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What are the key ISA reference values worth remembering? start learning
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Sea level: 1013.25 hPa and +15°C; tropospheric lapse rate about 2°C/1,000 ft; ISA tropopause about FL360 at −56.5°C.
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How do pressure, temperature and humidity affect air density? start learning
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Density decreases with lower pressure, higher temperature and higher humidity.
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What happens to true altitude when flying from high pressure to low pressure without correcting the altimeter? start learning
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True altitude becomes lower than indicated — “high to low, look out below”.
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What happens to true altitude in air colder than ISA? start learning
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True altitude is lower than indicated; the effect becomes more important with increasing height above the altimeter-setting source.
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Why is cold-temperature altimetry particularly important on instrument procedures? start learning
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The aircraft may be significantly lower than indicated relative to terrain and obstacles.
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What does a small temperature–dew-point spread tell you operationally? start learning
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Air is close to saturation, so cloud or fog becomes increasingly likely if further cooling occurs.
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What determines atmospheric stability? start learning
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How the environmental temperature lapse rate compares with the temperature change of a vertically displaced air parcel.
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What weather is generally associated with stable versus unstable air? start learning
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Stable: layered cloud, fog, poor visibility and smoother air; unstable: convection, cumulus/CB, showers and turbulence.
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What is a temperature inversion and why is it operationally relevant? start learning
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Temperature increases with height; the layer is strongly stable and can trap moisture, haze and pollutants beneath it.
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Why does rising unsaturated air cool? start learning
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Pressure decreases with height, so the parcel expands and cools adiabatically.
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Why does saturated rising air cool more slowly than unsaturated air? start learning
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Condensation releases latent heat.
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What mechanisms commonly force air to rise? start learning
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Convection, convergence, frontal lifting and orographic lifting.
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What is the basic mechanism of cloud formation? start learning
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Air cools to saturation and water vapour condenses or deposits on condensation nuclei.
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How do radiation fog and advection fog differ? start learning
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Radiation fog forms when the ground cools moist air near the surface; advection fog forms when moist air moves over a colder surface.
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What conditions favour radiation fog? start learning
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Clear or partly clear sky, moist near-surface air and light wind allowing strong nocturnal cooling.
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Why can too much wind prevent radiation fog? start learning
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Mixing prevents a shallow surface layer from cooling sufficiently to saturation.
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How do a typical warm front and cold front differ operationally? start learning
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Warm fronts usually produce widespread layered cloud and prolonged precipitation; cold fronts more often produce stronger lift, showers, convection and rapid weather changes.
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What is an occluded front? start learning
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A frontal system formed when a cold front catches a warm front and lifts the warm sector away from the surface.
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How do pressure-gradient force, Coriolis force and friction determine wind near the surface? start learning
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Pressure gradient drives air toward low pressure; Coriolis deflects it; surface friction slows the wind and makes it cross isobars toward lower pressure.
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Why does wind usually increase and veer with height above the surface in the Northern Hemisphere? start learning
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Friction decreases with height, strengthening the Coriolis balance and turning the wind toward the geostrophic direction.
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What is operationally important about the tropopause? start learning
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Jet streams, strong wind shear and CAT are frequently associated with it.
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start learning
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A narrow band of strong upper-level wind associated with strong horizontal temperature and pressure gradients.
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Where is CAT most likely around a jet stream? start learning
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In areas of strongest vertical and horizontal wind shear, often near the jet core and tropopause.
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What is clear-air turbulence? start learning
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Turbulence without significant visible cloud, commonly associated with strong wind shear near jet streams and upper-level frontal zones.
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What conditions favour mountain waves? start learning
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Stable air with strong wind crossing a mountain ridge approximately perpendicular to it.
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What visual clues suggest mountain-wave activity? start learning
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Lenticular clouds aloft and rotor clouds beneath the wave.
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Where is the greatest turbulence threat in a mountain-wave system? start learning
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In rotor and wave-breaking regions, especially on the lee side.
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What three ingredients are required for thunderstorms? start learning
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Moisture, atmospheric instability and a lifting mechanism.
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What are the three thunderstorm stages and their dominant vertical motion? start learning
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Developing: updrafts; mature: strong updrafts and downdrafts; dissipating: mainly downdrafts.
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What are the major operational hazards of a CB? start learning
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Severe turbulence, windshear/microbursts, hail, lightning, icing, heavy precipitation and possible rapid pressure/wind changes.
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Why can a thunderstorm be hazardous even outside the visible cloud? start learning
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Hail, turbulence, lightning and strong outflow/windshear can extend beyond the visible CB.
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What is a microburst and why is it particularly dangerous near the ground? start learning
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A concentrated downdraft with divergent outflow; during take-off or landing it can rapidly change headwind into downdraft and tailwind.
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What is the typical wind sequence when penetrating a microburst on final? start learning
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Increasing headwind, strong downdraft, then decreasing headwind or increasing tailwind.
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start learning
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A significant change in wind speed and/or direction over a short distance.
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Where should low-level windshear be particularly expected? start learning
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Near thunderstorms/outflows, fronts, strong inversions and terrain-induced flow.
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Why can virga indicate a windshear or microburst threat? start learning
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Evaporating precipitation cools the air, increasing its density and strengthening downdrafts.
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What conditions are required for airframe icing? start learning
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Supercooled liquid water must strike an aircraft surface whose temperature allows ice accretion.
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What is the practical difference between rime, clear and mixed ice? start learning
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Rime is rough/opaque from rapidly freezing small droplets; clear ice is denser and smoother from larger spreading droplets; mixed ice contains characteristics of both.
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Why are supercooled large droplets (SLD) particularly hazardous? start learning
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They can flow beyond normally protected leading-edge areas before freezing, producing rapid and extensive ice accretion.
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Why is freezing rain a severe icing threat? start learning
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It contains large supercooled droplets and can produce rapid clear-ice accretion over extensive surfaces.
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What are the main aerodynamic effects of wing icing? start learning
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Increased drag, reduced CLmax, lower critical AoA, higher stall speed and degraded controllability.
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Why can the stall warning characteristics of an iced wing change? start learning
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Ice alters the aerofoil and may cause separation and stall at a lower AoA than in the clean configuration.
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Why is TAT higher than SAT/OAT at high speed? start learning
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Air is heated by compression and conversion of kinetic energy as it is brought toward rest at the temperature probe.
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What is the operational difference between BECMG, TEMPO and FM in a TAF? start learning
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BECMG is a transition to new prevailing conditions; TEMPO describes temporary fluctuations; FM establishes new prevailing conditions from a specified time.
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What does PROB30 or PROB40 communicate in a TAF? start learning
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A 30% or 40% probability of the specified conditions occurring.
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For operational planning, when is a BECMG deterioration normally considered to apply? start learning
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From the beginning of the BECMG period.
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For operational planning, when is a BECMG improvement normally considered to apply? start learning
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From the end of the BECMG period.
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Why is TEMPO different from BECMG operationally? start learning
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TEMPO does not replace the prevailing forecast; it describes temporary conditions superimposed on it.
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What is the practical distinction between SIGMET, AIRMET and GAMET? start learning
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SIGMET warns of specified significant en-route hazards; AIRMET covers specified hazards mainly relevant to low-level flight; GAMET is a low-level area forecast.
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start learning
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A strategic picture of significant en-route weather such as CB, turbulence, icing, jet streams, tropopause and major hazardous phenomena.
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What is the current vertical coverage of the new-generation WAFS SIGWX product? start learning
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Why should METAR, TAF, SIGMET and SIGWX never be considered in isolation? start learning
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They describe different locations, scales, time periods and hazards; the operational picture comes from combining them and continuously reassessing changes.
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