Chapter 5 - Meteorology

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

5.1 Atmosphere Fundamentals

• Atmospheric composition and pressure behavior underpin all weather interpretation.
• Key concepts:
  • Pressure decreases with altitude
  • Temperature usually decreases with altitude in troposphere
  • Density depends on pressure and temperature
• ISA is a reference model used in performance calculations.

5.2 Pressure Systems and Wind

• Pressure gradient drives wind; Coriolis and friction modify direction/speed.
• Around pressure systems (Southern Hemisphere patterns should be understood for exam context).
• Stronger gradient -> stronger wind.
• Surface wind differs from gradient wind due to friction and terrain channeling.

5.3 Stability, Lapse Rates, and Vertical Motion

• Stability determines cloud type and turbulence tendency.
• Stable air favors stratiform cloud and smoother conditions.
• Unstable air favors convective cloud, showers, and turbulence.
• Inversions:
  • Can trap haze/smoke/pollutants
  • Can suppress convection
  • May create wind shear across inversion boundary.

5.4 Moisture, Cloud, and Precipitation

• Key moisture terms:
  • Relative humidity
  • Dew point
  • Saturation
• Cloud formation mechanisms:
  • Convective lifting
  • Orographic lifting
  • Frontal lifting
  • Convergence
• Cloud families and implications:
  • Cumuliform: vertical development, turbulence/showers
  • Stratiform: widespread cloud, reduced visibility/base.

5.5 Fronts and Air Masses

• Front types: cold, warm, occluded, quasi-stationary.
• Typical hazards:
  • Cloud/precip bands
  • Wind shifts/gusts
  • Turbulence and embedded convection
  • Visibility deterioration
• Frontal timing uncertainty matters; trend and movement are more valuable than single snapshot data.

5.6 Fog, Visibility, and Low Cloud

• Radiation fog (definition): fog formed overnight when ground loses heat by radiation under clear skies and light winds, cooling air to dew point.
  • Typical setup: clear night, moist surface, light wind, valley/low-lying terrain.
  • Typical burn-off: after sunrise as surface heating increases.
• Advection fog (definition): warm, moist air moving over colder surface and cooling to dew point.
  • More persistent than radiation fog; can continue all day.
• Upslope fog (definition): moist air forced upslope and cooled adiabatically to saturation.
• Steam/evaporation fog (definition): cold air over warmer water/wet surface causing rapid near-surface saturation.
• Low stratus vs fog: fog is cloud at surface; low stratus has cloud base above surface but creates similar operational limits.
• Visibility can degrade from haze, smoke, dust, precipitation, and low sun angle.
• For VFR, legal minima are minimums, not targets; operational margins should be higher.

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.
• Pilot actions:
  • Delay departure and monitor trend (METAR/SPECI updates, webcams/field observations).
  • Check nearby alternates for conditions and expected fog dissipation timing.
  • Avoid “scud running” below minima; wait for sustained improvement, not a short temporary break.
  • Re-brief takeoff/alternate plan because post-fog conditions can include low cloud layers and changing winds.
  • If already airborne and destination develops fog: divert early while fuel/time margins are strong.

5.7 Thunderstorms and Severe Convective Hazards

• Thunderstorm lifecycle: cumulus, mature, dissipating.
• Major threats:
  • Severe turbulence
  • Hail
  • Lightning
  • Microburst/downburst and gust fronts
  • Heavy precipitation and rapid visibility loss
• Avoidance principle:
  • Strategic avoidance, not tactical threading between cells.

5.8 Wind Shear and Turbulence

• Turbulence sources:
  • Mechanical (terrain/obstacles)
  • Thermal (surface heating)
  • Frontal/convective
  • Mountain wave/rotor
• LLWS cues:
  • Significant wind changes over short vertical distance
  • Frontal passages, nocturnal inversions, thunderstorms.

5.9 Icing (PPL Conceptual Depth)

• Structural, induction, and instrument icing categories.
• Conditions: visible moisture plus suitable temperature range.
• Operational message:
  • Plan to avoid icing environments in light GA without robust anti-ice capability.

5.10 Weather Products and Interpretation

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.

5.11 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 Common Meteorology Exam Traps

• Confusing weather minima legality with safe go/no-go judgment.
• Focusing on single weather report instead of trend.
• Ignoring temperature/dew point spread significance.
• Misreading forecast time groups and validity periods.
• Underestimating convective outflow and gust front effects.

5.13 Rapid Revision Checklist (Pre-Exam)

• Can explain stable vs unstable air and resulting cloud/weather.
• Can identify frontal weather implications from chart/symbol context.
• Can decode METAR/TAF and extract operational risk.
• Can explain thunderstorm hazards beyond lightning.
• Can apply a conservative diversion/no-go decision to scenario questions.

References (Primary)

• FAA PHAK (weather chapters and weather services context): https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak