Aviation Weather — FRNAS Aviation Library

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Publisher

FAA

Code

AC 00-6B

Level

Advanced

Category

Meteorology

About This Book

Aviation Weather (AC 00-6B) is the FAA's authoritative reference on meteorology for pilots. Weather is responsible for a significant portion of aviation accidents, and misunderstanding or underestimating weather is one of the most common errors pilots make. This advisory circular provides pilots with a thorough understanding of weather phenomena and how they affect flight.

Unlike basic weather theory in student pilot training, this book goes deep. It covers the full complexity of atmospheric phenomena — from microscale turbulence inside a cloud to mesoscale weather systems spanning hundreds of miles. It explains not just what weather looks like, but why it forms and how it behaves.

Reading weather products correctly depends entirely on understanding what they're measuring. This book is the foundation for that understanding.

Chapter Breakdown

Part 1

Weather Briefing Process

Three types of weather briefings: Standard (complete picture for a planned flight), Abbreviated (update an already-received standard briefing), Outlook (planning only, more than 6 hours before departure). Briefing sources: 1800wxbrief.com, ForeFlight, NOAA Aviation Weather Center (aviationweather.gov). Required elements: adverse conditions, synopsis, current conditions, en-route forecast, destination forecast, winds aloft, NOTAMs, ATC delays.

Part 2A

Atmosphere, Pressure & Temperature

Atmospheric layers: troposphere (weather occurs here, up to ~36,000 ft), tropopause, stratosphere. Standard lapse rate: dry adiabatic lapse rate (DALR) = 3°C per 1,000 ft; moist adiabatic lapse rate (MALR) = 1.5–2°C per 1,000 ft. Temperature inversions trap pollutants and produce excellent VFR visibility. High pressure (anticyclone): descending air, clear skies. Low pressure (cyclone): rising air, clouds, precipitation.

Part 2B

Wind, Fronts & Clouds

Wind shear: change in wind speed or direction with altitude; most hazardous during approach and departure. Jet stream: 100-200 knots at FL300-FL400, creates Clear Air Turbulence (CAT) on the polar side. Fronts: cold front (steep boundary, fast movement, squall lines possible 200 NM ahead), warm front (gradual slope, embedded thunderstorms possible, wide precipitation area). Cloud types: low (stratus), middle (alto), high (cirro), vertical development (cumulo).

Part 2C

Radar, Mountain Waves & Microbursts

Radar returns: green (light), yellow (moderate), red (heavy), purple (extreme, hail probable). Mountain wave turbulence: extends downwind of ridges to 100+ miles, indicated by lenticular clouds at mountain-top level. Rotors form below the wave crests and produce severe-to-extreme turbulence. Microbursts: intense downdraft up to 6,000 FPM, horizontal outburst up to 45 knots wind shear; maximum intensity within 5 minutes of initial contact, dissipates within 15 minutes.

Part 2D

Severe Hazards: Thunderstorms, Icing & CAT

Thunderstorm rule: circumnavigate by 20 NM minimum; never fly beneath an anvil. Structural icing: requires visible moisture + temp between +2°C and −20°C. Rime ice (rough, opaque, light aircraft), clear ice (heavy, smooth, very hazardous), mixed. SLD (Supercooled Large Droplets) freeze beyond the leading-edge protection zone. CAT: often unforecast, indicated by PIREPs; common near the jet stream, in mountain wave areas, and near strong fronts. Space weather: HF radio blackouts and GPS degradation during solar events.

Part 3

Aviation Weather Products

METAR: surface observation updated hourly (or more). TAF: terminal aerodrome forecast valid 24-30 hours. SIGMET (WS): non-convective severe weather affecting all aircraft; convective SIGMET (WST): thunderstorm activity affecting all aircraft. AIRMET Sierra (mountain obscuration/IFR), Tango (turbulence, low-level wind shear, strong surface winds), Zulu (icing and freezing level). GFA (Graphical Forecasts for Aviation): replaced FA, covers CONUS to FL450 with 15-hour forecasts.

Key Weather Thresholds

Dry Adiabatic Lapse Rate (DALR): 3°C / 1,000 ft

Unsaturated air cools at 3°C per 1,000 ft as it rises. When the environmental lapse rate exceeds the DALR, the atmosphere is unstable and convective development is likely.
Thunderstorm Avoidance: 20 NM Lateral Clearance

The FAA recommends a minimum of 20 NM lateral clearance from any thunderstorm cell. Severe turbulence, hail, and lightning have been encountered well beyond the visible cloud boundary.
AIRMET Types: Sierra, Tango, Zulu

Sierra = IFR conditions and mountain obscuration. Tango = moderate turbulence, sustained surface winds over 30 knots, low-level wind shear. Zulu = moderate icing and freezing levels. Valid for 6-hour periods; check before every IFR flight and VFR cross-country.
Structural Icing Conditions

Icing requires both visible moisture (cloud, precipitation, or fog with 1-mile visibility) AND temperature between +2°C and −20°C (most severe between −5°C and −15°C). Rime ice at −10°C to −20°C; clear ice near 0°C; most hazardous mix near −5°C to −10°C.

Who Should Read It

This book is required reading for instrument-rated pilots and beyond. It is especially important for pilots who fly in areas with complex weather patterns, those pursuing commercial or airline pilot careers, and anyone who wants to make truly informed weather decisions. Good weather judgment comes from deep understanding, and this is the book that builds it.

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