IFR weather information

Brief the whole flight in time and altitude

Start with the route, departure time, expected altitudes, destination/alternate arrival times, aircraft limitations and personal minimums. Obtain a complete aviation weather briefing using appropriate FAA/NWS or qualified provider resources. Flight Service can help interpret uncertain products; a compliant self-briefing is also possible. Looking at one favorable airport icon is not a route briefing.

Build three layers: the large-scale pattern and hazards; observations showing what is happening now; and forecasts covering each phase at the time the airplane will be there. Include winds, temperature/freezing levels, cloud bases/tops, ceilings/visibility, convection, icing, turbulence and escape options. Review airport and route NOTAMs separately; weather information alone does not establish airport usability.

For each product ask what, where, when, altitude and confidence. Distinguish observation time, issue time and valid time. Compare reports with forecasts and examine changes rather than selecting the most optimistic product. Rebrief before departure if delay or a new report changes exposure. Uncertainty that affects the only safe option should change the plan.

Aviation Weather Handbook (AWH) Chapter 3: use and acquisition of weather information · AIM 7-1-2, 7-1-3 and 7-1-5: services, products and briefing · §91.103: available information before flight

Read observations as local evidence

A METAR is a routine airport observation; a SPECI is a special observation when specified significant changes occur. Check station, UTC observation time, wind, visibility, weather, sky condition, temperature/dew point, altimeter and remarks. U.S. coded METAR winds are from true north; voice airport weather winds generally use magnetic north. Cloud heights in the report are hundreds of feet above the station/ground reference, not MSL cruise altitudes.

The ceiling is the lowest broken or overcast layer or vertical visibility into an obscuration. Scattered clouds are not a ceiling. Visibility in a U.S. METAR is statute miles; the reported surface visibility is not a guarantee of flight visibility along an approach or over terrain. Automated observing systems have coverage and sensor limitations; missing or not-detected weather is not proof of absence.

Fictional decoding example: METAR KEXM 081655Z 18012G20KT 2SM -RA BR BKN006 OVC015 08/07 A2992 describes an observation on day 8 at 1655Z, wind from 180° true at 12 knots gusting 20, two statute miles in light rain and mist, a 600-foot broken ceiling and 1,500-foot overcast layer, temperature 8°C/dew point 7°C, and 29.92 inHg setting. KEXM is an invented station for study. The small temperature/dew-point spread supports concern about saturation but does not forecast an exact future ceiling.

A PIREP supplies aircraft-observed conditions. Read its position, time, altitude, aircraft type and reported cloud, visibility, turbulence or icing details. A smooth report from a large airplane at 12,000 feet does not prove a small airplane's path at 6,000 feet is smooth. Few reports can mean few aircraft or sparse reporting, not benign weather. Report significant encounters and useful negative observations when workload permits.

AWH §§24.4–24.5: METAR/SPECI elements, limitations and aircraft reports · AIM 7-1-10 and 7-1-18: observations and PIREPs

Connect the map to the airport observations

A surface analysis organizes observed pressure, fronts and other surface information at its valid time. Locate highs, lows, pressure gradients and fronts, then compare the stations near the route. Closely spaced pressure contours suggest a stronger pressure-gradient contribution to wind, but terrain and friction alter the surface result. A front drawn at one time is not its forecast position at arrival.

Ceiling and visibility analysis (CVA) describes an analysis function: depicting observed/analyzed restrictions between stations. The legacy National CVA static imagery was retired; the current GFA observation interface provides gridded ceiling/visibility or flight-category analysis. It interpolates between observations and can miss local conditions. Use station reports, terrain and other evidence to interpret a colored area; do not read each grid cell as a measured airport report.

Radar principally shows precipitation reflectivity, not every cloud, turbulence area or icing condition. Terrain blockage, beam height, sampling and processing can hide precipitation or misrepresent its intensity at the aircraft's altitude. A clear radar image does not prove a route is free of fog, cloud, ice or turbulence. Satellite views help identify cloud patterns and trends, but a cloud-top temperature is not a complete cloud-base or hazard measurement.

AWH Chapter 25: analysis charts and their observation sources · AWC GFA Help: Gridded Flight Category Analysis and radar limitations · NWS notice: retirement of NCVA static imagery and replacement analysis · AIM 7-1-11: radar capabilities and limitations

Match each forecast to the question it answers

A TAF has a UTC issue time and valid period. A FM group starts a new prevailing forecast from the specified time; TEMPO describes temporary fluctuations during its period; PROB30 states a 30 percent probability of specified conditions under the product's rules. Do not average adverse groups out of the forecast. Interpret their coverage in the proposed ETA window and use the appropriate regulatory criteria when evaluating alternates.

The GFA combines observations, forecasts and advisories; a time-slider change can change the meaning of the layer. Check MSL versus AGL and any masked or unsupported region. Evaluate cruise, climb and descent exposure separately. A clear forecast at one selected level cannot establish a safe climb through an adverse lower layer.

Text FB winds use true directions, knots and Celsius at specified levels. For example, 2018+00 means wind from 200° true at 18 knots and 0°C; 9900 means light/variable under the product rule. The high-speed encoding and missing-temperature conventions have specific meanings—use the legend. Current gridded forecasts may use different models and time/altitude resolution; identify what the planning application actually uses.

Convective outlooks support early route/time choices. A low categorical threat is not a promise that every storm will be safe for a small airplane. Thunderstorm hazards and avoidance decisions are developed in the weather-hazards lesson.

AWH §§27.2–27.3: winds/temperatures aloft and TAFs · AWH §27.17.1: Convective Outlook · AWH Chapter 28: aviation weather tools and GFA · AWC GFA Help: forecast layers, time and altitude selection

Treat advisories as route and altitude warnings

AIRMETs/G-AIRMETs identify specified widespread hazards that can matter greatly to a small aircraft: IFR conditions/mountain obscuration, moderate turbulence or icing, and related wind/freezing-level information. The ACS retains the AIRMET term; current CONUS text AIRMETs have been replaced by G-AIRMETs. Alaska continues applicable AIRMET products. Use the current regional service instead of expecting an old national text format.

SIGMETs address significant nonconvective hazards such as severe icing, severe/extreme turbulence, volcanic ash and specified visibility-reducing dust/sandstorms. Convective SIGMETs identify qualifying convective situations, including specified lines, embedded/severe thunderstorms or areas of thunderstorms. Absence of a SIGMET does not establish that a storm or icing layer is suitable for this aircraft.

Read the phenomenon, valid period, affected horizontal area, altitude range, movement and changes. A route can miss the surface outline yet enter a developing hazard later. G-AIRMET forecast snapshots must be interpreted across the interval, not as a switch that creates weather only at the printed hour. A Center Weather Advisory (CWA) is a short-term advisory; it supplements other products rather than replacing the complete briefing.

Correlate advisories with observations, PIREPs and forecasts, then with actual aircraft capability and escape options. Do not wait for an advisory category to become severe before respecting a no-icing approval or personal minimum.

AWH Chapter 26: SIGMETs, AIRMETs and CWAs · AWC Help: current CONUS G-AIRMET replacement and advisory definitions · AIM 7-1-6: inflight aviation weather advisories

Explain why layers and convection form

Dry air is approximately 78 percent nitrogen and 21 percent oxygen, with smaller amounts of other gases. Water vapor varies with place and time; suspended particles provide condensation nuclei and can restrict visibility. The variable moisture and particles matter to weather even though their share of the atmosphere is small. The troposphere contains most everyday aviation weather. Temperature often decreases with altitude, but an inversion increases temperature with height. Compare an air parcel's temperature after displacement with the surrounding environment: if warmer and less dense it tends to keep rising; if colder it tends to return. This is the practical meaning of unstable versus stable air, not a label based only on whether the surface feels hot.

Surface heating, cooling aloft and lifting can favor instability and vertically developed clouds. Stable moist air favors layered clouds, persistent low ceilings and poor visibility when lifting/cooling occurs. Stable does not mean hazard-free: strong flow over mountains can produce wave activity, and an inversion can trap smoke/haze or accompany low-level wind shear.

Radiation, conduction and convection transfer heat. Land can warm and cool faster than nearby water, creating local circulations and changing fog potential. Rising air expands and cools; sinking air compresses and warms. These processes change the air's proximity to saturation, cloud formation and the freezing-level profile. Use forecasts and sounding/temperature information to assess the actual vertical structure rather than assuming one standard lapse rate applies.

AWH §4.2: atmospheric composition · AWH Chapter 5: temperature and heat transfer · AWH Chapter 12: lifting, cooling and clouds · AWH Chapter 13: stability, inversions and convection

Connect moisture, fronts and visibility to the flight

Water vapor can change to liquid droplets or ice. Dew point identifies the temperature at which the air reaches saturation for the relevant pressure/moisture content; cooling toward it or adding moisture promotes condensation. Evaporation consumes heat, while condensation releases latent heat and can support further cloud development. Precipitation depends on particle growth and the temperature profile through which it falls.

Air masses acquire temperature and moisture characteristics from their source regions. Fronts separate contrasting masses and force vertical motion. A warm front commonly brings broad layered cloud and prolonged precipitation as warm air rises over cooler air; a cold front can bring a narrower zone of stronger lifting and showers/thunderstorms when moisture and instability support them. Actual frontal weather depends on slope, motion, moisture and stability, so the symbol alone cannot forecast a safe corridor.

Cloud form and coverage help describe the process: layered stratus points toward widespread reduced ceiling; cumulus vertical development points toward localized lifting/instability. Cloud tops, bases and gaps vary along the route. Fog is a cloud at the surface. Radiation fog follows ground cooling under favorable conditions; advection fog occurs as moist air moves over a colder surface; upslope fog follows lifting/cooling over rising terrain. Precipitation can add moisture or cool near-surface air into fog. A departure can remain clear while a destination valley becomes obscured.

Mist/fog droplets reduce visibility; smoke, haze, dust and volcanic ash can do so without ordinary rain. Poor slant visibility toward a low sun can make acquiring the runway harder than the horizontal report suggests. Volcanic ash adds aircraft hazards beyond visibility. Evaluate the observation, forecast trend and terrain together; an instrument rating does not make a low ceiling or obscured mountain harmless.

AWH Chapter 6: water vapor, saturation and phase changes · AWH Chapter 11: air masses and fronts · AWH Chapter 12: clouds and lifting · AWH Chapter 14: precipitation · AWH Chapter 18: fog, mist and visibility restrictions

Carry the wind profile into performance and risk

Wind results from forces associated with pressure differences, Earth's rotation and friction; terrain and local heating modify it. Aloft, forecast direction/speed affect drift, groundspeed, time and fuel. Near the surface, gusts, crosswind and changing winds affect takeoff/landing and missed-approach planning.

Wind shear is a change in wind speed and/or direction over distance. Strong shear near the ground can alter indicated airspeed and the flightpath faster than a pilot can restore performance. Mountain wave forms when suitable stable flow crosses terrain; strong downdrafts, rotors and shear may exist even outside visible cloud. A forecast tailwind above the ridge does not establish a safe climb through the lower wind structure.

Compare winds/temperatures aloft, surface observations, low-level shear information, advisories and altitude-specific PIREPs. Choose a route and altitude that the airplane can actually reach and use safely. The next lesson develops the avoidance/escape implications of wave, shear and turbulence.

AWH Chapter 10: wind forces, local and adverse winds · AWH Chapter 16: mountain waves and adverse winds · AIM 7-1-18 and 7-1-24: reports, microbursts and wind shear

Separate useful awareness from a tactical guarantee

Installed weather radar samples precipitation ahead, subject to antenna tilt, range, attenuation, beam geometry and interpretation. A dark area behind a strong return may be radar shadow rather than a clear passage. A lightning display indicates detected electrical activity; it does not show every storm edge or prove the absence of convection. Learn the installation's manual and limitations before using either.

FIS-B and other datalink weather bring useful strategic information, but collection, processing and delivery delay make them unsuitable for close thunderstorm maneuvering. A displayed age may refer to transmission or mosaic time rather than the oldest contributing observation. Check product-time meanings, update/transmission intervals, reception, range and any no-data area. A retained picture can look normal after reception stops.

Inflight resources include current airport broadcasts, ATC/Flight Service assistance, PIREPs and onboard equipment. They complement a complete briefing and continuing pilot assessment. Weather-camera images are supplementary and viewpoint-limited. Neither a colored display nor a controller's precipitation description guarantees cloud, ice, turbulence or terrain clearance.

Common error: using a fresh tablet refresh to discount an older adverse PIREP. Compare the actual observation times, locations and altitudes. Seek clarification and preserve a safe option when the information conflicts.

AWH §§3.3.4–3.4: FIS-B and weather information limitations · AWH Chapter 15: radar and precipitation detection limits · AIM 7-1-9, 7-1-11 and 7-1-12: FIS, radar and ATC weather assistance · AIM 4-5-9: FIS-B service limitations

FLIGHT SCENARIO

What would change your plan?