Brunei DCA CPL(A) Meteorology cheat sheet

Composition by volume of dry air: nitrogen about 78%, oxygen about 21%, argon about 0.93%, carbon dioxide about 0.04%; water vapour varies from near zero to about 4%.

ICAO Standard Atmosphere: MSL pressure 1013.25 hPa, temperature +15 °C, density 1.225 kg/m³, temperature lapse rate 1.98 °C per 1000 ft (6.5 °C/km) in the troposphere.

Tropopause height averages about 36,000 ft at the equator and about 20,000 ft at the poles; it is higher in summer and lower in winter, and marks the base of the isothermal stratosphere.

Troposphere contains about 75% of atmospheric mass and nearly all water vapour and weather; temperature decreases with height up to the tropopause.

Ozone layer lies in the stratosphere (roughly 15-35 km); it absorbs ultraviolet radiation and causes stratospheric temperature to increase with height.

Pressure altitude is the altitude indicated on a subscale of 1013.25 hPa; true altitude is the actual height above MSL; density altitude is pressure altitude corrected for temperature.

Standard atmosphere is used for aircraft performance, altimetry and Mach/true airspeed computations; deviations from it are called ISA deviations.

Pressure decreases with height at about 1 hPa per 30 ft near MSL; the rate roughly halves at higher levels (about 1 hPa per 60 ft near FL180).

Altimeter set to QNH reads elevation when on the ground; set to QFE reads zero on the reference point; set to 1013.25 hPa reads pressure altitude (flight levels).

Fly from high pressure to low pressure without resetting the subscale: the aircraft is lower than indicated. Fly from low to high: the aircraft is higher than indicated.

Fly from warm air into cold air without resetting: the aircraft is lower than indicated (about 4% error per 10 °C of ISA deviation).

Density decreases with increasing temperature, increasing humidity and increasing altitude; all three reduce lift, engine power and propeller/thrust efficiency.

Density altitude = pressure altitude + 120 ft per 1 °C that the actual temperature exceeds ISA temperature at that level (approximation).

High QNH, low temperature and low humidity all increase air density; low QNH, high temperature and high humidity decrease it.

QNE is the standard setting 1013.25 hPa; transition altitude/level separate QNH and QNE usage.

Relative humidity = (actual vapour pressure / saturation vapour pressure) × 100%; it falls as temperature rises if moisture content is constant.

Dew point is the temperature to which air must be cooled at constant pressure to reach saturation; the spread between temperature and dew point indicates how close the air is to saturation.

Lifting condensation level approximates 125 m (about 400 ft) per 1 °C of temperature-dew point spread.

Environmental lapse rate (ELR) compared with dry adiabatic lapse rate (DALR ≈ 3 °C/1000 ft) and saturated adiabatic lapse rate (SALR ≈ 1.5 °C/1000 ft near MSL) determines stability.

Absolutely stable: ELR < SALR. Conditionally unstable: SALR < ELR < DALR. Absolutely unstable: ELR > DALR.

Latent heat released on condensation warms the rising parcel and offsets adiabatic cooling, favouring continued ascent in saturated air.

Stability is increased by cooling of the lower layers (nocturnal radiation, cold advection) and decreased by surface heating or lifting of the whole layer.

Temperature inversions trap moisture and pollutants beneath them and are associated with haze, fog and low-level wind shear.

Cloud families: high (cirrus, cirrostratus, cirrocumulus), middle (altostratus, altocumulus), low (stratus, stratocumulus, nimbostratus), and clouds with vertical development (cumulus, cumulonimbus).

Fair-weather cumulus have bases at the condensation level; flat bases indicate a well-mixed boundary layer and give a usable estimate of cloud base height.

Cumulonimbus contains strong updraughts and downdraughts, severe icing, hail, lightning and microbursts; avoid by at least 20 nm where possible.

Nimbostratus produces continuous steady precipitation and extensive low cloud; stratus produces drizzle and reduces visibility.

Fog forms when air is cooled to its dew point (radiation, advection, upslope) or when moisture is added at constant temperature (evaporation fog, frontal fog).

Radiation fog forms on clear, calm nights over land and usually dissipates after sunrise; advection fog forms when warm moist air moves over a colder surface and can persist.

Precipitation types: rain, drizzle (drops < 0.5 mm), snow, sleet, hail; freezing rain occurs when rain falls through a sub-freezing layer and freezes on impact.

Orographic lifting on the windward side of mountains produces cloud and heavy rain; the lee side is drier and often turbulent.

Air mass classification: cA, cP, mP, cT, mT; source regions are high-latitude land, high-latitude ocean, subtropical land and subtropical ocean.

Cold front: steep slope (about 1:50), narrow band of heavy showery precipitation, rapid passage, wind veers, temperature falls, pressure rises, visibility improves.

Warm front: shallow slope (about 1:150), broad band of stratiform cloud and steady precipitation ahead of the surface front, slow passage, wind veers, temperature rises.

Occluded front forms where a cold front overtakes a warm front; frontal occlusion often brings complex cloud and precipitation.

Stationary front lies nearly parallel to the upper flow and produces prolonged cloud and rain; a wave may develop into a frontal depression.

Mid-latitude depression life cycle: wave development, open stage, occlusion, decay; deepening is favoured by upper divergence and strong temperature gradients.

Anticyclone: descending air, subsidence inversion, light winds, clear skies, good visibility aloft but haze or fog near the surface; ridge lines and cols are weak pressure areas.

Tropical cyclone and monsoon trough patterns are covered separately but interact with mid-latitude systems through the subtropical ridge.

Geostrophic wind flows parallel to straight isobars with low pressure on the left in the Northern Hemisphere; speed is inversely proportional to latitude and directly proportional to pressure gradient.

Buys Ballot's law: with your back to the wind in the Northern Hemisphere, low pressure lies to your left.

Coriolis force increases with latitude and with wind speed; it is zero at the equator, so geostrophic balance fails in low latitudes.

Friction reduces wind speed near the surface and turns the wind towards low pressure, typically by 20-45° over land.

Sea breeze blows from sea to land by day; land breeze blows from land to sea at night; both are driven by differential heating and are strongest in the afternoon.

Anabatic (upslope) winds occur on sun-facing slopes by day; katabatic (downslope) winds occur at night and can be strong in valleys and near glaciers.

Mountain waves and rotors form downwind of ridges in stable, strong cross-mountain flow; associated with severe turbulence and lenticular cloud.

Jet streams are strong narrow bands of upper wind near the tropopause; the polar front jet is associated with the polar front and mid-latitude depressions.

Tropical cyclone formation requires sea surface temperature above about 26.5 °C, high low-level vorticity, low vertical wind shear and at least 5° latitude from the equator.

Tropical cyclone structure: eye, eyewall (strongest winds and heaviest rain), spiral rainbands; the eye is a region of subsidence, light wind and clear sky.

In the Northern Hemisphere tropical cyclones rotate anticlockwise and move generally west-northwest, then recurve poleward and eastward ahead of a mid-latitude trough.

Sectors of a tropical cyclone: dangerous semicircle (right in the Northern Hemisphere) and navigable semicircle; the path ahead of the storm is the most hazardous.

Monsoon: seasonal reversal of wind caused by differential heating of land and sea; the southwest monsoon affects Brunei from about May to September with heavy rain and squalls.

Sumatra squalls are lines of intense thunderstorms that move eastwards across the Strait of Malacca, often in the early morning, producing strong wind gusts and heavy rain.

Convective activity in the equatorial belt is greatest in the afternoon and evening over land; daytime heating plus high humidity triggers thunderstorms.

Tropical cyclone naming, warnings and SIGMET dissemination are handled by the responsible RSMC and the local meteorological authority.

Structural icing types: clear (rime-freezing rain, large droplets, strong adhesion), rime (small droplets, low liquid water content) and mixed; clear ice is the most hazardous.

Icing risk is greatest in stratiform cloud at temperatures between 0 °C and about -10 °C; below -20 °C supercooled water is scarce.

Freezing rain or freezing drizzle encountered in flight indicates severe icing risk in the layer below the warm front aloft.

Turbulence types: convective, mechanical (terrain and buildings), frontal, clear-air turbulence (CAT) and wake turbulence.

CAT is associated with jet streams, strong wind shear and the tropopause; it is often reported near the jet core and is invisible to the eye.

Thunderstorm hazards: severe turbulence, hail, lightning, microbursts, windshear, icing and reduced visibility; avoid the cell by at least 20 nm.

Microburst is a concentrated downdraught producing a sudden headwind then tailwind change; the resulting windshear can exceed the aircraft's performance capability.

Low-level windshear warnings are issued when wind changes of 15 kt or more over a short distance are detected; go-around is the standard recovery.

METAR is a routine surface observation, usually issued hourly; SPECI is issued when significant changes occur.

METAR wind group: dddffKT, with G for gusts; VRB for variable; 00000KT for calm; 9999 for visibility 10 km or more.

Weather groups in METAR: intensity (+/-), descriptor (MI, BC, SH, TS, FZ), precipitation (RA, SN, GR, GS), obscuration (BR, FG, HZ), other (PO, SQ, FC).

Cloud groups: FEW (1-2 oktas), SCT (3-4), BKN (5-7), OVC (8); vertical visibility VV/// indicates sky obscured.

TEMPO and BECMG in TAF: TEMPO is a temporary fluctuation, BECMG is a gradual permanent change; both are followed by a time group.

SIGMET is issued for weather significant to all aircraft, including severe turbulence, severe icing, volcanic ash, tropical cyclones and severe mountain waves; valid for 4 hours.

AIRMET is issued for moderate hazards affecting low-level aircraft, and is valid for 6 hours.

Low-level significant weather charts show fronts, cloud, turbulence and icing below FL150; upper-wind and temperature charts give winds and temperatures aloft.