Chapter 5 - Meteorology

These notes are exam-focused for CASA PPL meteorology, with operational interpretation emphasis for VFR decision making.

How to use this chapter

Study habits: Decode one METAR and TAF daily; mark trends on a GAF map. Sketch a cold front cross-section and a mountain-wave airflow diagram when revising hazards.

5.1 Atmosphere Fundamentals

Why this matters

Stability controls whether a small lifting force grows into CB or stays benign stratus — it drives your go/no-go below legal minima.

Definition — atmosphere: the envelope of gases surrounding Earth; weather occurs mainly in the troposphere (surface to about 10–16 km, higher at equator).

Composition (exam awareness)

Pressure, temperature, and density

• ISA (International Standard Atmosphere): reference model (15°C at sea level, 1013.25 hPa, standard lapse rates) used for performance charts and comparisons.

• QNH / QFE: subscale settings for altimeter — QNH gives height above mean sea level; critical for terrain clearance and circuit height discipline.

• FAA PHAK — atmosphere / weather intro
• Bureau of Meteorology — aviation weather

flowchart TD
    A[Surface heating and moisture] --> B[Vertical motion]
    B --> C{Stable or unstable?}
    C -- Stable --> D[Stratiform cloud / limited vertical growth]
    C -- Unstable --> E[Convective cloud / showers / turbulence]

5.2 Pressure Systems and Wind

Definition — pressure gradient: change in pressure over distance; air tends to move from higher to lower pressure (wind).

Definition — Coriolis effect: apparent deflection of moving air due to Earth’s rotation; in the Southern Hemisphere, flow is deflected to the left.

Definition — gradient wind: wind parallel to isobars aloft, balanced by pressure gradient and Coriolis (friction small).

Definition — surface wind: modified by friction and terrain; typically crosses isobars toward low pressure at surface.

Southern Hemisphere high and low (memory)

• Stronger isobar spacing → stronger pressure gradient → stronger wind.

• Sea breeze / land breeze: local pressure/temperature differences near coasts — can shift wind direction during the day.

• FAA PHAK — wind and pressure systems
• BOM — isobar and synoptic chart help

5.3 Stability, Lapse Rates, and Vertical Motion

Definition — atmospheric stability: resistance of a parcel of air to vertical displacement; determines whether lifted air continues rising or returns.

Definition — lapse rate: rate at which temperature decreases with height.

Stable vs unstable (parcel test — exam logic)

Inversion

Definition — inversion: temperature increases with height over a layer (reverse of normal lapse).

• FAA PHAK — stability and clouds

5.4 Moisture, Cloud, and Precipitation

Key moisture terms

Dew point spread = T − Td

Small spread → high humidity and increased fog/low cloud risk.

Lifting mechanisms (how clouds form)

Cloud families (operations)

• FAA PHAK — moisture and precipitation
• WMO cloud atlas (reference)

5.5 Fronts and Air Masses

Definition — air mass: large body of air with relatively uniform temperature and moisture properties (e.g. maritime tropical, continental polar).

Definition — front: boundary zone between two air masses with contrasting properties.

Front types (PPL summary)

Typical frontal hazards

• Cloud and precipitation bands reducing VMC.
• Wind shift and gusts at passage.
• Embedded convection along cold fronts in unstable air.
• Visibility reduction in rain, drizzle, and low cloud.

Exam and operational point

• Single chart time is a snapshot; trend (METAR sequence, TAF FM/BECMG/TEMPO) matters more than one observation.

• Plan alternates on forecast movement, not only current weather at destination.

• BOM — fronts and synoptic features
• FAA PHAK — air masses and fronts

flowchart LR
    W[Warm air mass] --> F[Frontal boundary]
    C[Cold air mass] --> F
    F --> H[Cloud / precip / wind shift]

5.6 Fog, Visibility, and Low Cloud

Definition — fog: cloud with base at the surface; visibility below 1000 m in aviation context (exact definitions vary by authority — use exam/AIP context).

Definition — visibility: greatest distance at which objects can be identified against background.

Fog types

Low stratus vs fog

Other visibility reducers

• Haze, smoke, dust, precipitation, low sun glare.
• VFR principle: legal minima are minimums; use personal margins above them.

Practical pilot actions: radiation fog example

• Scenario: pre-dawn departure from inland aerodrome after clear, calm night; T/Td spread has collapsed and visibility is dropping.
• Actions:
  • Delay and monitor trend (METAR/SPECI, webcams, field reports).
  • Check alternates and expected dissipation time.
  • Do not scud-run below minima; wait for sustained improvement.
  • Re-brief for post-fog low cloud and wind changes.
  • If destination fogs in while en route: divert early with fuel/time margins.
• FAA PHAK — fog and visibility

5.7 Thunderstorms and Severe Convective Hazards

Definition — thunderstorm: moist, unstable air with strong vertical motion producing cumulonimbus, lightning, and often heavy precipitation.

Lifecycle stages

Major threats (avoid penetration)

Avoidance principle

• Strategic avoidance: plan route and timing to stay clear by large margins (20+ NM from severe cells is a common training guideline; verify school/operator policy).

• Do not attempt to “thread” between mature cells under VFR.

• BOM — thunderstorms
• FAA AC 00-24 — thunderstorm avoidance

flowchart TD
    S[Storm cell on track] --> A{Can reroute with margin?}
    A -- Yes --> R[Reroute / delay]
    A -- No --> D[Do not launch / divert early]

5.8 Wind Shear, Microbursts, and Mountain Waves

Definition — turbulence: irregular air movement causing bumpiness and attitude/airspeed fluctuations.

Definition — wind shear: change in wind speed and/or direction over a short distance (horizontal or vertical).

Definition — LLWS (low-level wind shear): wind shear in the lower levels — critical on takeoff and approach.

Turbulence sources (summary)

Wind shear — types and pilot risk

Cues: airspeed fluctuations, unexpected sink or ballooning, wind reported on ATIS/METAR, WS group where published, rapidly changing surface wind at aerodrome.

Avoidance / response

• Brief wind at departure and destination; compare surface wind vs gradient aloft (winds aloft / GPWT).
• Use extra approach margin; be ready to go around early.
• Avoid takeoff/landing when convective outflow or frontal shear is active unless margins are large.

Microbursts

Definition — microburst: small-scale, intense downdraft that spreads outward near the surface, producing strong horizontal wind shear and severe performance loss (classic hazard below thunderstorms and some rain showers).

Avoidance strategies

• Do not take off or land under active cell or virga shaft; wait for cell passage.
• If encountered: maximum performance — pitch for recommended attitude, apply power per POH, do not trade altitude for airspeed indiscriminately; go around or reject takeoff if sufficient runway remains.
• Link to Chapter 7 (stabilized approach, early go-around).

flowchart TD
    M[Microburst suspected] --> G[Go-around or reject takeoff if able]
    G --> A[Fly through with POH escape attitude/power]
    A --> L[Do not turn back into cell]

Mountain waves and rotors

Definition — mountain wave: standing wave downwind of significant terrain when stable air flows over a ridge.

Definition — rotor: violent, chaotic turbulence in the roll cloud / rotor zone beneath the wave crest — extremely hazardous.

Avoidance strategies

• Preflight: check GAF/SIGMET for turbulence; winds aloft vs ridge orientation.
• Route: stay well clear of lee side of ranges when wave/rotor forecast; add altitude only if POH, oxygen, and rules allow — often better to reroute upwind of range.
• If severe turbulence encountered: reduce to manoeuvring speed (VA) per POH, maintain control, exit area laterally if possible.
• VFR: remain in smooth air with terrain clearance; do not scud along lee slopes.

LLWS summary table

CASA Exam Cues — shear / microburst / mountain wave

• Microburst = downdraft + outflow shear near surface — worst on approach/landing.
• Rotor is not “normal turbulence” — avoid lee side of ranges when waves forecast.
• Wind shear on approach: go-around early, do not salvage.
• Exam may describe airspeed increasing then decreasing on final — think shear/microburst, not instrument failure first.

• FAA PHAK — wind shear and turbulence
• BOM — aviation forecasts

5.9 Icing (PPL Conceptual Depth)

Definition — aircraft icing: accretion of ice on airframe, engine intakes, or instruments when flying in visible moisture below freezing.

Icing categories

Typical icing conditions (conceptual)

• Visible moisture (cloud, drizzle, rain in cold layer).
• Temperature at or below 0°C (supercooled droplets can exist slightly above 0°C in cloud).
• Light GA without certified icing protection: avoid forecast icing layers.

Operational message

• Check freezing level and cloud tops in briefing; stay VMC and clear of icing layers when possible.

• If unexpected icing: exit icing conditions (climb/descend/turn), inform ATC, divert.

• See Chapter 2 (AGK) for system failures and carb icing.
• FAA PHAK — icing
• CASA icing safety material

5.10 Weather Products and Interpretation

CASA Primary: BOM/Airservices briefing products below. PHAK Secondary: generic METAR/TAF decode rules (same symbols, verify Australian examples).

Ask yourself: Does the TAF trend support continuing, or does GAF show area turbulence/icing your route cannot avoid?

5.10.0 METAR vs TAF comparison table

5.10.1 METAR/SPECI - what each field means

• METAR: routine aerodrome weather observation at scheduled intervals.
• SPECI: special observation issued when significant weather changes occur between METAR times.
• Typical structure (order matters):
  • Report type: METAR or SPECI
  • Station identifier: e.g., YSSY
  • Time group (UTC): e.g., 301100Z (day 30, 1100 UTC)
  • Wind: e.g., 23012KT, variable wind VRB03KT, gusts 23012G22KT
  • Visibility: e.g., 9999 (10 km or more), lower values in meters
  • Runway visual range (when included): Rxx/....
  • Weather phenomena: -RA, +TSRA, BR, FG, HZ, DZ, GR
  • Cloud: FEW, SCT, BKN, OVC with heights (hundreds of feet AGL), and cloud type when relevant (e.g., CB, TCU)
  • Temperature/dew point: e.g., 18/16
  • QNH: e.g., Q1016
  • Recent weather and wind shear groups may appear in some formats
  • Trend section (where provided by state practice): NOSIG, BECMG, TEMPO, etc.

5.10.2 METAR quick decode example

• Example string:
  • METAR YSSY 301100Z 21008KT 9999 -RA SCT020 BKN035 19/17 Q1014
• Interpretation:
  • Sydney report at 1100 UTC
  • Wind from 210 at 8 kt
  • Visibility 10 km or more
  • Light rain
  • Scattered cloud at 2,000 ft, broken at 3,500 ft
  • Temperature 19 C, dew point 17 C (small spread indicates high humidity)
  • QNH 1014 hPa.

5.10.3 TAF - what each field means

• TAF: aerodrome forecast valid for a defined period.
• Typical structure:
  • Header and station identifier
  • Issue time (UTC)
  • Validity period (from/to UTC)
  • Forecast prevailing wind, visibility, weather, cloud
  • Change groups:
    • FM (from): rapid/step change from stated time
    • BECMG: gradual change in window
    • TEMPO: temporary fluctuations, expected to occur for short periods
    • PROB30/PROB40: probability groups (where used in local format)
    • INTER may appear in some systems as intermittent condition indicator
• TAF is forecast guidance, not observation; always compare against current METAR/SPECI.

5.10.4 TAF quick decode example

• Example string:
  • TAF YSCB 301100Z 3012/0100 33010KT 9999 SCT030
  • TEMPO 3014/3018 4000 SHRA BKN020
  • FM302200 02012KT 9999 SCT040
• Interpretation:
  • Issued at 1100 UTC for Canberra
  • Valid from day 30 1200 UTC to day 01 0000 UTC
  • Initial prevailing: wind 330/10 kt, visibility 10 km+, scattered cloud 3,000 ft
  • Temporarily between 1400-1800 UTC: visibility 4 km in showers, broken cloud 2,000 ft
  • From 2200 UTC: wind shifts to 020/12 kt and cloud lifts/scatters.

5.10.5 Practical METAR/TAF use in flight planning

• Do not read a single line in isolation. Build a weather picture:
  • Current conditions (METAR/SPECI)
  • Near-term forecast change timing (TAF groups)
  • En route and area-scale hazards (SIGMET/area forecasts/radar/satellite)
• High-value checks before VFR go/no-go:
  • Ceiling and visibility trend at departure, destination, and alternates
  • Wind trend vs runway and crosswind limits
  • Convective/fog timing overlap with planned arrival window
  • Temperature/dew-point spread trend for fog/low cloud risk.

5.10.6 Common METAR/TAF exam mistakes

• Confusing report issue time with validity period.
• Treating TEMPO as prevailing condition.
• Missing that FM replaces prior conditions from that time onward.
• Ignoring cloud amount significance (BKN/OVC) for practical ceiling.
• Not converting Zulu times correctly to local operation timeline.

5.10.7 Australian aviation weather products (priority briefing set)

Obtain current products via NAIPS (Airservices) and Bureau of Meteorology (BOM) aviation services. Names and formats evolve — confirm current product suite on briefing platforms.

Briefing stack (VFR cross-country — exam logic)

flowchart TD
    A[METAR/SPECI + TAF aerodromes] --> B[GAF area forecast]
    B --> C[SIGMET / AIRMET if issued]
    C --> D[GPWT winds and temps]
    D --> E[Radar / satellite / lightning]
    E --> F[NOTAM and personal minima check]

GAF (Graphical Area Forecast) — interpretation basics

• Coverage: Australian airspace divided into GAF areas (fewer sectors than old ARFORs).
• Validity: typically 6-hour blocks (standard times — check chart header for issue/valid UTC).
• Content (symbols): cloud amount/base, visibility, weather phenomena, turbulence and icing areas, freezing level — read legend every time.

• Use with: destination TAF (detail) + SIGMET/AIRMET (hazard amendment).

• BOM — Graphical Area Forecasts (GAF)
• BOM — GAF education guide (PDF)
• BOM — legacy ARFOR information

SIGMET vs AIRMET (Australia — conceptual)

• SIGMET validity and phenomenon codes are examinable at recognition level — read header (location, time, phenomenon).

• If SIGMET active for your route: treat as hard constraint unless you can remain clearly clear of phenomenon.

• BOM — SIGMET information

GPWT and graphical products

• GPWT: select route grid points; compare wind direction/speed and temperature at planned levels — headwind/tailwind fuel impact, icing level trends.
• Radar / satellite: identify convective development, frontal cloud bands, fog/stratus extent.
• Lightning / storm tracking: avoid cells showing rapid growth on approach to destination.

NAIPS briefing (operational)

• NAIPS (Airservices): integrated briefing — TAF, NOTAM, winds, often links to BOM graphical products.
• Airservices — flight briefing services

Worked briefing example (exam-style narrative)

• Route: YSCB → coastal aerodrome, afternoon arrival.
• METAR/TAF: destination TAF TEMPO lower vis and BKN cloud in arrival window.
• GAF: coastal sector shows scattered showers, moderate turbulence downwind of ranges.
• SIGMET: none.
• AIRMET: check issued — if present, compare to GAF.
• GPWT: tailwind en route improves ETA but headwind component at destination not relevant on ground.
• Decision: legal VMC possible but personal minima fail due to TEMPO and turbulence — delay or inland alternate per GAF sector.

CASA Exam Cues — Australian products

• GAF = area forecast (en route); TAF = aerodrome (destination timing).
• ARFOR is legacy — modern briefing uses GAF.
• SIGMET = significant hazard — avoid; do not confuse with AIRMET (advisory/amendment role).
• Always brief departure, en route, destination, alternate — not destination only.
• Convert all times to UTC then local for flight.

5.11 Practical VFR Weather Decision Framework

• Before flight:
  • Is weather legal?
  • Is weather operationally safe for your experience and aircraft?
  • Are alternates/diversion options robust?
• In flight:
  • Update using observations and ATC/FIS information
  • Decide early; avoid pressing into deteriorating conditions
  • Keep terrain/airspace escape options.

5.12 Pre-Exam Revision (Must Know · Nice to Know · Common Traps)

Sketch it: Cold vs warm front side-view; METAR/TAF decode boxes; lee-side mountain wave airflow.

Must know

• Stable vs unstable air; cloud types and typical hazards.
• METAR/TAF decode; trend (METAR now + TAF + GAF area).
• CASA Primary products: GAF, SIGMET, AIRMET, GPWT, NAIPS briefing role.
• Thunderstorm hazards (turbulence, hail, microburst/outflow, icing).
• Wind shear, microburst, mountain wave / rotor awareness.
• Dew point spread and fog/low cloud risk.

Nice to know

• ISA lapse rates; pressure altitude approximations.
• Occluded and stationary front nuances.
• ARFOR legacy context if mentioned in training.

Common traps

• Weather legal vs safe go/no-go confused.
• Single report without trend.
• TAF validity/time groups misread.
• TAF only — ignoring GAF area hazards.
• SIGMET vs AIRMET confused.
• Lee of ranges when GAF shows turbulence/mountain wave.
• Underestimating convective outflow and gust fronts.

5.13 Meteorology Formula Pack and Graphics

Core formulas (exam-useful)

Dew point spread = T - Td

Small spread suggests high humidity and increased fog/low cloud risk.

Pressure altitude ≈ Elevation + (1013 - QNH) × 30

(QNH in hPa, altitude in ft; approximation for quick mental checks.)

Density altitude ≈ Pressure altitude + 120 × (OAT - ISA temp)

(Approximation useful for planning sense-checks; POH charts remain primary.)

Graphic: weather decision trend logic

flowchart TD
    A[METAR/SPECI now] --> B[TAF trend]
    B --> C[Area hazards: radar, SIGMET, wind]
    C --> D{Trend improving?}
    D -- Yes --> E[Continue with margins]
    D -- No --> F[Delay, reroute, or divert early]

Front and hazard quick table

Cloud family and turbulence expectation

References

CASA Primary / Australian operational

• BOM — Graphical Area Forecasts (GAF): http://www.bom.gov.au/aviation/gaf/
• BOM — aviation forecasts and advisories: http://www.bom.gov.au/aviation/
• Airservices — NAIPS / flight briefing: https://www.airservicesaustralia.com/pilots/flight-briefing-services
• CASA weather and flight planning guidance: https://www.casa.gov.au/

PHAK Secondary / supplementary

• FAA PHAK (weather chapters): https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak
• ICAO meteorological service framework (Annex 3): https://www.icao.int/
• EASA weather information resources: https://www.easa.europa.eu/

IMPORTANT: Always verify with current official publications.