ATPL 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.

Standard atmosphere (ISA): MSL pressure 1013.25 hPa, temperature +15 °C, density 1.225 kg/m³; tropospheric lapse rate 6.5 °C/km up to 11 km (tropopause), then isothermal -56.5 °C up to 20 km.

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

Tropopause height varies: about 16 km over the equator, about 8-10 km over the poles; it is higher in summer and lower in winter.

Temperature inversion in the stratosphere is caused by ozone absorbing ultraviolet radiation; ozone concentration peaks near 20-25 km.

Pressure halves roughly every 5500 m in the lower atmosphere (rule of thumb); density halves at about the same rate.

Above 11 km in ISA, pressure and density continue to decrease although temperature is constant; altimeter errors grow with height above the tropopause.

The ionosphere reflects radio waves; the ozonosphere absorbs UV; the homosphere (below about 80 km) has near-uniform gas proportions.

QNH set on the subscale gives elevation above mean sea level; QFE gives height above the aerodrome reference point; QNE (1013.25 hPa) gives pressure altitude/flight level.

Pressure altitude = indicated altitude when subscale is 1013.25 hPa; true altitude differs from indicated altitude when temperature differs from ISA.

Cold air: true altitude is lower than indicated altitude (altimeter over-reads); warm air: true altitude is higher than indicated altitude.

1 hPa pressure change corresponds to about 27 ft near MSL and about 30 ft at higher elevations; 1 inHg equals 33.86 hPa.

Density altitude = pressure altitude corrected for temperature deviation from ISA; high density altitude reduces engine power, propeller thrust and lift, and increases take-off and landing distances.

High-to-low pressure transition (flying from high to low QNH) causes indicated altitude to be higher than true altitude; the 'high-to-low, look out below' rule.

Temperature decreases with height in the troposphere; in an inversion, temperature increases with height, which suppresses vertical mixing.

Diurnal temperature range is greatest over land under clear skies and light winds; cloud cover and wind reduce it.

Relative humidity = (actual vapour pressure / saturation vapour pressure) × 100%; it changes with temperature even when the water vapour content is constant.

Dew point is the temperature to which air must be cooled at constant pressure and moisture content to reach saturation; spread = temperature minus dew point.

When the dew point spread is 2 °C or less and the temperature is close to the dew point, fog or low cloud is likely.

Latent heat of vaporisation is about 2.5 × 10^6 J/kg; latent heat of fusion is about 3.34 × 10^5 J/kg; condensation releases latent heat and warms the air.

Sublimation is a direct change from ice to water vapour; deposition is water vapour to ice; both occur in clouds containing ice crystals.

Mixing ratio and specific humidity are conserved during adiabatic ascent; relative humidity increases as the air cools.

Saturation vapour pressure depends only on temperature and increases exponentially; warm air can hold much more water vapour than cold air.

Condensation requires not only saturation but also condensation nuclei; in very clean air, supersaturation of several percent may occur.

Environmental lapse rate (ELR) is the actual temperature decrease with height; DALR = 9.8 °C/km (3 °C/1000 ft), SALR is about 6 °C/km (1.8 °C/1000 ft) and varies with moisture.

Absolute instability: ELR > DALR (9.8 °C/km); absolutely stable: ELR < SALR; conditionally unstable: SALR < ELR < DALR.

A rising unsaturated parcel cools at the DALR; once saturated, it cools at the SALR, which is less than the DALR because of released latent heat.

Level of free convection (LFC) is the height at which a saturated parcel becomes warmer than the environment; the equilibrium level (EL) is where it again becomes colder.

Stable air suppresses vertical motion, produces stratiform cloud and steady precipitation, and limits turbulence and visibility problems to the surface layer.

Unstable air favours cumuliform cloud, showers, thunderstorms, gusty winds and good visibility between showers.

Temperature inversion aloft acts as a lid, trapping moisture and pollutants below it and causing haze or stratus; it also caps convective development.

Potential instability occurs when a layer has decreasing humidity with height; lifting the whole layer can make it unstable.

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

Radiation fog forms over land on clear, calm nights with high relative humidity and a small dew point spread; it usually dissipates after sunrise.

Advection fog forms when warm, moist air moves over a cold surface; it can occur over land or sea, is often persistent and may be accompanied by wind.

Frontal fog forms when rain falls through cooler air below and saturates it; steaming fog forms when cold air moves over a warm water surface.

Orographic cloud and upslope fog form when moist air is forced to rise over hills; the windward side is cloudy and the leeward side is drier (foehn effect).

Continuous rain is typical of nimbostratus; showers come from cumuliform clouds; drizzle comes from stratus; hail forms in cumulonimbus with strong updraughts.

Freezing rain occurs when raindrops fall through a warm layer and then a sub-freezing layer; supercooled water freezes on impact with the aircraft or ground (clear ice).

Snow forms when the entire temperature profile below the cloud is below 0 °C; sleet occurs when snow melts and then refreezes, or partially melts.

Air mass source regions: maritime tropical (mT), continental tropical (cT), maritime polar (mP), continental polar (cP), and arctic/antarctic (A).

Cold front: steep slope (about 1:50), narrow band of heavy showers and thunderstorms, rapid passage, wind veers and temperature falls sharply.

Warm front: shallow slope (about 1:150), wide band of stratiform cloud and steady rain ahead of the surface front, gradual changes, wind veers.

Occluded front: a cold front overtakes a warm front; warm sector is lifted off the ground; mixed weather with cloud and precipitation near the triple point.

Stationary front: little movement, prolonged cloud and precipitation; often associated with quasi-stationary lows.

Frontal passage indicators: wind veer, temperature change, pressure tendency (fall then rise for a cold front), dew point change and visibility change.

Polar front is the boundary between polar and tropical air; it is the main source region for mid-latitude depressions.

Squall line is a line of active thunderstorms ahead of or along a cold front, often with strong gusty winds and heavy rain.

In the Northern Hemisphere, wind blows anticlockwise around a low and clockwise around a high; friction causes the wind to cross the isobars towards the low.

Geostrophic wind is parallel to straight isobars above the friction layer; it results from the balance between the pressure gradient force and the Coriolis force.

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

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

Anabatic (upslope) winds blow up valley sides by day; katabatic (downslope) winds blow down at night; katabatic winds can be strong in polar regions.

Foehn wind is warm and dry on the leeward side of mountains; it can cause rapid temperature rises and clear skies.

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

Wind speed increases with height in the friction layer; the surface wind is typically about 2/3 of the geostrophic wind and is backed relative to it.

Turbulence types: mechanical (terrain/obstacles), thermal (convection), frontal, wind shear and clear air turbulence (CAT); CAT is common near jet streams and in mountain waves.

Wind shear is a change in wind speed and/or direction over a short distance; it is most critical at low level during take-off and landing (microburst, gust front).

Structural icing requires visible moisture and an airframe temperature at or below 0 °C; clear ice forms from large supercooled drops, rime ice from small drops.

Clear ice is heavy, uneven and dangerous; rime ice is rough and lighter; mixed ice is a combination; ice increases weight, drag and stall speed and reduces lift.

Freezing rain or freezing drizzle is the most severe icing hazard; carburettor icing can occur at outside air temperatures between about -10 °C and +25 °C with high humidity.

Thunderstorm life cycle: cumulus (updraughts), mature (updraughts and downdraughts, lightning, hail, heavy rain), dissipating (downdraughts dominate).

Microburst is a strong, localized downdraught that produces a damaging outflow; it causes a sudden headwind then tailwind shear and a loss of performance.

Volcanic ash and sandstorms damage engines and windscreens; mountain waves produce severe turbulence, downdraughts and lenticular cloud.

METAR: routine aerodrome weather report issued usually every 30 or 60 minutes; includes wind, visibility, runway visual range, weather, cloud, temperature, dew point, QNH and trend.

TAF: aerodrome forecast valid typically for 9, 24 or 30 hours; includes wind, visibility, weather, cloud and expected changes (BECMG, TEMPO, FM).

SPECI: special report issued when significant weather changes occur; SIGMET: warning of significant weather such as thunderstorms, turbulence, icing, volcanic ash or tropical cyclones.

METAR wind is given in degrees true and speed in knots; VRB means variable direction; G denotes gusts; 9999 means visibility 10 km or more.

Cloud amount codes: FEW (1-2 oktas), SCT (3-4), BKN (5-7), OVC (8); height is given in hundreds of feet above aerodrome level.

Weather codes: RA rain, SN snow, DZ drizzle, SH showers, TS thunderstorm, GR hail, FG fog, BR mist, HZ haze, FZ freezing, SQ squall, FC funnel cloud.

Significant weather charts (SIGWX) show fronts, pressure systems, jet streams, turbulence and icing areas and cloud; they are used for flight planning.

Wind and temperature aloft charts give forecast winds and temperatures at standard pressure levels (e.g., FL50, FL100, FL180, FL300, FL340) for route planning.

Intertropical convergence zone (ITCZ) is a belt of low pressure near the equator where trade winds converge; it brings heavy convective showers and thunderstorms.

Subtropical high-pressure belts (horse latitudes) are found near 30° latitude; they are associated with descending air, clear skies and light winds.

Trade winds blow from the subtropical highs towards the equator, deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Mid-latitude westerlies and the polar front dominate weather in temperate latitudes; depressions travel eastwards along the polar front.

Monsoon circulations reverse seasonally: summer onshore winds bring heavy rain, winter offshore winds bring dry conditions (e.g., South Asia, West Africa).

Seasonal weather: summer brings more convection, thunderstorms and tropical cyclones; winter brings fog, low stratus, icing and snow in temperate regions.

Tropical cyclones form over warm oceans (sea surface temperature above about 26.5 °C), between about 5° and 20° latitude, and do not form at the equator.

Flight planning: check METAR/TAF, SIGWX, wind aloft, freezing level, turbulence and icing forecasts, and alternate aerodrome weather; consider density altitude and fuel reserves.