IFR Weather — Icing, Storms & Briefings

Weather is the #1 Cause of Fatal GA Accidents

This module covers ACS IR.I.B knowledge areas K1–K4 and risk management R1–R2. References: Aviation Weather Handbook (FAA-H-8083-28A), Instrument Flying Handbook (FAA-H-8083-15B), AIM Chapter 7, AC 00-6B, AC 91-74B.

Key Takeaways

Stable air → stratiform clouds, steady precip, poor visibility, smooth air (classic IFR); unstable air → cumuliform, showers, turbulence, good vis between.
Standard lapse rate is 2°C/1,000 ft; a steep lapse rate destabilizes (convection), an inversion is extremely stable and traps moisture into low IFR.
A warm front is the classic extended-IFR maker — a 200+ NM band of stratus with gradually lowering ceilings; cold fronts are narrow but violent.
The worst icing sits just above the freezing level in stratiform cloud ahead of a warm front; wind shear and mountain-wave are the big wind hazards.

Atmospheric Stability & Fronts · ACS K3a–K3f

Atmospheric Stability & the IFR Pilot

Stable air resists vertical motion — produces stratiform clouds, steady precipitation, poor visibility, smooth air
Unstable air promotes vertical motion — produces cumuliform clouds, showery precipitation, good visibility (outside precip), turbulence
Temperature inversions = extremely stable — trap moisture, haze, pollution below the inversion
Surface-based inversions are the #1 cause of morning low IFR (LIFR) conditions
Conditionally unstable: stable until lifted to saturation, then becomes unstable — classic setup for embedded thunderstorms

DPE Tip

DPEs love asking: "What kind of weather would you expect in stable vs. unstable air?" Stable = stratus, poor vis, smooth. Unstable = cumulus, turbulence, good vis between showers.

FAA-H-8083-28A Ch. 4

Lapse Rates & Stability Indicators

Standard lapse rate: 2°C per 1,000 ft (3.5°F/1,000 ft)
If ambient lapse rate > 3°C/1,000 ft → absolutely unstable (expect strong thermals, CB development)
If ambient lapse rate < moist adiabatic (~1.5°C/1,000 ft) → absolutely stable
Increasing lapse rate (surface heating) → destabilization → afternoon convection
Decreasing lapse rate (radiational cooling) → stabilization → nighttime fog and low stratus

DPE Tip

Know the difference between dry adiabatic (~3°C/1,000 ft) and moist/saturated adiabatic (~1.5-2°C/1,000 ft). The moist rate is less because condensation releases latent heat.

FAA-H-8083-28A Ch. 4

Air Masses & Their Weather

cP (continental Polar): Cold, dry, stable — brings clear skies but bitter cold in winter. Can become unstable over warm water (lake-effect snow)
mP (maritime Polar): Cool, moist — Pacific mP brings low stratus and rain to the West Coast; Atlantic mP brings fog and drizzle to the Northeast
mT (maritime Tropical): Warm, moist, unstable — Gulf of Mexico mT is the primary moisture source for thunderstorms east of the Rockies
cT (continental Tropical): Hot, dry, unstable at low levels — forms over deserts, causes extreme turbulence and dust devils
Source region determines initial properties; modification occurs as the air mass moves over different surfaces

FAA-H-8083-28A Ch. 5

Frontal Systems — IFR Weather Generators

Warm front: Warm air rides up over cold air — wide band (200+ NM) of stratiform clouds, steady precip, gradual lowering ceilings, poor visibility — classic IFR setup
Cold front: Cold air undercuts warm — narrow band (50 NM) of cumuliform clouds, heavy rain, turbulence, rapid clearing behind
Stationary front: Neither air mass advancing — prolonged poor weather, low ceilings, fog for days
Occluded front: Cold front overtakes warm front — combines hazards of both; wide area of clouds and precip
Cold-type occlusion: Cold front lifts warm front — most severe weather at the surface

DPE Tip

A warm front is the most likely to produce extended IFR conditions. DPEs will present a scenario where ceilings are gradually lowering — that screams warm front approach.

FAA-H-8083-28A Ch. 5

Frontal Weather Sequences

Approaching warm front: Cirrus → cirrostratus → altostratus → nimbostratus → stratus/fog. Pressure falling, wind shifting
Warm front passage: Temperature rises, pressure levels, visibility may improve, wind shifts clockwise (veers)
Approaching cold front: Altocumulus → cumulonimbus. Pressure falling, temp steady/warm
Cold front passage: Temperature drops, pressure rises sharply, wind shifts clockwise (veers), visibility improves after front passes
Worst icing: Just above the freezing level in stratiform clouds ahead of a warm front — extensive areas of freezing rain

FAA-H-8083-28A Ch. 5

Clouds as Stability Indicators

Stratiform (stratus, nimbostratus, altostratus): Stable air — continuous, layered, smooth, steady precip
Cumuliform (cumulus, cumulonimbus, towering Cu): Unstable air — vertical development, turbulence, showery precip
High clouds (cirrus family, >20,000 ft): Ice crystals — indicate upper-level moisture, approaching warm front
Lenticular clouds: Mountain wave indicator — smooth appearance but severe turbulence nearby
Standing lenticular clouds (ACSL) in PIREPs = mountain wave turbulence — stay well above or avoid

DPE Tip

If a DPE shows you a METAR with "SCT025 OVC080" and asks what it implies — scattered low layer (stable boundary layer) with overcast above (approaching front or moist layer aloft).

FAA-H-8083-28A Ch. 3

Wind Hazards & Turbulence · ACS K3d–K3e

Wind Shear — The Invisible Threat

Wind shear: Any rapid change in wind speed or direction over a short distance — horizontal or vertical
Low-level wind shear (LLWS): Most dangerous within 2,000 ft AGL, especially on approach
Associated with: fronts, thunderstorms, temperature inversions, jet streams, sea/land breezes
Microburst-induced shear: Headwind → tailwind transition can cause loss of 40-80 ft/min climb performance in seconds
LLWAS (Low-Level Windshear Alert System) at many airports — ATC will advise but coverage has gaps
If you encounter wind shear on approach: GO AROUND immediately. Apply max power, pitch for Vref + wind additive

DPE Tip

Classic checkride scenario: "You're on an ILS approach and tower reports LLWAS alert, wind shear on final. What do you do?" — Missed approach. No hesitation.

FAA-H-8083-28A Ch. 7; AIM 7-1-27

Mountain Wave Turbulence

Caused by strong winds (25+ kts) blowing perpendicular to a mountain ridge
Turbulence extends well above ridge height — can reach into the stratosphere
Rotor zone (below ridge height, downwind side): Most severe turbulence — can exceed structural limits
Cap cloud on peak, lenticular clouds downwind = visual indicators of mountain wave
PIREP mountain wave reports use "MWAVE" — always check these when crossing terrain
Rule of thumb: Fly at least 50% above ridge height, or avoid entirely if moderate+ wave reported

FAA-H-8083-28A Ch. 7

Turbulence Categories & Reporting

Light: Slight, erratic changes in altitude/attitude — occupants may feel a slight strain against seatbelts
Moderate: Changes in altitude/attitude, but aircraft remains in positive control — occupants feel definite strain; unsecured objects dislodged
Severe: Large, abrupt changes — aircraft may be momentarily out of control; occupants forced violently against seatbelts
Extreme: Aircraft practically impossible to control — may cause structural damage. IMMEDIATE heading/altitude change required
Report turbulence location, intensity, type (chop/CAT/MWAVE), altitude, aircraft type — PIREPs are the ONLY real-time turbulence data

Level	Aircraft	Cabin
Light	Momentary slight, erratic changes in altitude and/or attitude. Rhythmic bumpiness without appreciable change is Light Chop	Slight strain against seat belts. Unsecured objects displaced slightly; walking is easy
Moderate	Similar to light but greater intensity. Changes occur but the aircraft remains in positive control at all times. Usually varies indicated airspeed	Definite strain against seat belts. Unsecured objects dislodged; walking is difficult
Severe	Large, abrupt changes in altitude and/or attitude, usually with large airspeed variations. Aircraft may be momentarily OUT OF CONTROL	Occupants forced violently against seat belts. Objects tossed about; walking impossible
Extreme	Aircraft violently tossed about and practically impossible to control. May cause structural damage	—

Duration is reported separately: Occasional is less than 1/3 of the time, Intermittent 1/3 to 2/3, Continuous more than 2/3.

DPE Tip

Don't conflate the scales: ICING is Trace / Light / Moderate / Severe, but TURBULENCE is Light / Moderate / Severe / Extreme — no "Trace," and it adds "Extreme."

AIM 7-1-21

Key Takeaways

Clear ice (large supercooled drops / freezing rain) is the most dangerous; structural icing needs visible moisture and roughly -15°C to +2°C.
Ice can raise stall speed 20–30% WITHOUT a stall warning — if the airplane feels sluggish in ice, act, don't wait for the horn.
A thunderstorm's mature stage (updrafts + downdrafts) is the most dangerous; stay 20 NM from severe cells, never fly under or between close cells, never top a building storm.
Microbursts give a headwind-then-tailwind shear on approach — max power, pitch up, go around; advection fog (warm moist air over a cold surface) can persist for days.

Aircraft Icing · ACS K3g–K3h

Types of Structural Icing

Clear ice: Hard, heavy, transparent — forms from large supercooled droplets that flow back before freezing. Most hazardous — difficult to see and remove
Rime ice: Rough, milky, opaque — forms from small supercooled droplets that freeze instantly on contact. Easier to remove with deice boots
Mixed ice: Combination of clear and rime — characteristics of both types. Common in rapidly changing conditions
Frost: Forms on cold-soaked surfaces when dew point is below freezing. Reduces lift up to 30% — NEVER take off with frost on wings
SLD (Supercooled Large Droplets): Extremely hazardous — can accumulate behind protected surfaces and in areas boots/TKS can't reach

DPE Tip

DPE question: "Which type of icing is most dangerous and why?" Clear ice — it's heavy, changes the airfoil shape, is hard to see, and is difficult to remove.

AC 00-6B Ch. 10; AC 91-74B

Icing Conditions & Temperature Envelope

Structural icing requires TWO ingredients: visible moisture AND temperature at or below 0°C
Most likely between -15°C and +2°C (supercooled water droplet range)
Peak icing risk: -10°C to 0°C — where supercooled liquid water content is highest
Below -15°C: Most moisture is ice crystals that bounce off the airframe (less icing risk)
Below -40°C: All moisture is ice crystals — no icing risk
Freezing rain = temperature inversion overhead — warmer air aloft is melting precipitation that refreezes on contact. Climbing MAY find warmer air, but verify with PIREPs

DPE Tip

Memory aid: "Minus 15 to Plus 2 — that's the icing range for you." If OAT is in this range AND you're in visible moisture, expect icing.

AC 00-6B Ch. 10

Effects on Aircraft Performance

Increased weight AND drag — decreased lift coefficient
Disrupted airflow over wings — effective stall speed increases (stalls at higher airspeed)
Stall warning may NOT activate — ice changes the airfoil shape before reaching the stall AOA
Unprotected surfaces (antennas, gear, pitot tube, static ports) accumulate ice rapidly
Pitot tube blockage → erroneous airspeed (reads low or zero). Turn on pitot heat proactively before entering known moisture
Propeller icing reduces thrust efficiency — RPM may decrease
Tailplane icing: Can cause pitch-down moment when flaps are lowered (NASA tailplane icing studies)

DPE Tip

Critical gotcha: Ice contamination can increase stall speed by 20-30% WITHOUT stall warning activation. If the aircraft feels sluggish in icing, DO NOT wait for the stall horn — take action.

AC 91-74B

Pilot Actions & Regulatory Framework

14 CFR §91.527: No person may fly under IFR into known or forecast icing unless equipped for it (applies to large/turbine aircraft)
For Part 91 small aircraft: No explicit FAR prohibition, but "known icing" is defined by the certification standards — if not certified for flight into known icing (FIKI), don't do it
Exit icing conditions immediately — change altitude or route. Every minute adds weight
Use all available anti-ice/deice equipment at the FIRST sign of accumulation
Increase approach speed 10-20% to account for increased stall speed from residual ice
Report icing via PIREP: Trace, Light, Moderate, Severe — include altitude, temperature, aircraft type
Pre-flight: Clean aircraft of ALL frost, ice, and snow before takeoff — no exceptions

DPE Tip

"Known icing" vs "forecast icing" — one of the most debated topics in aviation. The safe answer: if icing is reported or forecast AND you're not FIKI-equipped, find another route/altitude or don't go.

AC 91-74B; 14 CFR §91.527; AIM 7-1-22

Icing PIREPs — Intensity Scale

The current AIM defines the four intensities by a representative ACCRETION RATE on the unprotected outer wing — see the table below
Trace and Light both say "consider exiting"; Moderate says "as soon as possible"; Severe is the only one where exit is required by regulation
Severe is aircraft-dependent — it may occur at ANY accumulation rate when the rate or accumulation exceeds what your aircraft tolerates
Expect icing in visible moisture between +2 °C and −10 °C
When reporting: Include type (rime/clear/mixed), intensity, altitude, temperature, and aircraft type — a Cessna 172 accumulates differently than a King Air

Level	Rate	Means
Trace	Under ¼ in/hr	Ice becomes noticeable; accumulation slightly exceeds sublimation. Consider exiting before it worsens
Light	¼ to 1 in/hr	Requires occasional cycling of manual deicing systems. Consider exiting
Moderate	1 to 3 in/hr	Requires frequent cycling of manual deicing systems. Consider exiting as soon as possible
Severe	Over 3 in/hr	Ice protection systems FAIL to remove it and ice forms where it normally does not. By regulation, immediate exit is required

Rates are representative accretion on the unprotected part of the outer wing. Severe is aircraft-dependent — it can occur at any rate that exceeds what your airplane tolerates.

AIM 7-1-19

Thunderstorms & Convection · ACS K3i–K3j

Three Stages of Thunderstorms

Cumulus (Developing): Updrafts ONLY — building upward, rain not yet reaching surface. Looks harmless but can intensify rapidly
Mature: Most dangerous stage — both updrafts AND downdrafts coexist. Heaviest rain, hail, lightning, wind shear, microbursts
Dissipating: Downdrafts dominate — rain decreasing, anvil top spreading. STILL DANGEROUS — contains turbulence and wind shear
A single cell lasts 20-90 minutes. Multi-cell clusters and supercells can persist for hours
Three ingredients for TS development: moisture, instability (lifting mechanism), and a trigger (fronts, terrain, convergence, heating)

FAA-H-8083-28A Ch. 8; AC 00-6B Ch. 11

Thunderstorm Hazards for IFR Flight

Severe turbulence: Can exceed structural limits of GA aircraft (exceeds ±25 ft/sec gust loads)
Hail: Can occur in clear air up to 20 NM from the storm. Hail shafts can be invisible to radar
Lightning: Temporarily blinds pilots, can damage avionics, magnetize compass, punch holes in skin
Microbursts: 4,000 ft diameter, winds up to 150 kts — headwind-to-tailwind transition on approach
Tornadoes: Associated with supercell thunderstorms, extending from cloud to ground
Heavy precipitation: Obscures visibility, ingestion can cause engine flameout
Gust front: Leading edge of cold air outflow can extend 15-20 NM ahead of the storm

AIM 7-1-29; FAA-H-8083-28A Ch. 8

Microbursts — The Silent Killer

A microburst is a localized column of sinking air within a thunderstorm — produces damaging divergent winds at the surface
Size: 1-3 NM diameter. Duration: 5-15 minutes from first contact to dissipation
Peak intensity reached within 5 minutes of initial outflow — no time to react if on approach
Headwind-to-tailwind shear: Aircraft initially gains airspeed (pilot reduces power), then rapidly loses airspeed and altitude
Can produce winds up to 150 kts. Delta-V (total velocity change) can exceed 90 kts
Dry microbursts: Occur in arid regions — precipitation evaporates before reaching ground. No visual cue!
Wet microbursts: Occur in moist environments — visible rain shaft can indicate location
If encountered: Apply MAX power, pitch to 15° nose-up, accept stall warning — priority is arresting descent rate

DPE Tip

DPE scenario: "You're on approach and your airspeed suddenly increases by 15 kts, then decreases by 25 kts with a rapid descent rate. What's happening?" — Microburst. Go around NOW.

AIM 7-1-24; FAA-H-8083-15B Ch. 3

Thunderstorm Avoidance Rules

Maintain at least 20 NM from any severe thunderstorm or intense radar echo
Do NOT attempt to fly under a thunderstorm — turbulence and downdrafts extend to the surface
Do NOT fly between cells that are less than 40 NM apart — the gap may be filled with severe turbulence
Circumnavigate the entire area if storms are embedded (cannot identify individual cells)
Avoid the anvil — severe turbulence and hail extend well above and downwind
Do NOT rely solely on onboard radar or datalink weather — datalink has 5-20 minute latency!
At night, embedded thunderstorms are especially dangerous — no visual avoidance cues
Do NOT attempt to "top" a building thunderstorm — they can build at 6,000 ft/min

DPE Tip

Datalink weather (ADS-B In, XM) is NOT real-time. It shows where storms WERE, not where they ARE. Never use datalink for tactical avoidance — it's a strategic planning tool only.

AIM 7-1-29

Embedded Thunderstorms (Critical IFR Hazard)

Thunderstorms hidden within widespread stratiform cloud layers — cannot be seen visually
Most commonly associated with warm fronts and squall lines in moist, unstable air masses
SIGMET or Convective SIGMET will be issued when embedded thunderstorms are occurring or forecast
Onboard weather radar is the only real-time tool for detecting embedded TS — request vectors from ATC if not equipped
Continuous lightning illumination from within a cloud layer at night = embedded TS
If you inadvertently enter one: Maintain attitude, keep wings level, accept altitude deviations, slow to maneuvering speed (Va), don't chase the instruments

FAA-H-8083-15B Ch. 3; AIM 7-1-29

Fog, Visibility & Ground Icing · ACS K3k–K3l

Types of Fog — IFR Pilot's Primary Adversary

Radiation fog: Clear nights, light winds (<5 kts), moist air — forms in low-lying areas, burns off 1-3 hours after sunrise
Advection fog: Warm moist air moves over cold surface — can persist for DAYS, common on coasts. Wind 5-15 kts helps maintain it
Upslope fog: Air forced uphill cools adiabatically to dew point — common in mountains. Can be thick and persistent
Steam fog (arctic sea smoke): Cold air over warm water — shallow but can be dense. Common over lakes/rivers in fall
Precipitation-induced fog: Rain from warmer air aloft saturates cooler air below — common ahead of warm fronts
Freezing fog: Fog with temps below 0°C — deposits ice on aircraft and runway. Extremely hazardous for takeoff and landing

DPE Tip

When a DPE asks "what fog type can persist for days?" — advection fog. It doesn't need calm winds or clear skies, unlike radiation fog.

FAA-H-8083-28A Ch. 6

Visibility Considerations for IFR

Temp/dew point spread ≤3°C: Fog or low clouds likely developing. ≤1°C: almost certain
Flight visibility vs. ground visibility: §91.175 — YOU must have the required flight visibility to descend below MDA/DA
Prevailing visibility in METARs is "representative" — actual conditions vary across the airport environment
LIFR (Low IFR): Ceiling <500 ft AND/OR vis <1 SM — consider whether you're proficient enough to attempt
RVR (Runway Visual Range): Measured by transmissometers along the runway. Used for precision approaches when vis < 1 SM
Sector visibility: Some METARs report variable visibility by compass direction — check remarks section

FAA-H-8083-28A Ch. 6; AIM 7-1-11

Obstructions to Visibility

Haze (HZ): Fine particles, visibility typically 3-5 SM — slant range visibility much worse than reported ground vis
Smoke (FU): From fires, industrial sources — can reduce visibility to near zero. Check NOTAMs for wildfire areas
Volcanic ash (VA): Extremely hazardous — can cause engine failure (melts in combustion chamber, solidifies on turbine blades)
Dust/sand (DU/SA): Common in desert regions — haboobs can reduce vis to zero in minutes
Blowing snow (BLSN): Can produce whiteout conditions on approach and departure
Mist (BR): Vis 5/8 SM to 6 SM with moisture. Fog (FG): Vis < 5/8 SM. Know the difference for METAR decoding

DPE Tip

Memory aid for METAR vis obscurations: "BR = Brume (French for mist), FG = Fog, HZ = Haze." BR = vis ≥ 5/8, FG = vis < 5/8.

FAA-H-8083-28A Ch. 6; AIM 7-1-28

Frost & Ground Icing

Frost forms on aircraft surfaces when the skin temperature is at or below 0°C AND the dewpoint — NO visible moisture required
Even a thin layer of frost disrupts airflow — increases stall speed, reduces lift, reduces climb performance
14 CFR §91.527(a): "No pilot may take off when frost, ice, or snow is adhering to wings, control surfaces, or propellers"
Polished frost: The most deceptive — thin, transparent coating that may appear harmless but still disrupts laminar flow
Deicing vs. anti-icing: Deicing removes existing contamination; anti-icing prevents new accumulation. Know the holdover times
Cold-soaked fuel tanks: Fuel in wing tanks can cool wing skin below ambient — frost forms even above 0°C ambient when humidity is high

DPE Tip

"Can you take off with frost on the fuselage but not the wings?" Technically, the reg addresses wings, control surfaces, and propellers — but if there's frost ANYWHERE, it signals conditions exist for wing frost. Clean it all.

AC 91-74B; 14 CFR §91.527

Key Takeaways

Flight categories: VFR >3,000 & >5 SM; MVFR 1,000–3,000 and/or 3–5; IFR 500–999 and/or 1–2; LIFR <500 and/or <1 — judged on the WORST of ceiling/vis.
TAFs cover a 5 SM radius for 24–30 hours; watch FM/BECMG/TEMPO/PROB groups, and TAF/FB winds are TRUE (ATIS/AWOS are magnetic).
AIRMETs (moderate): Sierra = IFR/mountain obscuration, Tango = turbulence/winds, Zulu = icing; SIGMETs and Convective SIGMETs are severe.
PIREPs are the ONLY real-time source of actual cloud tops, icing, and turbulence — and datalink (FIS-B/XM) is 5–20 min old, strategic only.

METAR & TAF Decoding · ACS K1–K2

METAR Decoding — Beyond the Basics

Type: METAR (routine) or SPECI (special — issued when significant weather changes occur between routines)
Wind: First 3 digits = direction (true north), next 2-3 = speed, G = gusts. VRB = variable direction when speed <6 kts
Visibility: In statute miles (US). P6SM = "plus 6 SM." RVR reported in feet when vis drops below 1 SM
Weather: Intensity prefix (- light, moderate (no prefix), + heavy), descriptor (TS, SH, FZ, DR, BL, MI, PR, BC), type (RA, SN, FG, BR, HZ, etc.)
Clouds: SKC/CLR/FEW/SCT/BKN/OVC + altitude in hundreds of feet AGL. "CB" or "TCU" appended when observed
Ceiling: The LOWEST broken (BKN) or overcast (OVC) layer. FEW and SCT are NOT ceilings
Remarks (RMK): Critical info — peak wind, pressure tendency, lightning, variable ceiling/vis, maintenance data

DPE Tip

DPE will often hand you a raw METAR and ask: "What's the ceiling? Is this VFR, MVFR, IFR, or LIFR?" Practice decoding METARs until it's second nature.

AIM 7-1-28; AC 00-45H

TAF Decoding — The IFR Planning Backbone

Issued 4x daily for 24 or 30 hours. Covers a 5 SM radius around the airport
FM (From): Complete change in conditions at the specified time. Previous conditions no longer valid
TEMPO: Temporary fluctuation expected for <1 hour at a time, covering <50% of the period
BECMG: Gradual, permanent change expected over the 2-hour period specified
PROB30/PROB40: 30% or 40% probability of conditions during the period. Not used within first 9 hours of TAF
AMD: Amended forecast. COR: Corrected. RTD: Delayed. NIL: Missing
TAF winds are TRUE direction (unlike ATIS/AWOS which report magnetic)
NSW: No Significant Weather expected — cancels previous weather phenomena

DPE Tip

Critical for alternate planning: When does the TAF at your alternate show IFR? The TEMPO and PROB groups matter — if TEMPO shows IFR, you may need a different alternate even if the prevailing forecast is VFR.

AIM 7-1-28

Flight Categories — Know the Thresholds

VFR: Ceiling >3,000 ft AND visibility >5 SM
MVFR (Marginal VFR): Ceiling 1,000-3,000 ft AND/OR visibility 3-5 SM
IFR: Ceiling 500 to less than 1,000 ft AND/OR visibility 1 to less than 3 SM
LIFR (Low IFR): Ceiling <500 ft AND/OR visibility <1 SM
These are based on the WORST condition — if ceiling is 600 ft (IFR) but vis is 8 SM (VFR), category is IFR
On GFA and other charts: VFR = green, MVFR = blue, IFR = red, LIFR = magenta

AIM 7-1-7

Advisories, PIREPs & Forecasts · ACS K2

AIRMETs (WA) — Widespread Moderate Hazards

AIRMET Sierra (S): IFR conditions (ceilings <1,000 ft and/or vis <3 SM over >50% of area) and/or mountain obscuration
AIRMET Tango (T): Moderate turbulence, sustained surface winds ≥30 kts, low-level wind shear (non-convective)
AIRMET Zulu (Z): Moderate icing and freezing level information
Valid for 6 hours, reissued every 6 hours, amended as needed
Significant to ALL aircraft, but especially light GA aircraft — a "moderate" hazard in an AIRMET is defined for light aircraft
If you see AIRMET Zulu covering your route: evaluate altitude options to stay above freezing level or below the icing layer

DPE Tip

Memory aid: "SaTurday = Sierra-Tango-Zulu" — S for Sierra (IFR/mtn), T for Tango (Turbulence), Z for Zulu (icing/freeZing).

AIM 7-1-6

SIGMETs (WS) — Severe Hazards

SIGMET: Severe icing not associated with thunderstorms, severe or extreme turbulence, dust/sandstorms reducing vis <3 SM, volcanic ash
Valid for 4 hours (6 hours for volcanic ash)
Convective SIGMET (WST): Severe thunderstorms (surface winds ≥50 kts, hail ≥¾″, tornadoes)
Also issued for: Embedded TS, line of TS, area of TS ≥40% of an area
Convective SIGMETs valid for 2 hours, updated hourly. Issued for Eastern, Central, and Western US
Any convective SIGMET implies: severe or greater turbulence, severe icing, and low-level wind shear

DPE Tip

Key distinction: AIRMETs = moderate hazards, widespread. SIGMETs = severe hazards, any size area. If a SIGMET covers your route, you probably shouldn't go — it's "significant" for a reason.

AIM 7-1-6

PIREPs (UA/UUA) — The Only Real-Time Reports

UA = Routine pilot report. UUA = Urgent pilot report (severe icing, severe turbulence, etc.)
ONLY real-time source of: actual cloud tops/bases, icing type/intensity, turbulence type/intensity, flight conditions between reporting stations
Format: /OV (location) /TM (time) /FL (altitude) /TP (aircraft type) /SK (sky condition) /WX (weather) /TA (temperature) /WV (wind) /TB (turbulence) /IC (icing) /RM (remarks)
ATC may solicit PIREPs — especially during approaches in changing conditions
YOU should file: When encountering conditions different from forecast, especially icing, turbulence, or cloud tops. Contact FSS (122.0) or ATC
Absence of PIREPs does NOT mean conditions are good — it may mean nobody is flying there

DPE Tip

DPE favorite: "How would you determine cloud tops for your IFR flight?" — PIREPs are the only way. Forecasts estimate, but PIREPs are actual observations from pilots who were there.

AIM 7-1-20

Winds & Temperatures Aloft Forecast (FB)

Issued 2x daily, valid at 6, 12, and 24 hours
Stations at 3,000, 6,000, 9,000, 12,000, 18,000, 24,000, 30,000, 34,000, 39,000 ft MSL
No wind reported for levels within 1,500 ft of station elevation (too close to surface turbulence)
Temperature not reported for the 3,000 ft level
Format: 4-digit wind direction/speed (9900 = light and variable). Above 100 kts: add 50 to direction, subtract 100 from speed
Negative temperatures indicated by "1" in the first digit of temperature field (e.g., "2722-12" = 270° at 22 kts, -12°C)
Critical for: flight planning fuel burn, icing avoidance (check temps at planned altitude), wind component calculations

AIM 7-1-11; AC 00-45H

Graphical Forecast for Aviation (GFA) & Convective Outlook

GFA: Web-based interactive tool at aviationweather.gov — replaced the legacy Area Forecast (FA)
Provides: clouds, visibility, precipitation, TS, turbulence, icing — observations + forecasts out to 18 hours
Layers can be toggled: Surface, Low, Mid, High. Click on any grid point for detailed info
Use GFA alongside METARs and TAFs for the complete picture — GFA fills the gaps between reporting stations
Convective Outlook (AC): Issued by Storm Prediction Center — shows probability of thunderstorms. Marginal/Slight/Enhanced/Moderate/High risk areas
If "Moderate" or "High" risk covers your route → plan for significant delays or cancellation

DPE Tip

Many DPEs will show you a GFA and ask you to interpret it for your route. Practice using aviationweather.gov/gfa before the checkride — know what each layer shows.

AIM 7-1-4; AIM 7-1-7

Center Weather Advisories (CWA) & AIRMETS G-AIRMET

CWA: Issued by ARTCC (Center) weather service units — more localized and time-critical than AIRMETs/SIGMETs
Valid for 2 hours. Often the FIRST advisory product for rapidly developing conditions
May be issued before a SIGMET when conditions are developing but criteria not yet fully met
G-AIRMET: Graphical version of AIRMETs — issued every 6 hours with 3-hour snapshots out to 12 hours
G-AIRMET is more precise geographically than text AIRMETs — shows exact boundaries of hazard areas
Both available on aviationweather.gov — check before filing and again right before departure

AIM 7-1-6

Key Takeaways

Start with a Standard briefing; an Outlook briefing (6+ hrs out) is planning-only — get a Standard/Abbreviated before departure.
Work the go/no-go top-down: big picture → departure → en route (PIREPs / AIRMETs / SIGMETs) → destination → alternate → personal minimums → fuel.
An alternate is required unless the destination meets the 1-2-3 rule (≥2,000 ft & ≥3 SM, ±1 hr of ETA); standard alternate mins are 600-2 / 800-2.
Legal minimums are the floor — set higher personal minimums, run IMSAFE and PAVE, and beat get-there-itis.

Briefings, Go/No-Go & Decision Making · ACS R1–R2, S1–S4

Types of Weather Briefings

Standard Briefing: Request when you have not received a previous briefing. Most complete — covers all elements. Always start here.
Abbreviated Briefing: When updating a previous briefing, or need only specific information. Tell the briefer what you already know.
Outlook Briefing: For flight planned 6+ hours in the future. General weather trends — you MUST get a Standard or Abbreviated before departure.
Sources: 1800wxbrief.com (Leidos), ForeFlight (FAA-approved), aviationweather.gov, or call 1-800-WX-BRIEF
Always tell the briefer: aircraft type, IFR/VFR, departure point, route, destination, ETD, altitude, duration
Standard briefing includes: adverse conditions, VFR flight not recommended (if applicable), synopsis, current conditions, en route forecast, destination forecast, winds aloft, NOTAMs, ATC delays

DPE Tip

On the checkride: You'll be asked to present a weather briefing for your planned cross-country. Use a logical flow: big picture (synopsis) → departure → en route → destination → hazards → decision. Just like a CFI would brief.

AIM 7-1-2

Go/No-Go Decision Framework

Step 1: Get the big picture — frontal positions, pressure systems, where is the weather moving?
Step 2: Check departure conditions — METAR, TAF, departure procedure requirements (ODP minimums)
Step 3: Analyze en route weather — PIREPs (icing? turbulence?), AIRMETs/SIGMETs, GFA cloud/precip layers
Step 4: Evaluate destination — TAF, approach minimums vs. forecast weather, trends (improving or deteriorating?)
Step 5: Alternate planning — Required? Does alternate weather meet the 1-2-3 or 600-2 rule? (See regulations module)
Step 6: Personal minimums check — Am I current? Proficient? Rested? Is this within MY personal minimums, not just legal minimums?
Step 7: Fuel — Can I get there, attempt the approach, miss, fly to alternate, and have 45 min reserve?

DPE Tip

The DPE is evaluating your DECISION-MAKING process, not just your weather knowledge. Walk through your reasoning out loud. "Based on these METARs, TAFs, and PIREPs, here is my plan and here is why I believe this flight is safe."

FAA-H-8083-15B Ch. 1; AC 60-22 (ADM)

Personal Minimums — Your Safety Buffer

Legal minimums are the FLOOR, not the target — personal minimums should always be higher
Consider ceiling, visibility, crosswind, icing, currency/proficiency, aircraft equipment, terrain, and night factors
New instrument pilot suggestion: Start with minimums well above published (e.g., 1,000 ft ceiling / 3 SM vis) and lower gradually with experience
Raise your minimums when: Flying an unfamiliar aircraft or approach, night IFR, mountainous terrain, limited recent IFR time
The IMSAFE checklist for the PILOT: Illness, Medication, Stress, Alcohol, Fatigue, Eating (nutrition)
The PAVE checklist: Pilot, Aircraft, enVironment, External pressures — evaluate each before every flight
Document your personal minimums in writing and review them periodically — easier to follow a plan than to make decisions under pressure

DPE Tip

If a DPE asks "what are your personal minimums?" — have a definitive answer ready. "I fly to 600/2 as a minimum, but I'll raise that to 1,000/3 for unfamiliar approaches or nighttime." Shows maturity.

FAA-H-8083-15B Ch. 1; AC 60-22

Alternate Airport Requirements (§91.169)

An alternate is required if the destination forecast (1 hour before to 1 hour after ETA) shows: ceiling <2,000 ft above the airport OR visibility <3 SM
This is the "1-2-3 Rule": 1 hour before/after, 2,000 ft ceiling, 3 SM visibility
Alternate minimums (standard): Precision approach — 600 ft ceiling / 2 SM vis. Non-precision — 800 ft ceiling / 2 SM vis
Non-standard alternate minimums: Shown by a triangle with "A" (▲A) on approach plates — check the actual values
Some airports are "NA" (Not Authorized) as alternates — usually due to lack of weather reporting
You can file without an alternate if destination has weather reporting AND forecast meets 1-2-3 rule — but always have a backup plan
Fuel requirement: Enough to fly to destination, attempt approach, fly to alternate, and then fly 45 minutes at normal cruise

DPE Tip

"Walk me through the alternate requirements for your planned IFR flight." — This is a checkride staple. Show the TAF, apply the 1-2-3 rule, pick an alternate, verify its minimums, compute fuel.

14 CFR §91.169; AIM 5-1-8

In-Flight Weather Updates & Diversion

HIWAS: Hazardous In-Flight Weather Advisory Service — broadcast on selected VORs. Listen on en route VOR frequencies
Flight Watch: Discontinued in 2015. Use 122.0 to contact FSS for PIREPs and updates
ATC: Request current weather at your destination or alternate. Controllers can relay METARs, PIREPs, and SIGMET info
ADS-B In: If equipped, provides FIS-B (Flight Information Service - Broadcast) with METARs, TAFs, SIGMETs — remember: 5-20 min latency
Diversion triggers: Weather worse than forecast at destination, icing or turbulence exceeding aircraft/pilot capabilities, fuel remaining below personal minimums
If diverting: Advise ATC immediately, request amended clearance to your alternate or nearest suitable airport, declare emergency if fuel is critical
Never let "get-there-itis" override your training — the cemetery is full of pilots who pressed on into weather they shouldn't have

AIM 7-1-2; FAA-H-8083-15B Ch. 1

Onboard Weather Equipment — Uses & Limitations

Airborne weather radar: Active system — shows precipitation intensity in real-time. Best for tactical TS avoidance. Tilt management is critical
Stormscope/lightning detection: Passive system — plots electrical discharges. Shows where lightning IS, not where rain is. Good supplement to radar
ADS-B In (FIS-B): Mosaic NEXRAD, METARs, TAFs, SIGMETs, PIREPs displayed on MFD. STRATEGIC use only (5-20 min latency)
XM/SiriusXM Weather: Similar to FIS-B but subscription-based. Also strategic, not tactical. 5-15 min latency
CRITICAL: Neither FIS-B nor XM weather is real-time — NEVER use for tactical avoidance (threading between cells)
Best practice: Use datalink for strategic routing (planning), airborne radar for tactical avoidance (maneuvering)
Icing detection: Some aircraft have ice detection systems (icing rate probes). Most GA aircraft rely on visual checks and OAT gauge

DPE Tip

One of the most common DPE questions for ForeFlight users: "Is the weather data on your iPad real-time?" NO. It's delayed. You cannot use it to pick gaps between thunderstorm cells.

AIM 7-1-11; AC 00-63A

Go Deeper

Question about icing envelopes, reading a TAF, or an IFR go/no-go call? Ask Kit for a CFI-level explanation.

“Walk me through a go/no-go weather decision for an IFR cross-country”

Training aid only — verify all data against your POH and current FAA publications.