Meteorology
Meteorology tests whether you can read a forecast, understand the atmosphere over West Africa, and know which weather will hurt you. Expect METAR/TAF decoding, icing, thunderstorm hazards and tropical weather.
Pressure, wind and altimetry
- ISA: 15 °C and 1013.25 hPa at MSL, lapse rate about 1.98 °C per 1 000 ft. QNH reduces to sea level, QFE to the aerodrome, QNE is standard.
- Wind veers with height in the northern hemisphere and backs in the southern; surface friction reduces speed and turns the wind towards low pressure.
- Flying from high to low pressure, or high to low temperature, without resetting the subscale puts you lower than indicated.
West African weather
- The Intertropical Discontinuity separates dry, dusty Harmattan air from moist monsoon air; its position controls the rainy season and the Harmattan haze that reduces visibility over Ghana in December to February.
- Squall lines ahead of the ITD bring strong gusts, heavy rain and microbursts; tropical thunderstorms are most frequent in the afternoon and evening.
Cloud, icing and hazards
- Rime icing forms in stratiform cloud; clear icing in cumuliform cloud with large droplets; freezing rain means severe clear icing.
- Thunderstorm life cycle: cumulus, mature (updraughts and downdraughts, hail, lightning, microburst), dissipating. Avoid by at least 20 nm; do not attempt to overfly the tops.
- Wind shear and microburst: increasing headwind, then downdraught, then tailwind. Go around on any doubt.
Worked example
DGAA 121200Z 21012KT 8000 -RA BKN008 OVC020 24/23 Q1010 NOSIG: wind 210° at 12 kt, 8 km visibility, light rain, broken at 800 ft, overcast at 2 000 ft, temperature 24 °C, dewpoint 23 °C, QNH 1010 hPa. A spread of 1 °C means nearly saturated air, so fog or low stratus is likely; with a broken layer at 800 ft an instrument approach is required.
Traps
Reading 9999 as 9 999 m instead of 10 km or more; confusing QNH with QFE when computing height; assuming NOSIG cancels a TEMPO group; and forgetting that icing needs visible moisture with a temperature near or below 0 °C.