CPL 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 is variable, 0-4% by volume.

Permanent gases are well mixed to about 80 km (homosphere); above the homopause (heterosphere) diffusive separation makes composition height-dependent.

Standard atmosphere (ISA): MSL pressure 1013.25 hPa, temperature 15 °C, density 1.225 kg/m³, lapse rate 1.98 °C/1000 ft (6.5 °C/km) to 36,090 ft (11 km).

ISA tropopause: 36,090 ft, temperature -56.5 °C; from 11 km to 20 km temperature is isothermal at -56.5 °C, then rises in the stratosphere.

Troposphere holds about 75-80% of atmospheric mass and nearly all water vapour; it is the layer of weather, decreasing temperature and strong vertical mixing.

Temperature inversion layers (e.g. tropopause, surface nocturnal inversions) are stable and suppress vertical motion; lapse rate sign reverses within them.

Pressure altitude is the altitude indicated when the subscale is set to 1013.25 hPa; density altitude is pressure altitude corrected for non-standard temperature.

Ozone layer peaks near 20-25 km in the stratosphere and absorbs ultraviolet radiation, warming the stratosphere and producing the temperature increase with height there.

Temperature falls with height in the troposphere at about 2 °C per 1000 ft; a standard-day aircraft at 10,000 ft sits in air near -5 °C.

Pressure decreases with height at roughly 1 hPa per 30 ft near MSL; the decrease is exponential, so 500 hPa lies near 18,000 ft and 250 hPa near 34,000 ft.

Altimeter setting rule: flying from high pressure to low pressure without resetting the subscale causes the aircraft to fly lower than indicated (high-to-low, look out below).

Flying from high to low temperature without resetting causes true altitude to be lower than indicated; cold air means the pressure surface is compressed.

Density altitude rises with high elevation, high temperature, high humidity and low pressure; it increases take-off run, reduces climb rate and raises true airspeed for a given IAS.

True altitude = indicated altitude corrected for altimeter setting error and temperature error; absolute altitude is height above terrain (AGL).

QNH set on the subscale gives elevation when on the ground; QFE gives height above the reference datum, usually aerodrome level; QNE is the standard setting 1013.25 hPa.

Pressure systems: a 1 hPa change of altimeter setting corresponds to about 30 ft of altitude error near sea level.

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

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

Saturated adiabatic lapse rate is about 1.5 °C/1000 ft near the surface and approaches the dry rate (3 °C/1000 ft) aloft; the dry adiabatic rate is 3 °C/1000 ft.

Environmental lapse rate (ELR) compared with the adiabatic rates determines stability: ELR < SALR is absolutely stable, ELR > DALR is absolutely unstable, in between is conditionally unstable.

Latent heat: condensation releases about 2,260 J per gram of water; evaporation absorbs the same, which is why saturated ascent cools more slowly.

Mixing ratio and specific humidity are conserved in adiabatic vertical motion; relative humidity is not conserved.

The lifted condensation level (LCL) is the height at which rising air reaches saturation; convective cloud bases sit near the LCL.

Stable air resists vertical displacement and returns to its level; unstable air continues to rise once displaced, favouring showers, turbulence and gusty surface winds.

Cloud families by height: high (cirrus, cirrostratus, cirrocumulus, above about 20,000 ft), middle (altostratus, altocumulus, 6,500-20,000 ft), low (stratus, stratocumulus, below 6,500 ft) and vertical-development clouds (cumulus, cumulonimbus).

Cumuliform clouds indicate instability; stratiform clouds indicate stability; lenticular and cap clouds indicate strong wind over terrain or a stable layer.

Continuous precipitation falls from nimbostratus; showers from cumuliform cloud; drizzle from stratus; hail from cumulonimbus with strong updraughts.

Radiation fog forms over land on clear, calm nights with high relative humidity and a nocturnal inversion; it dissipates with surface heating or wind.

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

Steam fog forms when cold air moves over warm water; frontal fog forms along a frontal zone by rain evaporating into cooler air below.

Fog requires a high condensation nucleus count, cooling to dew point and light wind; a surface wind of about 5 kt favours fog, strong wind disperses it.

Orographic cloud and upslope fog form when moist air is forced to ascend terrain and cools adiabatically to its condensation level.

Air mass source regions: maritime tropical (warm, moist), continental tropical (hot, dry), maritime polar (cool, moist), continental polar (cold, dry), continental arctic (very cold, dry).

Cold front: steep slope, fast movement, narrow band of heavy showers, squalls and cumulonimbus, rapid clearing and a wind veer with a temperature drop behind it.

Warm front: shallow slope, slow movement, extensive stratiform cloud and steady rain ahead, poor visibility, gradual clearing and a wind veer after passage.

Occluded front forms where a cold front catches a warm front; weather depends on whether it is cold or warm occlusion.

Stationary front lies nearly parallel to the upper flow with little movement and can persist for days with prolonged cloud and rain.

Mid-latitude depression life cycle: wave development on a polar front, deepening with frontal occlusion, then filling; associated with a low-pressure centre and fronts.

Troughs and ridges: a trough is a region of relatively low pressure favouring ascent, cloud and precipitation; a ridge favours subsidence and fine weather.

Tropical revolving storms need sea-surface temperature above about 26.5 °C, high humidity, low vertical wind shear and at least 5° latitude from the equator for Coriolis turning.

Geostrophic wind flows parallel to isobars with low pressure to the left in the Southern Hemisphere; the balance is between pressure gradient force and Coriolis force.

Surface friction backs the wind towards low pressure (cross-isobar angle of about 30° over land, 10-20° over sea) and reduces its speed relative to the geostrophic value.

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

Sea breeze blows from sea to land by day as land heats faster; land breeze blows from land to sea at night; the sea breeze front can trigger cumulus or showers.

Anabatic (upslope) winds occur on sun-warmed slopes by day; katabatic (downslope) winds occur at night as cold dense air drains into valleys.

Föhn or lee waves: stable air crossing a ridge produces lenticular cloud, rotors and severe downdraughts on the lee side; indicate mountain-wave turbulence.

Upper-level winds: the jet stream lies near the tropopause, is strongest in winter, and its position steers surface lows and the movement of fronts.

Wind veers when direction changes clockwise and backs when it changes anticlockwise; veering with height indicates warm advection.

Thunderstorm requirements: sufficient moisture, conditional instability, and a lifting trigger; the mature stage produces hail, lightning, strong gusts and microbursts.

Microburst: a small, intense downdraught causing a headwind then tailwind shear near the surface; escape by maximum power, pitch up, and do not chase the airspeed indicator.

Structural icing needs visible moisture and ambient temperature between 0 °C and about -20 °C; clear ice from large supercooled drops, rime from small drops.

Freezing rain indicates a warm layer aloft above a sub-freezing layer near the surface and carries severe icing risk.

Carburettor icing can occur with temperatures between about -10 °C and +30 °C and high relative humidity, even in clear air.

Clear-air turbulence is most likely near jet streams, in strong wind shear and near mountain waves; it is undetectable by airborne radar.

Wake turbulence vortices sink and drift with the wind; stay above the preceding aircraft's flight path and land beyond its touchdown point.

Wind shear and low-level jets are reported as significant weather; PIREP and low-level wind shear alert systems give warning of shear on approach and departure.

METAR: routine aerodrome report issued typically at hourly intervals; SPECI is a special report issued on significant weather change.

METAR wind is reported in degrees true and speed in knots; VRB indicates a variable direction, and 9999 means 10 km or more visibility.

Cloud in METAR is reported as amount (FEW 1-2 oktas, SCT 3-4, BKN 5-7, OVC 8) and height in hundreds of feet above aerodrome level; NSC means no significant cloud.

QNH in METAR is the altimeter setting that reads aerodrome elevation on the ground; temperature and dew point are reported in whole degrees Celsius.

TAF: aerodrome forecast with validity period, wind, visibility, weather and cloud; change groups BECMG, TEMPO and probabilistic groups describe expected variations.

GAFOR or area forecasts and SIGMET advise of significant weather such as thunderstorms, severe turbulence, icing and volcanic ash; AIRMET covers less severe phenomena.

PIREP/UA reports actual conditions aloft from pilots: turbulence, icing, cloud tops and bases, wind and temperature, and are a key supplement to forecasts.

Trend forecasts appended to METAR give a landing forecast for the next 2 hours using BECMG, TEMPO or NOSIG.

Surface synoptic chart: isobars at 4 hPa intervals, fronts, pressure centres; closely spaced isobars mean strong wind.

Significant weather (SIGWX) chart shows fronts, cloud, turbulence, icing and jet streams for a specified level band and validity time.

Upper-air charts (e.g. 500 hPa, 200 hPa) show contours, temperature and wind; the 500 hPa chart indicates steering flow for surface systems.

Prognostic charts carry a valid time; always compare issue time and validity with the planned flight time and note forecast uncertainty.

Low-level significant weather charts cover below about 24,000 ft and depict cloud, turbulence, icing, fronts and freezing level.

Freezing level and 0 °C isotherm charts identify where icing risk begins; the tropopause height chart shows where the jet stream and CAT are likely.

Route planning: compare forecast winds aloft for headwind or tailwind, check alternate aerodrome weather against minima, and note TAF validity and trend.

Volcanic ash and tropical cyclone advisories are operational hazards; ash must be avoided entirely, and cyclone forecast tracks define no-fly zones.

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.

Permanent gases are well mixed to about 80 km (homosphere); above the homopause (heterosphere) diffusive separation makes composition height-dependent.

Standard atmosphere (ISA): MSL pressure 1013.25 hPa, temperature 15 °C, density 1.225 kg/m³, lapse rate 1.98 °C/1000 ft (6.5 °C/km) to 36,090 ft (11 km).

ISA tropopause: 36,090 ft, temperature -56.5 °C; from 11 km to 20 km temperature is isothermal at -56.5 °C, then rises in the stratosphere.

Troposphere holds about 75-80% of atmospheric mass and nearly all water vapour; it is the layer of weather, decreasing temperature and strong vertical mixing.

Temperature inversion layers (e.g. tropopause, surface nocturnal inversions) are stable and suppress vertical motion; lapse rate sign reverses within them.

Pressure altitude is the altitude indicated when the subscale is set to 1013.25 hPa; density altitude is pressure altitude corrected for non-standard temperature.

Ozone layer peaks near 20-25 km in the stratosphere and absorbs ultraviolet radiation, warming the stratosphere and producing the temperature increase with height there.

Temperature falls with height in the troposphere at about 2 °C per 1000 ft; a standard-day aircraft at 10,000 ft sits in air near -5 °C.

Pressure decreases with height at roughly 1 hPa per 30 ft near MSL; the decrease is exponential, so 500 hPa lies near 18,000 ft and 250 hPa near 34,000 ft.

Altimeter setting rule: flying from high pressure to low pressure without resetting the subscale causes the aircraft to fly lower than indicated (high-to-low, look out below).

Flying from high to low temperature without resetting causes true altitude to be lower than indicated; cold air means the pressure surface is compressed.

Density altitude rises with high elevation, high temperature, high humidity and low pressure; it increases take-off run, reduces climb rate and raises true airspeed for a given IAS.

True altitude = indicated altitude corrected for altimeter setting error and temperature error; absolute altitude is height above terrain (AGL).

QNH set on the subscale gives elevation when on the ground; QFE gives height above the reference datum, usually aerodrome level; QNE is the standard setting 1013.25 hPa.

Pressure systems: a 1 hPa change of altimeter setting corresponds to about 30 ft of altitude error near sea level.

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

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

Saturated adiabatic lapse rate is about 1.5 °C/1000 ft near the surface and approaches the dry rate (3 °C/1000 ft) aloft; the dry adiabatic rate is 3 °C/1000 ft.

Environmental lapse rate (ELR) compared with the adiabatic rates determines stability: ELR < SALR is absolutely stable, ELR > DALR is absolutely unstable, in between is conditionally unstable.

Latent heat: condensation releases about 2,260 J per gram of water; evaporation absorbs the same, which is why saturated ascent cools more slowly.

Mixing ratio and specific humidity are conserved in adiabatic vertical motion; relative humidity is not conserved.

The lifted condensation level (LCL) is the height at which rising air reaches saturation; convective cloud bases sit near the LCL.

Stable air resists vertical displacement and returns to its level; unstable air continues to rise once displaced, favouring showers, turbulence and gusty surface winds.

Cloud families by height: high (cirrus, cirrostratus, cirrocumulus, above about 20,000 ft), middle (altostratus, altocumulus, 6,500-20,000 ft), low (stratus, stratocumulus, below 6,500 ft) and vertical-development clouds (cumulus, cumulonimbus).

Cumuliform clouds indicate instability; stratiform clouds indicate stability; lenticular and cap clouds indicate strong wind over terrain or a stable layer.

Continuous precipitation falls from nimbostratus; showers from cumuliform cloud; drizzle from stratus; hail from cumulonimbus with strong updraughts.

Radiation fog forms over land on clear, calm nights with high relative humidity and a nocturnal inversion; it dissipates with surface heating or wind.

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

Steam fog forms when cold air moves over warm water; frontal fog forms along a frontal zone by rain evaporating into cooler air below.

Fog requires a high condensation nucleus count, cooling to dew point and light wind; a surface wind of about 5 kt favours fog, strong wind disperses it.

Orographic cloud and upslope fog form when moist air is forced to ascend terrain and cools adiabatically to its condensation level.

Air mass source regions: maritime tropical (warm, moist), continental tropical (hot, dry), maritime polar (cool, moist), continental polar (cold, dry), continental arctic (very cold, dry).

Cold front: steep slope, fast movement, narrow band of heavy showers, squalls and cumulonimbus, rapid clearing and a wind veer with a temperature drop behind it.

Warm front: shallow slope, slow movement, extensive stratiform cloud and steady rain ahead, poor visibility, gradual clearing and a wind veer after passage.

Occluded front forms where a cold front catches a warm front; weather depends on whether it is a cold or warm occlusion.

Stationary front lies nearly parallel to the upper flow with little movement and can persist for days with prolonged cloud and rain.

Mid-latitude depression life cycle: wave development on a polar front, deepening with frontal occlusion, then filling; associated with a low-pressure centre and fronts.

Troughs and ridges: a trough is a region of relatively low pressure favouring ascent, cloud and precipitation; a ridge favours subsidence and fine weather.

Tropical revolving storms need sea-surface temperature above about 26.5 °C, high humidity, low vertical wind shear and at least 5° latitude from the equator for Coriolis turning.

Geostrophic wind flows parallel to isobars with low pressure to the left in the Southern Hemisphere; the balance is between pressure gradient force and Coriolis force.

Surface friction backs the wind towards low pressure (cross-isobar angle of about 30° over land, 10-20° over sea) and reduces its speed relative to the geostrophic value.

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

Sea breeze blows from sea to land by day as land heats faster; land breeze blows from land to sea at night; the sea breeze front can trigger cumulus or showers.

Anabatic (upslope) winds occur on sun-warmed slopes by day; katabatic (downslope) winds occur at night as cold dense air drains into valleys.

Föhn or lee waves: stable air crossing a ridge produces lenticular cloud, rotors and severe downdraughts on the lee side; indicate mountain-wave turbulence.

Upper-level winds: the jet stream lies near the tropopause, is strongest in winter, and its position steers surface lows and the movement of fronts.

Wind veers when direction changes clockwise and backs when it changes anticlockwise; veering with height indicates warm advection.

Thunderstorm requirements: sufficient moisture, conditional instability, and a lifting trigger; the mature stage produces hail, lightning, strong gusts and microbursts.

Microburst: a small, intense downdraught causing a headwind then tailwind shear near the surface; escape by maximum power, pitch up, and do not chase the airspeed indicator.

Structural icing needs visible moisture and ambient temperature between 0 °C and about -20 °C; clear ice from large supercooled drops, rime from small drops.

Freezing rain indicates a warm layer aloft above a sub-freezing layer near the surface and carries severe icing risk.

Carburettor icing can occur with temperatures between about -10 °C and +30 °C and high relative humidity, even in clear air.

Clear-air turbulence is most likely near jet streams, in strong wind shear and near mountain waves; it is undetectable by airborne radar.

Wake turbulence vortices sink and drift with the wind; stay above the preceding aircraft's flight path and land beyond its touchdown point.

Wind shear and low-level jets are reported as significant weather; PIREP and low-level wind shear alert systems give warning of shear on approach and departure.

METAR: routine aerodrome report issued typically at hourly intervals; SPECI is a special report issued on significant weather change.

METAR wind is reported in degrees true and speed in knots; VRB indicates a variable direction, and 9999 means 10 km or more visibility.

Cloud in METAR is reported as amount (FEW 1-2 oktas, SCT 3-4, BKN 5-7, OVC 8) and height in hundreds of feet above aerodrome level; NSC means no significant cloud.

QNH in METAR is the altimeter setting that reads aerodrome elevation on the ground; temperature and dew point are reported in whole degrees Celsius.

TAF: aerodrome forecast with validity period, wind, visibility, weather and cloud; change groups BECMG, TEMPO and probabilistic groups describe expected variations.

GAFOR or area forecasts and SIGMET advise of significant weather such as thunderstorms, severe turbulence, icing and volcanic ash; AIRMET covers less severe phenomena.

PIREP/UA reports actual conditions aloft from pilots: turbulence, icing, cloud tops and bases, wind and temperature, and are a key supplement to forecasts.

Trend forecasts appended to METAR give a landing forecast for the next 2 hours using BECMG, TEMPO or NOSIG.

Surface synoptic chart: isobars at 4 hPa intervals, fronts, pressure centres; closely spaced isobars mean strong wind.

Significant weather (SIGWX) chart shows fronts, cloud, turbulence, icing and jet streams for a specified level band and validity time.

Upper-air charts (e.g. 500 hPa, 200 hPa) show contours, temperature and wind; the 500 hPa chart indicates steering flow for surface systems.

Prognostic charts carry a valid time; always compare issue time and validity with the planned flight time and note forecast uncertainty.

Low-level significant weather charts cover below about 24,000 ft and depict cloud, turbulence, icing, fronts and freezing level.

Freezing level and 0 °C isotherm charts identify where icing risk begins; the tropopause height chart shows where the jet stream and CAT are likely.

Route planning: compare forecast winds aloft for headwind or tailwind, check alternate aerodrome weather against minima, and note TAF validity and trend.

Volcanic ash and tropical cyclone advisories are operational hazards; ash must be avoided entirely, and cyclone forecast tracks define no-fly zones.