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Instrument rating · ACS IR.I.B · lesson pack

Weather Information (Instrument)

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Why this one matters

Instrument pilots don’t avoid clouds, but they still have to avoid ice, thunderstorms and weather below what they can safely fly. Accidents follow pressing into deteriorating weather, picking up ice with nowhere to go, or trusting old datalink pictures near storms. A thorough briefing and a firm plan B are the defense.

1 · Know these cold

  • •An alternate is required unless the destination has a standard or special Part 97 instrument approach and, from 1 hour before to 1 hour after your ETA, the forecast is a ceiling at least 2,000 feet above the airport elevation and visibility at least 3 statute miles.(14 CFR 91.169(b))
  • •At the alternate, judged at your ETA there: its published (triangle-A) alternate minimums if it has them; otherwise 600-foot ceiling and 2 miles for a precision approach, 800 and 2 for a nonprecision one (LPV and other APV approaches count as nonprecision here). An airport marked alternate "NA" can't be filed. If you actually divert, land with the approach's normal 91.175 minimums.(14 CFR 91.169(c))
  • •Structural icing comes from supercooled liquid droplets striking an airframe at or below 0°C; the risk is greatest from about 0°C to -10 to -20°C. Check the freezing level, AIRMET Zulu (moderate) and icing SIGMETs (severe), PIREPs and the icing forecasts. Your POH/AFM limitations say whether flight into known icing is prohibited, and 91.9 makes them binding.(Aviation Weather Handbook (FAA-H-8083-28A); Your airplane's POH/AFM; 14 CFR 91.9)
  • •Avoid severe thunderstorms, or any storm with intense radar echoes, by at least 20 miles, especially under the anvil; don't fly between intense echoes less than 40 miles apart.(AC 00-24, Thunderstorms; AIM 7-1-26)
  • •Datalink NEXRAD is a mosaic: the weather shown can be 15-20 minutes older than the age displayed. Use it for strategic avoidance only, never to pick a gap between cells.(AIM, Chapter 7, Meteorology; NTSB Safety Alert SA-017)
  • •Get a complete briefing from Flight Service or approved sources, and check every phase: departure, en route, destination and alternate.(AIM, Chapter 7, Meteorology)
  • •Set personal weather minimums and a divert plan before you go; decide early when the weather is worse than forecast.(Risk Management Handbook (FAA-H-8083-2A))

2 · The lesson

Sources and products
  • Use Flight Service and the National Weather Service’s Aviation Weather Center products: METARs and SPECIs, PIREPs, TAFs, the Graphical Forecasts for Aviation, surface analysis, winds and temperatures aloft, AIRMETs, SIGMETs and convective SIGMETs.(AIM, Chapter 7, Meteorology)
  • Ceiling and Visibility Analysis (CVA) shows where IFR and low IFR are now; freezing-level and icing products show where the ice is; the Storm Prediction Center's Convective Outlook shows where storms are expected.(Aviation Weather Handbook (FAA-H-8083-28A))
  • AIRMETs (moderate hazards): Sierra, IFR and mountain obscuration; Tango, moderate turbulence, surface winds of 30 knots or more, low-level wind shear; Zulu, moderate icing and freezing levels. SIGMETs: severe icing, severe or extreme turbulence, dust or sandstorms, volcanic ash. Convective SIGMETs: thunderstorm lines, embedded or widespread storms, tornadoes, large hail.(Aviation Weather Handbook (FAA-H-8083-28A); AIM 7-1-6)
  • Briefings: an outlook briefing more than 6 hours out, a standard briefing before departure, and abbreviated briefings to update it, en route included.(AIM 7-1-4)
  • In flight: ATIS and ASOS/AWOS at the arrival airport, ATC, Flight Service, PIREPs and datalink weather. Datalink NEXRAD can be 15-20 minutes older than its age label: it shows where precipitation was, not where it is.(AIM, Chapter 7, Meteorology; NTSB Safety Alert SA-017)
IFR meteorology
  • Stability decides the weather: stable air gives stratus and steady precipitation with smooth air; unstable air gives cumulus, showers and turbulence.(Aviation Weather Handbook (FAA-H-8083-28A))
  • Fronts, low pressure and moist air bring low ceilings; fog forms when the temperature-dewpoint spread closes, especially overnight.(Aviation Weather Handbook (FAA-H-8083-28A))
  • Structural icing: supercooled water droplets freeze on contact with an airframe at or below 0°C, most likely between 0 and -20°C; freezing rain under a warm layer is the most dangerous. Know the freezing level and the tops.(Aviation Weather Handbook (FAA-H-8083-28A))
  • Fog: radiation fog forms on clear, calm nights and burns off after sunrise; advection fog comes with moist air moving over cold ground or water and can last all day; upslope fog forms as moist air rises up terrain. Watch a closing temperature-dewpoint spread.(Aviation Weather Handbook (FAA-H-8083-28A))
  • Frost on the wing disrupts lift: remove it all before takeoff. Haze, smoke and precipitation reduce visibility on the approach as well as en route.(Aviation Weather Handbook (FAA-H-8083-28A))
  • Thunderstorms bring severe turbulence, icing, hail, lightning and wind shear. Microbursts are intense downdrafts, under a mile or two across, that can produce a sudden airspeed gain and then a strong tailwind and sink near the ground. Stay at least 20 miles from severe storms, and don’t fly under an anvil.(AC 00-24, Thunderstorms)
  • Low-level wind shear without thunderstorms comes with fronts, temperature inversions, strong surface winds over terrain and mountain wave.(Aviation Weather Handbook (FAA-H-8083-28A))
  • Mountain wave and strong winds aloft can cause severe turbulence and downdrafts near terrain.(Aviation Weather Handbook (FAA-H-8083-28A))
Alternates and the go/no-go decision
  • The 1-2-3 rule: an alternate is required unless, from 1 hour before to 1 hour after ETA, the destination forecast is at least 2,000 and 3. Use the alternate’s published minimums, or 600-2 (precision) and 800-2 (nonprecision).(14 CFR 91.169)
  • An alternate with no instrument approach must allow descent from the MEA, approach and landing under basic VFR.(14 CFR 91.169(c))
  • Decide before you go: personal ceiling and visibility minimums, icing (no known icing in a non-certified airplane), convective activity, and where you’ll divert.(Risk Management Handbook (FAA-H-8083-2A))
  • Onboard weather has limits. Datalink radar is delayed. A lightning detector shows only discharges: a building cell may show nothing, and heavy activity can be misplotted. Onboard radar is attenuated by heavy rain, so a cell can hide behind another (shadowing), and its picture depends on tilt. Keep an out.(AIM, Chapter 7, Meteorology; Aviation Weather Handbook (FAA-H-8083-28A))

Want it all on one page, with scenarios? Read the full lesson.

3 · Say it out loud

Answer each one out loud, then check yourself and mark how it went.

  1. Remember Recall it

    “When do you need an alternate on an IFR flight plan?”

  2. Understand Explain why

    “Why can’t you use datalink NEXRAD to fly between storm cells?”

  3. Apply Use it on a flight

    “The TAF shows 1,500 broken and 4 miles at your ETA. What does that mean for your flight plan?”

4 · Common mistakes

  • Checking only the destination.

    The alternate and en route weather matter too.

    Instead: Brief every phase.

  • Ignoring the freezing level.

    Icing in the clouds you’re planning to fly in.

    Instead: Plot the freezing level against your altitudes and tops.

  • Trusting datalink weather like radar.

    It’s delayed.

    Instead: Use it for the big picture, not tactical avoidance.

5 · Quick check

Answer, then reveal. Flag anything you want to see again.

  1. 1. What are the standard alternate minimums for a precision approach?

  2. 2. And for a nonprecision approach?

  3. 3. What two things are required for structural icing?

  4. 4. How far should you stay from severe thunderstorms?

  5. 5. Which AIRMET covers icing?

  6. Scenario6. Destination 1,800 overcast, 5 miles at ETA. Alternate needed?

Trained it? Now prove it: practice weather information (instrument) out loud with our examiner.

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Connect it

One situation that runs across this lesson and its neighbors in the same ACS area. Decide first, then check.

Connect it · Preflight Preparation

Your last six approaches were 7 months ago. The forecast for your destination one hour either side of your arrival is ceilings 1,200 feet and visibility 3 miles.

Can you file and fly this, and what must the plan include? Decide, then see the worked answer

Not as pilot in command under IFR yet. You need six instrument approaches, holding, and intercepting and tracking courses within the preceding 6 calendar months. You're in the second 6 months, so you can still regain currency without an IPC: fly the missing tasks in simulated instrument conditions with a safety pilot, or in an approved FFS, FTD or ATD, or with an instructor. Until you're current you can't act as PIC under IFR or in weather below VFR minimums, and once the second 6 months have passed, only an IPC restores it. Once current: under 91.169 you need an alternate unless the destination has a standard instrument approach and, from 1 hour before to 1 hour after your ETA, the forecast is at least a 2,000-foot ceiling above airport elevation and 3 SM visibility. Either value falling short triggers it, so the 1,200-foot ceiling alone means you need an alternate here, even though the 3 miles meets the rule. At the alternate, published alternate minimums govern (and 'NA' means it can't be used); where none are published, the forecast at your ETA there must be at least 600-2 for a precision approach or 800-2 for a non-precision one. An airport with no instrument approach works only if the forecast allows a descent from the MEA, an approach and a landing under basic VFR. Fuel covers the destination, then the alternate, then 45 minutes at normal cruise.

14 CFR 61.57(c)-(d), 91.167, 91.169

Brings together: Pilot Qualifications · Weather Information · Cross-Country Flight Planning

Teaching this? The instructor lesson plan →

Study aid, not FAA guidance. Sources: Aviation Weather Handbook (FAA-H-8083-28A); AC 00-24; Aeronautical Information Manual (AIM); 14 CFR Part 91; Instrument Rating ACS (FAA-S-ACS-8C). Checked against current sources 2026-11-13.