Aviation Weather Briefings

Weather Information Sources

• The FAA offers the Flight Service program to provide weather briefings to pilots
• Briefings are available through Flight Service Stations (FSS) via 1-800-WX-BRIEF and online at 1800wxbrief.com
• Commercial weather data may differ substantially from NWS products
• Pilots should be cautious of weather services from non-FAA/NWS entities or online sources

Types of Weather Briefings

• Abbreviated briefing: Used to supplement existing data, update a previous briefing, or obtain one or two specific items
• Outlook briefing: Used for planning purposes when departure is six or more hours from the briefing time
• Inflight briefing: Used to update a preflight briefing while in flight
• Standard briefing: Includes strategic weather decision-making support, such as route selection around hazardous weather areas

Flight Information Service (FIS)

• Supports strategic weather decision-making with timely information
• Should not replace individual preflight weather and flight planning briefings
• Pilots can get updated weather data en route via FSS on 122.2 MHz, appropriate RCO frequencies, datalink weather (cockpit FIS-B display), and ATC.

Aviation Routine Weather Report (METAR)

• Is an hourly surface observation of airport weather conditions.
• Routine METAR: Transmitted every hour
• Aviation Selected Special Weather Report (SPECI): Updates METAR for rapidly changing conditions, aircraft mishaps, etc.

METAR Elements

• Type of report: METAR or SPECI
• Wind: 5-digit group (6 if over 99 knots); the first three digits show wind direction in tens of degrees relative to true north and the next two/three indicate speed in knots.
• Visibility: Surface visibility in statute miles with fractions and "SM" as needed
• RVR: Runway Visual Range, included as required
• Weather phenomena: Qualifiers and weather phenomena
• Sky condition: Amount/height/type (as required), or indefinite ceiling/height (vertical visibility) is recorded in feet
• Temperature/dew point: Two-digit format in Celsius, separated by a solidus (/); temperatures below zero are prefixed with "M"
• Altimeter: Four-digit format representing inches of mercury (tens, units, tenths, hundredths), prefixed with "A"

METAR Remarks (RMK)

• Operationally significant weather phenomena, location, beginning/ending times, and direction of movement.
• Example: METAR KLAX 140651Z AUTO 00000KT 1SM R35L/4500V6000FT -RA BR BKN030 10/10 A2990 RMK AO2

Weather Observing Programs

• Manual observations: Staffed by FAA personnel
• PIREPs: Available from ATC, FSS, and online via ADDS (aviationweather.gov/airep)

Terminal Aerodrome Forecasts (TAFs)

• Concise statements of expected meteorological conditions significant to aviation
• Apply to within 5 statute miles of the airport's runway complex
• Use the same weather codes as METARs

Graphical Forecasts for Aviation (GFA)

• Web-based graphics providing observations, forecasts, and warnings available from 14 hours in the past to 18 hours in the future
• Covers the CONUS from the surface up to FL480
• Includes forecasts for wind, icing, and turbulence.
• Includes NWS textual weather observations, forecasts, and warnings out to 15 hours
• Icing, turbulence, and wind gridded products are three-dimensional
• Other gridded products are two-dimensional and may represent a composite of a three-dimensional weather phenomenon or a surface weather variable, like horizontal visibility.

GFA Forecast Tab

• Offers forecasts for TAF, CIG/VIS (ceiling and visibility), clouds, PCPN/WX (precipitation and weather), TS (thunderstorms), winds, turbulence, and ice

GFA Obs/Warn Tab

• Displays weather data for the current time and the previous 14 hours (rounded to the nearest hour)
• Provides observations/warnings for METAR,PCN/WX (precipitation and weather), and CIG/VIS (ceiling and visibility)
• Also provides PIREPs and RAD/SAT (radar and satellite) information

Inflight Aviation Weather Advisories

• Forecasts to advise enroute aircraft of potentially hazardous weather development
• Include SIGMET (WS), convective SIGMET (WST), AIRMET (WA), and center weather advisory (CWA)
• Heights are referenced MSL, except ceilings (CIG) which indicate AGL

Convective SIGMET (WST)

• Valid for up to 2 hours.
• Reports severe thunderstorms with surface winds ≥ 50 knots, hail ≥ 3/4 inch, or tornadoes
• Also reports embedded thunderstorms, lines of thunderstorms, or heavy precipitation affecting ≥ 40% of a 3,000 square mile area

SIGMET (WS)

• Advises of weather potentially hazardous to all aircraft
• Unscheduled products valid for 4 hours, 6 hours for tropical cyclones and volcanic ash
• Issued in CONUS for severe icing (non-thunderstorm related), severe/extreme turbulence (non-thunderstorm related), widespread dust/sandstorms lowering visibility below 3 miles, and volcanic ash.

AIRMETs

• AIRMET Sierra: Describes IFR conditions and/or extensive mountain obscurations
• AIRMET Tango: Describes moderate turbulence, sustained surface winds ≥ 30 knots, and/or nonconvective low-level wind shear
• AIRMET Zulu: Describes moderate icing and provides freezing level

Winds and Temperatures Aloft Forecasts (FB) - FAA-H-8083-28

• Favorable altitude based on winds and flight direction.
• Areas of possible icing noted using temperatures from 2°C to -20°C
• Temperature inversions noted where temperature increases with altitude
• Turbulence noted by observing abrupt changes in wind direction and speed.

Center Weather Advisory (CWA)

• Aviation warning used by aircrews to anticipate and avoid adverse weather en route or at terminals.
• Not a flight planning product
• Reflects current conditions and short-range forecasts (up to 2 hours)
• Valid for a maximum of 2 hours; will include an outlook statement if conditions are expected to continue beyond that time.

Convective Outlook

• Includes surface analysis chart, weather depiction chart, short-range surface prognostic chart, significant weather prognostic chart, convective outlook chart, constant pressure analysis chart, and freezing level graphics.

Surface Analysis Chart

• Analyzed chart of surface weather observations
• Depicts sea level pressure, highs, lows, ridges, troughs, front locations, and boundaries
• Produced eight times daily

Flight Rule Definitions

• LIFR (Low IFR): Ceiling < 500 feet and/or visibility < 1 mile
• IFR: Ceiling 500 to < 1,000 feet and/or visibility 1 to < 3 miles
• MVFR (Marginal VFR): Ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 miles
• VFR: Ceiling > 3,000 feet

Constant Pressure Charts

• Pilots can approximate air temperature, winds, and temperature-dewpoint spread, and depict highs, lows, troughs, and ridges aloft
• Issued twice daily from data observed at 00Z and 12Z
• Include: 850 mb (5,000 ft), 700 mb (10,000 ft), 500 mb (18,000 ft), 300 mb (30,000 ft), and 200 mb (39,000 ft) levels
• The initial analysis and 3-hour forecast graphics are updated hourly and the 6, 9, and 12 hour forecasts are updated every three hours.

Earth's Atmosphere

• Composed of numerous gases: nitrogen, oxygen, argon, and carbon dioxide make up 99.998% of all gases
• Most weather occurs in the troposphere, extending up to approximately 36,000 feet
• In the troposphere, gases become thinner with height, and temperature decreases from about 15°C (59°F) to -56.5°C (-70°F)

Standard Atmosphere Lapse Rates

• Temperature lapse rate: Temperature decreases by approximately 3.5°F (2°C) per 1,000 feet up to 36,000 feet,remaining constant to 80,000 feet
• Pressure lapse rate: Pressure decreases by approximately 1 inHg per 1,000 feet of altitude gain to 10,000 feet

Atmospheric Stability

• Stable: Resists vertical motion; disturbances dampen out
• Unstable: Small air movements increase, leading to turbulence and convective activity; can cause significant turbulence, extensive vertical clouds, and severe weather

Determining Atmospheric Stability

• Unstable air: Indicated by a rapid, uniform temperature decrease with altitude (approaching 3°C per 1,000 feet)
• Stable air: Indicated by a temperature that remains unchanged or decreases only slightly with altitude
• Instability: Suspect when air near the surface is warm and moist

Effects of Stable and Unstable Air

• Stable: Stratiform clouds, smooth turbulence, steady precipitation, fair visibility
• Unstable: Cumuliform clouds, rough turbulence, showery precipitation, poor visibility

Wind Formation

• Caused by air density differences due to temperature changes
• The pressure change makes the atmosphere move vertically and horizontally.

Forces Affecting Wind

• Pressure gradient force (PGF)
• Coriolis force
• Friction

Isobars

• Connect areas of equal or constant barometric pressure on a weather chart

Pressure Gradient Force

• Drives wind from high to low pressure areas, attempting to equalize pressure differences
• Steep gradient (closely spaced isobars) indicates higher wind speeds
• Shallow gradient (widely spaced isobars) indicates lower wind speeds

Coriolis Force

• Deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
• Acts at a right angle to wind direction and is directly proportional to wind speed.

Local Winds

• Can affect aircraft in flight, including sea breezes

Wind Shear Detection

• Terminal forecasts: Mentioned if low-level wind shear (LLWS) may be present, as well as severe thunderstorms, heavy rain, hail, and wind gusts
• METARs: Checked to find thunderstorms, rain, or blowing dust, plus warming trends, gusty winds, or cumulonimbus clouds
• Severe weather watch reports, SIGMETs, and convective SIGMETs: indicate severe convective weather
• LLWAS (low-level windshear alert system) reports: These systems are installed at 110 airports across the U.S. and designed to detect wind shifts between outlying stations and a reference centerfield station
• PIREPs-Reports: They give real-time indication of wind shear with sudden airspeed changes during takeoff, approach, or landing

Lens-Shaped Clouds

• Presence indicates a mountain wave, atmospheric disturbance formed when stable air flows over a mountain
• Mountain waves cause severe to extreme turbulence above and downwind of mountains
• Cloud formations from mountain waves include cap clouds, cirrocumulus standing lenticular (CCSL), altocumulus standing lenticular (ACSL), and rotor clouds.
• Clouds may be absent if air is too dry

Air Moisture

• Depends on the air temperature
• Every 20°F increase in temperature doubles the air's moisture capacity
• A 20°F decrease cuts the moisture capacity in half

Humidity and Dew Point

• Relative humidity is a percentage of water vapor in the air compared to the air's capacity at a given temperature and pressure
• Dew point is the temperature to which air must be cooled at constant pressure to allow water vapor to condense into water (dew)

Precipitation Types

• Include drizzle, rain, freezing rain, freezing drizzle, snow, snow grains, ice crystals, ice pellets, hail, and small hail/snow pellets

Precipitation Formation

• Requires water vapor, sufficient lift to condense the vapor into clouds, and a growth process that allows cloud droplets to grow large and heavy

Required Cloud Thickness

• Significant precipitation formations require clouds to be at least 4,000 feet thick

Air Flow Around Systems (Northern Hemisphere)

• Low pressure: Inward, upward, and counterclockwise
• High pressure: Outward, downward, and clockwise

Weather in Relation to Pressure Systems

• Low-pressure systems: Rising air, cloudiness, precipitation, and bad weather
• High-pressure systems: Descending air, dissipation of cloudiness, and good weather

Front Types

• Cold front: Cold, dense air advances, replacing warmer air.
• Occluded front: Fast-moving cold front catches a slow-moving warm front
• Warm front: Warm air flows over colder air

Stationary Fronts

• Occur when air masses are relatively equal, and the boundary remains stationary
• Induce localized weather for days with typically a mixture of warm and cold fronts

Cloud Formation

• Forms when water vapor condenses in rising air currents, or when the lowest fog layer evaporates
• Rising currents must be present to create vertically deep clouds and significant precipitation

Cloud Vertical Extent Determination

• The stability of the atmosphere primarily determines the type and vertical extent of clouds

Basic Cloud Forms

• Cirriform: High-level clouds
• Stratiform: Latin for 'layer' or 'blanket', these clouds consist of a featureless lower layer that can cover the entire sky, with bases usually only a few hundred feet above the ground.

Turbulence Causes

• Convective currents (convective turbulence).
• Obstructions in wind flow (mechanical turbulence).
• Wind shear.

Turbulence Intensity Levels

• Light : Slight, erratic changes in altitude and/or attitude (pitch, roll, or yaw)
• Moderate : Changes in altitude and/or attitude occur, but the aircraft remains in positive control; usually causes variations in indicated airspeed.
• Severe: Large, abrupt changes in altitude and/or attitude; causes large variations in indicated airspeed; aircraft is momentarily out of control.
• Extreme: The aircraft is violently tossed about and is practically impossible to control, resulting in structural damage

Clear Air Turbulence (CAT)

• Requires sufficient water vapor , and an unstable lapse rate to start the storm process in motion.

Thunderstorm Lifecycle Stages

• Cumulus: Characterized by a strong updraft
• Mature: Precipitation begins falling and a downdraft develops
• Dissipating: Downdrafts characterize the stage, and the storm dies rapidly

Icing Types

• Structural
• Induction system
• Instrument

Structural Ice Types

• Clear icing: Also called glaze ice; glossy, clear, or translucent formed by the relatively slow freezing of large, supercooled water droplets
• Clear icing more often occurs in environments with warmer temperatures, higher liquid water contents, and larger droplets.
• Note: Rime icing occurs at temperatures colder than -15°C, clear ice above -10°C, and mixed ice in between.

Freezing Level

• Lowest altitude in the atmosphere where air temperature reaches 0°C
• If not equipped with deicing, leave precipitation or climb to where temperature is above freezing, or land at the nearest airport

Temperature Inversion

• Temperature increases with height, which reverses the normal decrease with height
• Can permit warm rain to fall through cold air, leading to icing conditions
• Ground-based inversions favor poor visibility by trapping fog, smoke, and other restrictions

Fog Formation

• Forms when the temperature and dewpoint of the air becomes nearly identical
• Cooling air beyond the dewpoint (radiation, advection, or upslope fog). Add moisture (frontal or steam fog)

Fog Types

• Radiation fog: Clear skies, little/no wind, small temperature-dew point spread, forms at night or near daybreak.
• Advection fog: Moist air moves over colder ground/water, common in coastal areas, any time of day or night.
• Upslope fog: Moist, stable air cools adiabatically as it rises up sloping terrain.
• Freezing fog: Occurs below 0°C (32°F); supercooled liquid water droplets freeze on exposed surfaces.

Frost Formation

• Ice crystal deposits formed by sublimation when the temperature and dewpoint are below freezing
• Forms on an aircraft sitting outside on a clear night due to radiation cooling

Visibility Obstructions

• Include fog, mist, haze, smoke, precipitation, blowing snow, dust storm, sandstorm, and volcanic ash