Aviation Weather Information and Briefings

Weather Information Sources

• The FAA offers the Flight Service program, providing weather briefings through Flight Service Stations (FSS) by phone at 1-800-WX-BRIEF and online at 1800wxbrief.com
• Commercial providers may offer weather products differing substantially from NWS products with formatting and layout modifications, but without material changes to the weather information

Weather Data and Briefings

• Pilots should be aware of weather services from non-FAA/NWS sources or online
• Abbreviated briefings are for supplementing data, updating previous briefings, or gathering limited information
• Outlook briefings are requested when the departure time is six or more hours away
• Inflight briefings are for updating preflight briefings during flight

Standard Weather Briefing

• A standard briefing includes providing a path through weather hazards
• FIS supports strategic weather decisions (20+ minute timeframe), like route selection around hazards
• FIS should not replace individual preflight weather and flight planning

Obtaining Updated Weather Information En Route

• Pilots can contact FSS on 122.2 MHz and appropriate RCO frequencies for updated weather
• Datalink weather provides cockpit display of FIS-B information
• ATC can provide weather updates when workload permits

METAR (Aviation Routine Weather Report)

• A METAR is an hourly surface observation of airport conditions
• METAR Types comprise of routine METARs transmitted hourly, and aviation selected special weather reports (SPECI) for rapidly changing conditions, aircraft mishaps, etc.

Basic Elements of a METAR

• A METAR report includes the report type - METAR or SPECI
• Remarks section indicates the type of precipitation sensor
• Wind is reported as a 5-digit group (6 if over 99 knots) with the first three digits indicating direction in tens of degrees relative to true north, less than 100 degrees are preceded by a zero
• The next two or three digits showing speed in knots
• Visibility is reported as surface visibility in statute miles with fractions and the letters SM
• Runway visual range (RVR) is also included as required
• Weather phenomena is broken into qualifiers and the phenomena itself
• Sky condition reports Amount/height/type (as required) or indefinite ceiling/height (vertical visibility), heights recorded in feet AGL
• Temperature/dew point group shows a 2-digit format in whole degrees Celsius, separated by a solidus (/), temperatures below zero are prefixed with M
• Altimeter shows a 4-digit format for tens, units, tenths, and hundredths of inches of mercury, prefixed with A
• Remarks include significant weather phenomena, location, timing, and movement

Weather Observing Programs

• Manual observations are performed by FAA personnel at airport locations
• PIREPs can be found via ATC, FSS, and the ADDS website

Terminal Aerodrome Forecasts (TAFs)

• A TAF is a concise forecast of expected meteorological conditions significant to aviation within 5 SM of an airport's runway complex
• TAFs use the same weather codes as METAR reports and indicate expected weather at ETA if the destination lacks a TAF

Graphical Forecasts for Aviation (GFA)

• The GFA provides web-based graphics with observations, forecasts, and warnings viewable from 14 hours past to 18 hours future, covering CONUS up to FL480
• GFA offers wind, icing, and turbulence forecasts, NWS textual data, and gridded products
• Selecting the "Forecast" tab provides TAF, CIG/VIS, Clouds, and PCPN/WX forecasts
• Selecting "Obs/Warn" shows current and past weather data, including METAR, PCPN/WX, and CIG/VIS

Inflight Aviation Weather Advisories

• Inflight Aviation Weather Advisories includes SIGMET (WS), convective SIGMET (WST), AIRMET (WA), and the center weather advisory (CWA)
• All heights are referenced MSL, except ceilings (CIG) are AGL

Convective SIGMET

• Convective SIGMETs apply for up to 2 hours and include information on severe thunderstorms with surface winds ≥50 knots, hail ≥3/4 inch diameter and tornadoes, embedded thunderstorms, lines of thunderstorms, or heavy precipitation affecting ≥40% of 3,000 square miles

SIGMET (WS)

• A SIGMET advises of weather potentially hazardous to all aircraft
• SIGMETs are valid for 4 hours, 6 hours for tropical cyclones and volcanic ash
• Issued for severe icing (not thunderstorms), severe/extreme turbulence (not thunderstorms), widespread dust/sandstorms reducing visibility below 3 miles, and volcanic ash

AIRMET Types

• The three types of AIRMETS are: Sierra, Tango, and Zulu
• AIRMET Sierra describes IFR conditions and/or extensive mountain obscurations
• AIRMET Tango describes moderate turbulence, 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)

• Winds and temperatures aloft forecasts (FB) give information regarding:
• Most favorable altitude based on winds and flight direction
• Areas of possible icing (air temperatures of +2°C to -20°C)
• Temperature inversions (temperature increase with altitude)
• Turbulence (abrupt changes in wind direction and speed at different altitudes)

Center Weather Advisories (CWA)

• A Center Weather Advisory (CWA) is used by aircrews to anticipate/avoid adverse en route/terminal weather, reflecting current conditions and short-range forecasts (within 2 hours)
• CWAs are valid for up to 2 hours; include a statement if conditions last longer

Convective Outlook Charts

• Convective Outlook charts includeSurface analysis chart, Weather depiction chart, Short-range surface prognostic chart, Significant weather prognostic chart, Convective outlook chart, Constant pressure analysis chart, Freezing level graphics

Surface Analysis Chart

• A surface analysis chart analyzes surface weather observations
• It depicts sea level pressure, highs, lows, ridges, troughs, fronts, drylines, outflow boundaries, sea-breeze fronts, and convergence lines
• The chart is produced eight times daily

Weather Definitions

• LIFR (Low Instrument Flight Rules)- ceiling <500 feet and/or visibility <1 mile
• IFR (Instrument Flight Rules)- ceiling 500 to <1,000 feet and/or visibility 1 to <3 miles
• MVFR (Marginal Visual Flight Rules)- ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 miles
• VFR (Visual Flight Rules)- ceiling >3,000 feet

Atmospheric Conditions

• Pilots can use charts to estimate air temperature, wind, and temperature-dewpoint spread along a route
• They depict highs, lows, troughs, and ridges aloft
• Charts are from data at 00Z and 12Z
• Initial analysis and 3-hour forecasts updated hourly, 6/9/12 hour updated every three hours

Earth's Atmosphere

• The Earth’s atmosphere consists of nitrogen, oxygen, argon, and carbon dioxide
• These gases make up 99.998% of all gases
• Most weather occurs in the troposphere
• It begins at the surface and extends to 36,000 feet with decreasing temperature

Standard Atmosphere Lapse Rates

• The standard temperature lapse rate is 2°C per 1,000 feet (3.5°F) up to 36,000 feet; then constant to 80,000 feet
• The standard pressure lapse rate is 1 inHg per 1,000 feet up to 10,000 feet

Atmospheric Stability

• Atmospheric stability affects resistance to vertical motion
• Stable air resists vertical movement, unstable air promotes it
• Surface warmth and moisture indicate potential instability

Signs of Instability

• Uniformly decreasing temperature as you climb (approaching 3°C per 1,000 feet) indicates unstable air
• Temperature remaining unchanged or decreases only slightly indicates stable air, and suspect instability if air near the surface is warm and moist

Stable vs Unstable Air

• Stable air produces stratiform clouds, smooth turbulence, steady precipitation and fair to good visibility
• Unstable air produces cumuliform clouds, rough turbulence, showery precipitation and poor visibility

Cause of Wind

• Air density differences from temperature changes cause pressure changes
• This Pressure causes vertical and horizontal motion

Forces affecting Wind

• The three forces affecting wind are Pressure gradient force (PGF), Coriolis force, and friction

Impact of Isobars

• Isobars connect equal barometric pressure areas
• Pressure gradient forces are created by pressure differences,
• PGF makes wind blow to equalize the differences
• A steep gradient (close isobars) means higher wind speeds; shallow gradient (distant isobars) means slower speeds

Coriolis Force

• The Coriolis force deflects winds right in the northern hemisphere and left in the southern hemisphere
• It acts at right angles to wind direction and is proportional to wind speed

Local Winds

• Example of local winds that may affect an aircraft in flight is: sea breeze

Impact of Wind Shear

• Examine terminal forecasts for LLWS (low-level wind shear) or severe weather
• Check METARs for thunderstorms, rain, or blowing dust
• Review severe weather watch reports, SIGMETS, and convective SIGMETS
• Low Level Windshear Alert System (LLWAS) at 110 airports detects wind shifts
• PIREPs reveal airspeed changes during departure or approach

Mountain Wave Clouds

• Lens-shaped cloud over mountains indicates a mountain wave
• Which is atmospheric wave from stable air over a mountain ridge
• Mountain waves cause mechanical turbulence, producing clouds like cap clouds, cirrocumulus standing lenticular (CCSL), and altocumulus standing lenticular (ACSL), however, they may be absent if the air is too dry

Atmospheric Moisture

• The amount of moisture in the air is dependent on temperature, every 20°F increase doubles moisture capacity, a similar decrease halves it

Humidity and Dew Point

• Relative humidity expresses water vapor in air compared to air's max capacity
• Dew point is the temperature at which air must cool at constant pressure to allow water vapor to condense into water (dew)

Precipitation Types

• Various precipitation types include:
• drizzle
• rain
• freezing rain
• freezing drizzle
• snow
• snow grains
• ice crystals
• ice pellets
• hail
• snow pellets

Requirements for form Precipitation

• Water vapor
• Sufficient lift
• Growth Process, significant precipitation needs 4,000 ft thick clouds

Air Pressure around the world

• Low pressure systems have inward, upward, and counterclockwise air flow in the Northern Hemisphere
• High pressure systems have outward, downward, and clockwise air flow in the Northern Hemisphere

Weather Conditions for Flight Systems

• Low-pressure systems lead to rising air, cloudiness, precipitation, and bad weather
• High-pressure systems lead to descending air, dissipation of cloudiness, and good weather

Fronts

• A cold front occurs when cold, dense air advances and replaces warmer air
• An occluded front occurs when a fast-moving cold front catches up with a slow-moving warm front
• A warm front occurs when a warm air mass flows over a colder air mass
• A stationary front occurs when two air masses are relatively equal, influencing local weather for days

Cloud Formation

• Clouds form through water vapor condensation in rising air or fog evaporation
• Rising air forms deep clouds capable of heavy precipitation

Vertical Extent of Clouds

• Atmospheric stability determines cloud type and vertical extent

Basic Cloud Forms

• Cirriform clouds are high-level clouds
• Stratiform consist in layered clouds-stratus layer

Main Causes of Turbulence

• Turbulence is caused by convective currents, obstructions in the wind flow, and wind shear

Turbulence Intensity

• Light turbulence causes slight, erratic changes in altitude and/or attitude
• Moderate turbulence causes changes in altitude and-or attitude occur, the aircraft remains in positive control at all times, varying airspeed
• Severe turbulence causes larger, abrupt changes in altitude and/or attitude, airspeed varies
• Extreme turbulence causes the aircraft to be violently tossed, practically impossible to control, it may cause structural damage

Clear Air Turbulence (CAT)

• Clear air turbulence (CAT) requires sufficient water vapor, an unstable lapse rate, and an initial upward boost

Thunderstorm Life Cycle Phases

• Cumulus-Characterized by a strong updraft
• Mature-The precipitation from cloud base shows a downdraft, and a cell has entered the mature stage
• Dissipating-Downdrafts characterize the dissipating stage, and the storm dies rapidly

Main Types of Aircraft Icing

• Aircraft encounters structural, induction system, and instrument icing

Structural Ice Types

• Clear icing conditions exist more often in an environment with warmer temperatures, higher liquid water contents, and larger droplets
• Rime icing tends to occur at temperatures colder than -15°C, clear ice when the temperature is warmer than -10°C, and mixed ice at temperatures in between depending on liquid water content, droplet size, varies

Freezing Level in relation to Structural Icing

• The freezing level is the lowest altitude where air temperature reaches 0°C
• One could leave the area of precipitation or go to an altitude where the temperature is above freezing
• If neither option is available, immediate landing

Temperature Inversion

• A temperature inversion is a temperature increase with height
• An inversion aloft allows warm rain through cold air, causing icing
• A ground-based inversion favors poor visibility by trapping fog/smoke

Fog Formation

• Fog forms when air temperature and dew point become identical through cooling or moisture addition

Fog Variations

• Radiation fog forms under clear skies, little wind, and small temperature-dew point spread
• Advection fog forms when moist air moves over colder ground/water, common in coastal areas
• Upslope fog forms as moist, stable air cools while moving upslope
• Freezing fog occurs when temperature falls to 0°C (32°F) or below, freezing supercooled droplets on exposed surfaces

Visibility Obstructions

• Weather and physical obstructions may include:
• fog
• mist
• haze
• smoke
• precipitation
• blowing snow
• dust storm
• sandstorm
• volcanic ash