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Instrument rating · ACS IR.I.B

Weather Information (Instrument)

Instrument weather planning asks different questions than VFR: Can I get in and out? Do I need an alternate? Where is the ice? Where are the storms? What will I do if it’s worse than forecast?

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.

Can’t miss

  • •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))

What you should know

Sources and products K1 · K2 · K4

  • 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 K3

  • 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 R1 · R2

  • 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))

Scenarios

Decide what you would do first, then see the worked answer.

Tops are forecast at 9,000 feet, the freezing level is 5,000, and your airplane has no ice protection. Your MEA is 6,000.

Can you go? See the worked answer

You’d be in cloud above freezing level at the MEA: forecast icing. Without approval for known icing, don’t go, or find a route with lower MEAs below the freezing level or out of the clouds.

Aviation Weather Handbook (FAA-H-8083-28A)

En route, datalink shows a gap between two red cells.

Do you go through it? See the worked answer

No. The picture is minutes old; the gap may have closed. Stay 20 miles from severe storms, divert or ask ATC.

AC 00-24, Thunderstorms

Arriving, the destination is below minimums and the alternate is falling.

What should you have planned? See the worked answer

An alternate with good weather, fuel for the destination, the alternate and 45 minutes beyond, and a point to decide early. Divert now while you have options.

Risk Management Handbook (FAA-H-8083-2A)

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.

Now train it

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Sources

  • Aviation Weather Handbook (FAA-H-8083-28A): IFR weather, icing, thunderstorms
  • AC 00-24: Thunderstorms
  • Aeronautical Information Manual (AIM): Chapter 7, Meteorology
  • 14 CFR Part 91: 91.169
  • Instrument Rating ACS (FAA-S-ACS-8C): Area I

Study aid, not FAA guidance. Regulations change; check the current text at eCFR and the ACS. Checked against current sources 2026-11-13.