least 26.5 °C to about 50 m depth, latitude of 5° or more for Coriolis, low vertical wind shear, upper-level divergence and a pre-existing disturbance.

August break

A two to four week lull in rainfall along the West African coast in July–August, linked to a southward shift of the monsoon trough and cooler Gulf of Guinea water; it is not a true dry season.

Clear versus rime icing

Clear (glaze) icing forms from large supercooled drops between 0 °C and −10 °C and spreads far back over the airframe; rime icing forms from small drops between −10 °C and −20 °C and is brittle and milky.

Microburst

A concentrated downdraft beneath a thunderstorm, less than 4 km wide and lasting minutes, causing a headwind gain followed by a severe tailwind loss; it is a leading cause of wind shear accidents.

Fog definition

Fog is reported when visibility falls below 1 000 m. Mist (BR) covers 1 000–5 000 m, while haze (HZ) reduces visibility through suspended dry particles rather than water droplets.

METAR element order

Wind, visibility, RVR, present weather, cloud, temperature and dewpoint, QNH, then trend; the observation is issued half-hourly or hourly and all times are in UTC.

CAVOK meaning

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.

SIGMET validity

A warning of significant en-route hazards such as thunderstorms, tropical cyclones, volcanic ash, severe turbulence and severe icing, valid for up to four hours, or six hours for ash and cyclones.

Station model pressure coding

The three-figure pressure gives the last digits in tenths of a hectopascal with a 9 or 10 prefix added: 998 means 999.8 hPa and 012 means 1001.2 hPa.

Wind barb increments

Half a barb is 5 kt, a full barb 10 kt and a pennant 50 kt; the shaft points towards the direction from which the wind blows, matching the reported wind direction.

500 hPa height chart

Contours of geopotential height at about 5.5 km, drawn at 40 or 60 gpm intervals; closely packed contours mean strong wind, and low heights indicate a cold trough with ascent and weather.

Harmattan

A dry, dust-laden north-easterly trade wind from the Sahara that affects Nigeria from December to February, bringing very low humidity, a large diurnal temperature range and visibility-reducing dust haze.

Nigerian rainy season onset

The monsoon reaches the coast around February–March and the far north around June–July; withdrawal begins in the north in September and reaches the coast by November.