Aviation Meteorology: Complete RPL & PPL Study Guide

Understanding the atmosphere is fundamental to safe flight and is a core topic in both the RPL and PPL CASA examinations. This section covers the structure of the atmosphere, the International Standard Atmosphere, air density, atmospheric stability, humidity, fog formation, and the effect of pressure changes on altimetry.

Structure of the Atmosphere

The Earth's atmosphere is divided into vertical layers. Most significant weather phenomena affecting aviation occur in the lowest layer, known as the troposphere. The troposphere extends from the surface to approximately 36,000 ft (11 km) at mid-latitudes, though it is higher at the equator (~55,000 ft) and lower at the poles (~25,000 ft). It is capped by the tropopause, a boundary layer above which lies the stratosphere. Temperature generally decreases with altitude in the troposphere, but remains approximately constant or increases slightly in the lower stratosphere.

International Standard Atmosphere (ISA)

Aviation performance calculations rely on the International Standard Atmosphere (ISA), which provides a baseline for comparing actual atmospheric conditions. ISA assumes the following sea level values:

• Temperature: 15°C
• Pressure: 1,013.25 hPa (commonly rounded to 1,013.2 hPa in Australian references)
• Density: 1.225 kg/m³
• Temperature lapse rate: 1.98°C per 1,000 ft (approximately 2°C per 1,000 ft) up to 36,090 ft
• Pressure lapse rate: approximately 1 hPa per 30 ft near sea level (this decreases with altitude)

When actual conditions differ from ISA, pilots describe conditions as ISA+ or ISA− (for example, ISA+10 means the actual temperature is 10°C warmer than the ISA value for that altitude). Variations from ISA directly affect aircraft performance, including takeoff distance, rate of climb, and true airspeed. This is frequently tested in the CASA exam.

Air Density and Density Altitude

Air density is a critical factor in aviation. Air density decreases (that is, the air becomes thinner) with:

• Increasing altitude (lower pressure)
• Increasing temperature
• Increasing humidity (moist air is less dense than dry air at the same temperature and pressure, because water vapour molecules are lighter than nitrogen and oxygen molecules)

Lower air density reduces engine power output, propeller efficiency, and aerodynamic lift, resulting in degraded aircraft performance. Density altitude is pressure altitude corrected for non-standard temperature, and is used to determine aircraft performance. A high density altitude indicates reduced performance.

Atmospheric Stability

Atmospheric stability describes the atmosphere's tendency to resist or encourage vertical motion of air parcels. Understanding stability is vital for predicting weather and turbulence.

• A stable atmosphere resists vertical motion. Air that is displaced vertically tends to return to its original level. A moist, stable atmosphere is most likely to result in the formation of stratus cloud (layered cloud), poor visibility, drizzle, and fog.
• An unstable atmosphere encourages vertical motion. Displaced air continues to rise or sink. A moist, unstable atmosphere favours the development of cumulus and cumulonimbus clouds, thunderstorms, heavy rain, and turbulence.

The stability of the atmosphere is largely determined by the environmental lapse rate (ELR) — the actual rate of temperature decrease with altitude. When the ELR is greater than the dry adiabatic lapse rate (DALR, approximately 3°C per 1,000 ft), the atmosphere is absolutely unstable. When the ELR is less than the saturated adiabatic lapse rate (SALR, approximately 1.5°C per 1,000 ft, though it varies), the atmosphere is absolutely stable. Between these values, the atmosphere is conditionally unstable.

Humidity, Dew Point, and Temperature Inversions

Understanding the relationship between temperature and moisture content is essential for predicting cloud formation, fog, and visibility. The key concepts are:

• Relative humidity: the amount of water vapour in the air expressed as a percentage of the maximum amount the air could hold at that temperature. Air is saturated at 100% relative humidity.
• Dew point: the temperature to which air must be cooled (at constant pressure) to become saturated. When the air temperature and dew point converge, condensation — cloud or fog formation — is likely.
• Temperature inversion: a layer in which temperature increases with altitude rather than decreasing. Inversions act as a lid, trapping moisture, pollutants, and haze below, reducing visibility and inhibiting convection.

Fog Formation

Fog formation is of particular concern to pilots. The prime conditions for radiation fog (early morning fog) include:

• Clear skies (allowing maximum radiative cooling of the ground overnight)
• Light winds (approximately 2–8 kt — enough to gently mix the cooling air, but not strong enough to disperse it)
• High relative humidity or a small temperature–dew point spread
• Moist ground or proximity to bodies of water
• Long nights (more common in autumn and winter)

Other fog types pilots should be aware of include advection fog (warm moist air moves over a cooler surface), steam fog (cold air moves over warmer water), and upslope fog (moist air is forced up rising terrain).

Pressure Changes and Altimetry

Pilots should understand the effect of pressure changes on altimetry. When flying from an area of high pressure to low pressure without resetting the altimeter subscale (QNH), the altimeter will over-read — the aircraft will be lower than indicated. The memory aid is: "High to low, look out below." Similarly, flying from warm air into cold air without correction will cause the altimeter to over-read.

Wind behaviour is a core topic in aviation meteorology for both the RPL and PPL exams. This section covers pressure gradients, Buys Ballot's law, wind direction changes, and diurnal variation — all of which are frequently tested in the CASA exam.

Pressure gradients and isobars

Wind is primarily driven by pressure gradients — the difference in atmospheric pressure between two areas. On a synoptic weather map, isobars drawn very close together indicate that winds will be strong in that area. The closer the isobars, the steeper the pressure gradient, and the stronger the resulting wind.

Buys Ballot's law

The relationship between pressure and wind direction is described by Buys Ballot's law, which helps pilots assess the approximate location of high and low-pressure systems relative to their position. Understanding this law allows you to mentally place pressure systems on a synoptic chart based on observed wind direction alone.

Wind direction changes: veering and backing

Pilots must differentiate between varying wind phenomena. A gust is a brief surge in wind speed, while a squall is a longer-lasting sudden increase. Wind direction changes are described as either veering (changing in a clockwise direction) or backing (changing in a counter-clockwise direction).

Diurnal variation in surface winds

Surface winds are often influenced by diurnal variation — changes in wind strength that occur over a 24-hour period due to solar heating. As the sun heats the surface during the day, surface winds typically increase; they tend to ease again overnight as temperatures drop. This is an important consideration when planning flight times and assessing conditions at aerodromes.

Clouds are classified into 10 genera based on their height (high, medium, low), vertical extent, and characteristics (cumuliform, stratiform, or a combination). Understanding this classification system is essential for pilots to predict associated weather and hazards.

Cloud height categories

• High clouds (above ~20,000 ft): Cirrus (Ci), Cirrostratus (Cs), and Cirrocumulus (Cc) — composed of ice crystals.
• Medium clouds (~6,500–20,000 ft): Altostratus (As) and Altocumulus (Ac) — may contain supercooled water droplets, creating an icing risk.
• Low clouds (surface–~6,500 ft): Stratus (St), Stratocumulus (Sc), and Nimbostratus (Ns) — associated with reduced visibility and low ceilings.
• Clouds with significant vertical extent: Cumulus (Cu), Towering Cumulus (TCu), and Cumulonimbus (Cb) — these can extend from low levels through to the upper troposphere and are associated with severe turbulence, heavy precipitation, icing, windshear, and thunderstorms.

Cloud characteristics

Cumuliform clouds develop vertically through convection in unstable air. They are associated with turbulence, gusty winds, and shower-type precipitation. Stratiform clouds develop horizontally in stable air and are associated with widespread, steady precipitation and reduced visibility.

Meteorologists use standard abbreviations for cloud types in forecasts and reports (e.g., Cu, Cb, Ci, Ns, Ac). Pilots must recognise these abbreviations in METARs, TAFs, and SIGMET/AIRMET information. This is frequently tested in the CASA exam.

Precipitation types

• Drizzle (DZ): Very small, closely spaced water droplets falling from stratiform clouds. Reduces visibility significantly.
• Rain (RA): Larger water drops that may be continuous (stratiform origin) or intermittent.
• Showers (SH): Precipitation from cumuliform clouds, characterised by sudden onset, rapid changes in intensity, and relatively brief duration.
• Virga: Precipitation that falls from a cloud base but evaporates before reaching the ground. Can indicate windshear and turbulence beneath the cloud.
• Hail (GR/GS): Ice precipitation associated with Cumulonimbus clouds — a serious hazard to aircraft structure and engines.
• Snow (SN): Relevant at altitude and in certain Australian regions, particularly during winter operations near elevated terrain.

Hazards for pilots

Cumulonimbus (Cb) is the most hazardous cloud type, producing severe turbulence, hail, lightning, heavy rain, windshear, microbursts, and possible tornadoes. Pilots must avoid Cb clouds by a safe margin. Nimbostratus (Ns) produces prolonged, continuous precipitation with very low cloud bases and poor visibility — this is a significant consideration for VFR pilots.

Reduced visibility from any precipitation can cause conditions below Visual Meteorological Conditions (VMC) minima, requiring pilots to remain vigilant and prepared for diversions. Virga beneath clouds can produce dangerous windshear on approach or departure.

Cloud formation

Conditions necessary for cloud formation include the lifting of moist air (through convection, orographic lift, frontal lift, or convergence), cooling to the dew point, and the presence of condensation nuclei. Understanding these processes helps pilots anticipate cloud development during flight planning and in-flight decision-making.

Aviation weather hazards pose significant risks to flight safety. Pilots must recognise signs of thunderstorms, windshear, mountain waves, low-level jet streams, and downdrafts. Understanding these hazards — and how to avoid or manage them — is essential knowledge for both the RPL and PPL CASA exams.

Thunderstorms and microbursts

Thunderstorms bring severe hazards including microbursts, heavy icing, hail, lightning, and extreme turbulence. Pilots should maintain a safe distance from thunderstorm cells — at least 10 NM from severe cells. Microbursts can produce intense downdrafts and outbursts of damaging winds at the surface, with performance-decreasing windshear that may exceed the aircraft's climb capability.

Windshear

Windshear is a sudden change in wind speed and/or direction over a short distance. It is particularly dangerous during takeoff and approach phases. Sources of windshear include thunderstorm gust fronts, microbursts, frontal passages, low-level jet streams, and terrain-induced effects.

Mountain waves

Mountain waves form on the lee side of mountain ranges when stable air flows over elevated terrain. They can produce severe turbulence, particularly in rotor zones beneath the wave crests. Lenticular clouds are a visual indicator of mountain wave activity. Pilots should be alert to mountain wave conditions when crossing ranges such as the Great Dividing Range in strong wind conditions.

Visibility hazards

Visibility can be severely reduced by phenomena such as fog, mist, haze, dust, sand, smoke, and precipitation. Pilots must understand the difference between these phenomena:

• Fog (FG) — visibility reduced to below 1,000 m by water droplets
• Mist (BR) — visibility between 1,000 m and 5,000 m by water droplets
• Haze (HZ) — reduced visibility caused by fine dry particles
• Dust (DU) and sand (SA) — widespread reduced visibility from suspended particles
• Smoke (FU) — visibility reduction from bushfire or industrial smoke

In METAR present weather reports, specific codes are used to identify observed weather phenomena. For example, the code PO in a METAR indicates dust/sand whirls (dust devils) as a currently observed present weather phenomenon. It is important to note that PO is a METAR observation code that may appear in both observations and forecasts. Understanding the difference between METAR (actual observed conditions) and TAF (forecast conditions) codes is essential for CASA exams.

Airframe icing

Airframe icing is a serious hazard that increases drag, reduces lift, adds weight, and can block pitot-static systems and carburettor intakes. Pilots must differentiate between the types of icing:

• Hoar frost — forms on cold surfaces (e.g., parked aircraft overnight) by direct sublimation; must be removed before flight as it disrupts airflow over wings
• Rime ice — rough, opaque ice formed by rapid freezing of small supercooled water droplets; typically found in stratiform cloud
• Clear (glaze) ice — smooth, transparent, heavy ice formed by slow freezing of large supercooled water droplets; the most dangerous form, typically found in cumuliform cloud and freezing rain
• Mixed ice — a combination of rime and clear ice

Icing is most likely between 0°C and −20°C, with the greatest risk near 0°C where large supercooled water droplets are present. Pilots should avoid flying in known icing conditions in aircraft not certified for flight in icing, and should know how to exit icing conditions promptly — typically by changing altitude or routing.

Carburettor icing

Carburettor icing can occur even in ambient temperatures as high as 38°C with moderate humidity, due to the cooling effect of fuel vaporisation and pressure drop in the venturi. Pilots should use carburettor heat as recommended in the aircraft's Pilot Operating Handbook.

Turbulence categories

Pilots should be able to recognise the main categories of turbulence and their causes:

• Convective turbulence — caused by thermal activity and thunderstorms
• Mechanical turbulence — caused by wind flowing over rough terrain or obstacles
Clear air turbulence (CAT) — associated with jet streams and wind shear at altitude
• Wake turbulence — caused by wingtip vortices from preceding aircraft

Prior to flight, obtaining an appropriate weather briefing is mandatory. Pilots use various forecasts and reports to assess conditions along the planned route and at departure, destination, and alternate aerodromes. Under Civil Aviation Safety Regulations (CASR) Part 91 (General Operating and Flight Rules), pilots must not commence a flight unless they have assessed all available meteorological information relevant to the intended flight.

Key Meteorological Products

• TAF (Terminal Area Forecast): A forecast of weather conditions expected at a specific aerodrome, typically covering a period of 24 or 30 hours. Cloud heights in a TAF are always given in Height Above Ground Level (AGL). TAFs are issued for aerodromes that meet certain traffic criteria, and are updated regularly and amended (indicated by TAF AMD) when forecast conditions change significantly.
• GAF (Graphical Area Forecast): The GAF replaced the old text-based ARFOR (Area Forecast) in November 2017. The GAF provides a graphical depiction of forecast weather conditions across defined areas of Australia, including cloud, visibility, weather, turbulence, icing, and freezing level information. The GAF is divided into geographic areas with associated text panels describing expected conditions, and is issued four times daily, covering periods of validity relevant to the flying day. Pilots must be proficient in interpreting GAF charts and their associated area text descriptions.
• GPWT (Grid Point Wind and Temperature): Provides forecast upper winds and temperatures at specific grid points and flight levels. Used in conjunction with the GAF for flight planning, particularly for calculating headings, groundspeeds, and fuel requirements. GPWTs are typically provided at standard flight levels (e.g., 2,000 ft, 5,000 ft, 7,000 ft, 10,000 ft, 14,000 ft AMSL and above).
• METAR (Meteorological Aerodrome Report): A routine observation of actual weather conditions at an aerodrome, issued at scheduled intervals (typically every 30 minutes at major aerodromes or every 60 minutes at others). METARs report actual conditions including wind, visibility, weather phenomena, cloud, temperature, dewpoint, and QNH.
• SPECI (Special Meteorological Report): An unscheduled observation issued when weather conditions change significantly between routine METAR reports — for example, a sudden drop in visibility, a wind shift, the onset or cessation of significant weather phenomena such as thunderstorms, or a cloud base falling below certain thresholds. SPECI reports use the same format as METARs.
• SIGMET and AIRMET: SIGMETs warn of significant meteorological phenomena hazardous to all aircraft (e.g., severe turbulence, volcanic ash, severe icing, thunderstorm activity). AIRMETs warn of phenomena hazardous particularly to light aircraft operating at lower levels (e.g., moderate turbulence, moderate icing, mountain waves, widespread low cloud or reduced visibility). These are issued by the Bureau of Meteorology (BoM) and should be checked as part of every pre-flight briefing.
• ATIS (Automatic Terminal Information Service): A continuous broadcast of recorded aerodrome information at busier aerodromes, including current weather observations, runway in use, and relevant NOTAMs. Pilots should obtain the current ATIS before contacting ATC.

Decoding METARs and TAFs

Decoding meteorological reports is a key pilot skill. The following are the most important codes and conventions you need to understand.

• In a METAR or TAF, the visibility code 9999 indicates that visibility is 10 km or more. This does not specify the exact visibility beyond 10 km — only that it meets or exceeds 10 km.
• Wind is reported as a three-digit true direction and speed in knots (e.g., 27015KT means wind from 270° true at 15 kt). Gusts are indicated with a 'G' followed by the gust speed (e.g., 27015G25KT). In a METAR, winds are reported in degrees true; however, tower-delivered winds (from ATC or ATIS for runway operations) are given in degrees magnetic. Variable winds below 3 kt may be reported as VRB (e.g., VRB02KT).
• Visibility is reported in metres. For example, 3000 means 3,000 m visibility. Visibility below 1,000 m may trigger conditions below VFR minima.
• Cloud amounts are reported using the codes: FEW (1–2 oktas), SCT (scattered, 3–4 oktas), BKN (broken, 5–7 oktas), and OVC (overcast, 8 oktas). Cloud types of significance (e.g., CB for cumulonimbus or TCU for towering cumulus) are appended where applicable. SKC (sky clear) or NSC (nil significant cloud) may also appear.
• CAVOK may be used when visibility is 10 km or more, no cloud below 5,000 ft or below the highest minimum sector altitude (whichever is greater), no cumulonimbus or towering cumulus, and no significant weather.
• Temperature and dewpoint are reported in degrees Celsius (e.g., 18/12 means temperature 18°C, dewpoint 12°C). A small temperature–dewpoint spread indicates high humidity and the potential for fog or low cloud formation.
• QNH is reported as a four-digit pressure value in hectopascals (e.g., Q1013 means QNH 1013 hPa).

Change Groups in Australian TAFs

Australian domestic civil TAFs use specific change indicators as defined by the BoM and CASA. It is important to note that the BoM no longer uses the BECMG (Becoming) change group in Australian domestic civil TAFs. The BECMG indicator was phased out and replaced by the FM (From) indicator for permanent changes. The current change groups used in Australian TAFs are:

• INTER: Indicates intermittent fluctuations in conditions expected to last less than 30 minutes in each instance during the specified period. These fluctuations are expected to occur on and off throughout the stated time window — for example, INTER periods of reduced visibility in rain showers. The conditions described may or may not occur, but if they do, each occurrence lasts less than 30 minutes.
• TEMPO: Indicates temporary fluctuations in conditions expected to last between 30 and 60 minutes in each instance during the specified period. TEMPO conditions are more sustained than INTER conditions but still temporary. The base conditions are expected to return after each occurrence.
• FM (From): Indicates a permanent change in conditions expected from the specified time. The FM group replaces all previously forecast conditions with a new set of conditions from that time onward. In Australian TAFs, FM is the sole indicator used for permanent changes, having replaced the former BECMG group. For example, FM081200 means that from 0812 UTC, the conditions following will prevail permanently for the remainder of the TAF period or until the next FM group.

Interpreting TAF Change Groups

This is frequently tested in the CASA exam. When interpreting TAFs for exam purposes or flight planning, keep the following rules in mind.

• The base conditions stated at the beginning of a TAF (or after an FM group) are the prevailing forecast conditions.
• INTER and TEMPO conditions represent temporary deviations from the base conditions during the specified time period.
• When assessing weather for alternate requirements, the worst-case scenario from all applicable groups (base conditions plus INTER or TEMPO) during the relevant time period must be considered.
• If INTER or TEMPO conditions forecast weather below VFR minima or below landing minima, the pilot must plan accordingly — for example, by nominating an alternate aerodrome or carrying additional fuel.

Using Forecasts for Flight Planning

Pilots must evaluate all available meteorological information to make sound go/no-go decisions. This includes the following considerations.

• Determining whether Visual Meteorological Conditions (VMC) will be maintained along the route and at aerodromes. For VFR operations, minimum VMC requirements must be met as prescribed in CASR Part 91 and the relevant AIP ENR sections.
• Assessing whether an alternate aerodrome is required based on TAF conditions at the destination. If the TAF (including INTER and TEMPO groups) indicates conditions may fall below landing minima during the expected time of arrival, an alternate must be nominated and fuel carried to reach it.
• Calculating fuel requirements correctly based on forecast and actual conditions. Under the CASR Part 91 Manual of Standards (MOS), the VFR fuel requirements for piston-engine aeroplanes are as follows:
  • Taxi fuel — sufficient fuel for engine start, taxi, and run-up.
  • Flight fuel — fuel required to fly from departure to destination, accounting for forecast wind and temperature conditions.
  • Fixed reserve — 30 minutes at normal cruise consumption for Day VFR, or 45 minutes at normal cruise consumption for Night VFR.
  • Alternate fuel — if an alternate aerodrome is required, fuel to fly from the destination to the alternate aerodrome.
  • Holding fuel — fuel for any anticipated holding at the destination or alternate.
  • Additional fuel — any extra fuel the pilot deems necessary for the safe conduct of the flight (e.g., due to weather uncertainty, potential diversions, or navigation uncertainties).
• Identifying hazards such as thunderstorms, turbulence, icing, and low-level wind shear from the GAF, SIGMETs, and AIRMETs. These hazards may necessitate route changes, altitude changes, or a decision not to fly.
• Checking the freezing level (from the GAF) relative to planned cruise altitude to assess icing risk.
• Reviewing NOTAMs in conjunction with weather products to identify temporary airspace restrictions, aerodrome closures, or navigation aid outages that may interact with weather-related decisions.

VFR Fuel Planning Summary for Piston-Engine Aeroplanes

The following summarises the minimum fuel required for a VFR flight in a piston-engine aeroplane under CASR Part 91 MOS. Each component must be calculated and the total carried at departure.

• Taxi fuel
• + Flight fuel (departure to destination)
• + Alternate fuel (if alternate required)
• + Holding fuel (if applicable)
• + Fixed reserve: 30 minutes (Day VFR) or 45 minutes (Night VFR)
• + Additional fuel (at pilot's discretion for safety)
• = Minimum total fuel required at departure

There is no variable (percentage) reserve for VFR piston-engine operations. The 10% variable reserve is an IFR requirement for piston-engine aeroplanes and does not apply to VFR flights. Pilots must always ensure the total fuel on board at departure is sufficient to cover all required fuel components.

Obtaining Weather Briefings

Weather briefings can be obtained via the NAIPS (National Aeronautical Information Processing System) internet service, which provides access to all current TAFs, METARs, GAFs, GPWTs, NOTAMs, SIGMETs, and AIRMETs. NAIPS allows pilots to generate a comprehensive pre-flight briefing package tailored to their planned route and flight time. Pilots should obtain a full briefing and review all relevant products before every flight.

Other sources of weather information include:

• Aerodrome forecasts and weather displays at flying school briefing rooms and approved aerodrome weather stations.
• Bureau of Meteorology (BoM) website and Aviation Weather pages for synoptic charts, satellite imagery, and radar imagery.
• ATIS broadcasts at towered aerodromes for current conditions.
• In-flight weather updates via radio from ATC, Flight Service, or AWIS (Aerodrome Weather Information Service) at selected aerodromes.

Pilot Responsibilities and Weather Decision Making

A pilot must not commence a VFR flight unless they are satisfied, based on all available meteorological information, that VMC will prevail along the route and at the destination (and alternate, if required) for the expected time of arrival. During flight, if weather conditions are encountered that are worse than forecast, the pilot must take appropriate action — which may include diverting to an alternate aerodrome, returning to the departure aerodrome, or landing at the nearest suitable aerodrome. Continued VFR flight into IMC (Instrument Meteorological Conditions) is one of the leading causes of general aviation accidents in Australia.

The following additional considerations apply to weather-related decision making.

• Personal minima: Student and low-hour pilots should establish personal weather minima that are higher than the legal VFR minima. This provides an additional safety margin while experience is being built.
• Deteriorating conditions en route: If conditions deteriorate below VMC en route, a pilot must not continue into IMC. Options include performing a 180° turn to return to VMC, diverting to a suitable aerodrome, or landing at a suitable location if safe to do so. Pre-planning escape routes is a sound practice.
• Fog and low stratus: When the temperature–dewpoint spread is 3°C or less and decreasing, fog or low stratus formation is likely, especially around dawn. Pilots should factor this into planning for early morning or late afternoon flights.
• Thunderstorm avoidance: Pilots should maintain a clearance of at least 10 NM from any thunderstorm cell and should never attempt to fly beneath a thunderstorm due to severe turbulence, wind shear, and microburst hazards.

Want to master TAF reading? See our dedicated guide: How to Read a TAF.

Related Resources

• How to Read a TAF
• Navigation
• RPL Meteorology Blog