CPL Meteorology (Sri Lanka CAA Pilot Licence Examinations) 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 is variable, 0-4% by volume.

Standard atmosphere (ISA) sea-level values: temperature 15 °C, pressure 1013.25 hPa, density 1.225 kg/m³; lapse rate 1.98 °C per 1000 ft up to 36,090 ft (11 km).

ISA tropopause: 36,090 ft (11,000 m), temperature -56.5 °C; above it the isothermal layer holds -56.5 °C to about 65,600 ft (20 km).

Troposphere contains about 75% of atmospheric mass and nearly all water vapour; weather, clouds and most turbulence occur here.

Stratosphere is stable and dry; the ozone layer absorbs ultraviolet radiation and warms the upper stratosphere.

Mesosphere temperature falls to about -90 °C near 85 km; thermosphere temperature rises strongly but air density is negligible for aircraft.

Pressure halves roughly every 18,000 ft in the lower atmosphere (rule of thumb: 500 hPa near 18,000 ft, 250 hPa near 34,000 ft).

Temperature decreases with height in the troposphere; an inversion is a layer where temperature increases with height and suppresses vertical motion.

Density altitude is the altitude in ISA at which air density equals the actual density; it rises with high temperature, high airfield elevation and low pressure.

A 1 °C error in temperature corresponds to roughly 120 ft of density-altitude error at a fixed pressure altitude.

Pressure altitude = indicated altitude with subscale set to 1013.25 hPa; true altitude = indicated altitude corrected for pressure error and temperature error.

Rule of thumb: true altitude differs from indicated by about 4 ft per 1000 ft per 1 °C of ISA deviation (high temperature gives true altitude above indicated).

High density altitude degrades take-off and climb performance, lengthens the take-off run and reduces engine power output.

QNH is the pressure reduced to mean sea level; QFE is the pressure at the airfield reference point; QNE is 1013.25 hPa used above the transition altitude.

Cold-air correction: with extremely low temperature, indicated altitude reads higher than true altitude, so minimum safe altitudes must be increased.

Density (ρ) falls as temperature rises at constant pressure (ρ = P/RT); humidity also lowers density because water vapour is lighter than dry air.

Relative humidity = (actual vapour pressure ÷ saturation vapour pressure at that temperature) × 100%; it falls as temperature rises at constant moisture content.

Dew point is the temperature to which air must cool at constant pressure to reach saturation; dew-point spread is a direct measure of how close the air is to saturation.

Mixing ratio is the mass of water vapour per unit mass of dry air and does not change with temperature or pressure unless condensation or evaporation occurs.

Environmental lapse rate (ELR) below 1.5 °C/1000 ft indicates absolute stability; between 1.5 and 3 °C/1000 ft is conditionally unstable; above 3 °C/1000 ft is absolutely unstable.

Saturated adiabatic lapse rate (SALR) is about 1.5 °C/1000 ft in warm air and approaches the dry adiabatic rate (3 °C/1000 ft) in very cold air.

A parcel is stable if lifted air returns to its original level, unstable if it continues rising, and neutral if it stays at the level to which it is displaced.

Stability increases with cooling at the surface, warming aloft, and subsidence; it decreases with surface heating and cold-air advection aloft.

Latent heat release in ascending saturated air slows its cooling (SALR < DALR), which is why saturated ascent can become self-sustaining once instability is triggered.

Cloud families: low (surface to 6500 ft: stratus, stratocumulus, cumulus, nimbostratus), middle (6500-20,000 ft: altostratus, altocumulus), high (above 20,000 ft: cirrus, cirrostratus, cirrocumulus).

Cumulonimbus has a base in the low or middle level and tops that can exceed 40,000 ft; it contains severe turbulence, icing, hail, lightning and microbursts.

Stratiform cloud indicates stable air and produces steady, continuous precipitation; cumuliform cloud indicates instability and produces showery precipitation.

Nimbostratus produces continuous moderate to heavy precipitation and is associated with warm fronts and frontal systems.

Ice crystals and supercooled water droplets coexist in the temperature band from 0 °C to about -20 °C, favouring aircraft icing and precipitation growth.

Freezing rain occurs when precipitation falls through a warm layer aloft and then into a sub-freezing layer near the surface; it produces severe clear icing.

Supercooled large droplets (SLD) form where the droplet-size distribution is dominated by large droplets, typically in freezing drizzle or freezing rain.

Cloud base can be estimated from surface temperature and dew-point spread: convective cloud base (ft) ≈ 400 × (temperature − dew point in °C).

Radiation fog forms on clear, calm nights with a small dew-point spread; it is most common in autumn and winter over land and usually dissipates after sunrise.

Advection fog forms when warm, moist air moves over a colder surface; it can persist day and night and is common over the sea and coastal areas.

Upslope fog forms when moist air is forced up a slope; frontal fog forms at a frontal surface where rain evaporates into cooler air below.

Steam fog forms when cold air moves over a warm water surface; it is common over lakes and seas in autumn and winter.

Fog requires a high relative humidity (typically 95-100%) and either cooling to the dew point or added moisture; a small dew-point spread is the key indicator.

Visibility in fog is reported in metres or statute miles; RVR (runway visual range) is the distance a pilot can see along the runway and is reported separately.

Haze and mist are obscurations caused by suspended particles and droplets; mist gives visibility of 1000-5000 m, haze generally gives a more uniform, milky appearance.

Blowing snow, dust and sand reduce visibility in the same way as fog but are reported as separate present-weather phenomena.

Air-mass classification: cA (continental arctic), cP (continental polar), cT (continental tropical), mP (maritime polar), mT (maritime tropical); source regions determine temperature and moisture.

A cold front typically moves at 20-35 kt and produces a narrow band of cumuliform cloud, showers, squalls and a sharp wind veer after passage.

A warm front typically moves at 10-15 kt and produces a broad band of stratiform cloud and steady precipitation ahead of the surface front.

A stationary front lies nearly parallel to the upper-level flow and can persist for days, producing prolonged cloud and precipitation.

An occluded front forms where a cold front catches up with a warm front; the warm sector is lifted off the surface.

Frontal passage: pressure falls before the front and rises after; temperature falls after a cold front and rises after a warm front; wind veers in the Northern Hemisphere.

Frontal weather is most intense where the frontal surface is steep (cold front) and least intense where it is shallow (warm front).

A dry line marks the boundary between moist and dry air masses and is a common trigger for severe convection.

Synoptic chart symbols: isobars join equal pressure, H marks high pressure, L marks low pressure, triangles mark cold fronts, semicircles mark warm fronts.

Isobar spacing indicates pressure gradient: closely spaced isobars mean strong wind, widely spaced isobars mean light wind.

In the Northern Hemisphere, wind flows anticlockwise around a low and clockwise around a high, with surface friction causing inflow toward the low.

A ridge is an elongated area of high pressure; a trough is an elongated area of low pressure; a col is a neutral saddle point between pressure systems.

A low-pressure system (depression) typically brings cloud, precipitation and strong winds; an anticyclone brings subsidence, clear skies and light winds.

Forecast validity: TAF gives aerodrome forecast, GAMET gives area forecast for low-level flights, SIGMET warns of significant weather, AIRMET covers moderate hazards.

METAR gives observed aerodrome weather; SPECI is a special report issued when conditions change significantly; trend forecasts may follow METAR.

Model forecast skill decreases with lead time; beyond about 7 days, synoptic detail is unreliable and only broad patterns should be used.

Gradient wind is the theoretical wind that blows parallel to straight isobars, balancing pressure-gradient force and Coriolis force; it underestimates actual wind in curved flow.

Surface friction reduces wind speed to about two-thirds of the gradient wind over land and turns the wind about 30° toward the low.

In the Northern Hemisphere, wind veers with height (backs with height in the Southern Hemisphere) due to decreasing friction aloft.

Sea breeze blows from sea to land by day; land breeze blows from land to sea by night; both are driven by differential heating.

Anabatic (upslope) winds blow up valleys by day; katabatic (downslope) winds blow down valleys at night and can be strong in mountainous terrain.

Föhn (foehn) wind is warm, dry downslope wind on the lee side of a mountain range, produced by adiabatic warming of descending air.

Jet streams are fast, narrow upper-level currents near the tropopause; the polar-front jet is associated with the polar front and strong wind shear.

Wind direction is reported in degrees true (meteorological) from which the wind blows; wind speed is reported in knots in aviation products.

Wind shear is a change in wind speed or direction over a short distance, either horizontally or vertically; it is a major cause of low-level accidents.

Low-level wind shear (LLWS) is reported when shear is expected within 2000 ft of the surface and can be associated with fronts, thunderstorms or strong surface winds.

Microburst is an intense, localized downburst with a diameter of about 4 km or less; it produces severe headwind-to-tailwind shear on final approach.

Mechanical turbulence occurs near the surface due to obstacles and uneven terrain; it increases with wind speed and decreases with height.

Thermal turbulence results from convective currents over heated surfaces and is strongest on hot afternoons over land.

Clear air turbulence (CAT) occurs in the upper troposphere, often near jet streams and in regions of strong wind shear, without visible cloud.

Wake turbulence is generated by wingtip vortices; the greatest hazard is behind heavy aircraft on take-off and landing, especially in light winds.

Turbulence intensity is reported as light, moderate, severe or extreme; severe turbulence causes large, abrupt changes in altitude and airspeed.

Thunderstorm life cycle: cumulus stage (updrafts only), mature stage (updrafts and downdrafts, most severe weather), dissipating stage (downdrafts dominate).

Thunderstorm ingredients: sufficient moisture, conditional instability, and a lifting trigger such as a front, convergence line or orographic lifting.

Hazards include hail, lightning, severe turbulence, icing, microbursts, downbursts, tornadoes and heavy rain reducing visibility.

Lightning can strike up to about 10 nautical miles from the parent thunderstorm; avoid flying near or under cumulonimbus.

A squall line is a line of thunderstorms, often along or ahead of a cold front, producing strong gusty winds and heavy precipitation.

Tornadoes form in the most intense supercells; they are small-scale and short-lived but can produce extreme wind and pressure changes.

Hail forms in strong updrafts where supercooled water freezes and accumulates in layers; large hail can cause structural damage to aircraft.

Downdrafts and microbursts produce sudden wind shifts on approach; go-around is the standard response to an unstabilized approach.

Clear ice forms from large supercooled droplets that spread and freeze slowly, producing a heavy, irregular, hard deposit; it is the most dangerous type.

Rime ice forms from small supercooled droplets that freeze quickly on impact, producing a rough, milky, brittle deposit that is lighter than clear ice.

Mixed ice is a combination of clear and rime ice; it is common in cumuliform cloud and can be severe.

Icing severity increases with liquid water content, droplet size and airspeed; the temperature band from 0 °C to -20 °C is the most hazardous.

Freezing rain and freezing drizzle produce severe icing; the presence of these phenomena at flight level is a strong signal to avoid the area.

Mountain waves form when stable air crosses a mountain barrier with wind speed increasing with height; they can produce severe turbulence and lenticular clouds.

Lee waves and rotors can extend hundreds of kilometres downwind of mountains; avoid flying in the lee of mountains in strong wind conditions.

Cold-weather hazards include carburettor icing (most likely at temperatures from -10 °C to +20 °C with high humidity), airframe icing and reduced engine performance.

Tropical cyclone formation requires sea-surface temperature of at least 26.5 °C, high humidity, low vertical wind shear and Coriolis force (not within about 5° of the equator).

Tropical cyclone stages: tropical disturbance, tropical depression (winds below 34 kt), tropical storm (34-63 kt), hurricane/typhoon (64 kt and above).

The eye is a region of calm, subsiding air surrounded by the eyewall, which contains the strongest winds and heaviest rain.

Tropical cyclones weaken rapidly after landfall due to loss of moisture supply and increased surface friction.

Storm surge is the abnormal rise of sea level near the coast, caused mainly by strong onshore winds and low pressure; it is a major cause of coastal flooding.

In the Northern Hemisphere, tropical cyclones rotate anticlockwise and typically track westward, then recurve poleward and eastward.

Extreme weather includes severe thunderstorms, tornadoes, hail, heavy rain and strong winds; all require avoidance or diversion.

Hurricane season in the North Atlantic runs from June to November, with peak activity from August to October.

METAR is a routine aerodrome weather report issued typically every 30 or 60 minutes; it includes wind, visibility, weather, cloud, temperature, dew point and pressure.

TAF is an aerodrome forecast valid for 9-30 hours; it uses change groups BECMG (becoming), TEMPO (temporary) and FM (from) to indicate forecast changes.

SIGMET warns of significant weather such as severe turbulence, icing, volcanic ash, tropical cyclones and severe squall lines.

AIRMET (or GAMET) covers moderate hazards and is issued for areas where SIGMET criteria are not met.

Wind and temperature aloft forecasts (FD) give forecast winds and temperatures at specified levels and are used for flight planning.

PIREP is a pilot report of actual conditions encountered; it is valuable for confirming or updating forecasts, especially for turbulence and icing.

VOLMET broadcasts current aerodrome weather on HF or VHF for in-flight use; ATIS provides recorded aerodrome information before departure or approach.

QNH, QFE and QNE are altimeter settings; QNH is used below the transition altitude, QNE (1013.25 hPa) above it.