Ch 16

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Ch 16 Aviation Weather Resources
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Ch 16 Aviation Weather Resources

• Section A The Weather Forecasting Process
• Collecting Weather Data
• Processing Weather Data
• Making the Forecast
• Forecasting Methods
• Forecasting Accuracy

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• Section B Aviation Weather Forecast Products
• Forecast Products in Text Format
• Terminal Aerodrome Forecast (TAF)
• Area Forecast (FA)
• In-flight Weather Advisories (WS, WST, WA)
• Transcribed Weather Broadcasts (TWEB)
• Winds and Temperatures Aloft Forecast (FD)
• Other Advisories, Watches, and Warnings

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• Forecast Products in Graphic Format
• Significant Weather Prognostic Chart
• Forecast Winds and Temperatures Aloft Chart

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• Section C Aviation Weather Information Sources
• FAA Flight Service Stations (FSS)
• Continuous Broadcasts of Weather Information
  (TWEB, HIWAS)
• Other Weather Information Sources
• Weather on the Internet

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• Weather information and forecasts are beneficial
  in numerous ways
• Prediction of warm temperatures or the change of
  rain helps us decide whether to plan a picnic or
  carry an umbrella for the day
• Forecasts of severe weather such as blizzards,
  thunderstorms, or hurricanes help communities
  prevent property damage and save lives

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• As a pilot, weather influences your life in a
  unique way
• Determinations regarding weather conditions must
  be made before every flight, and crucial weather
  decisions may have to be made in-flight

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• A wide variety of weather resources are available
  to assist you in this decision-making process
• We describe the on-going process of collecting,
  transmitting, and processing weather data to
  produce a weather forecast
• We then consider various formats of aviation
  weather forecasts, and the sources of aviation
  weather information

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• When you complete Chapter 16, you should
  understand how weather information is compiled
  and processed, and some basic concepts of
  forecasting
• You should also be familiar with the variety of
  aviation weather forecasts available to you, as
  well as the sources of aviation weather
  information to help you make safe preflight and
  in-flight decisions

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• Section A The Weather Forecasting Process
• There are three important steps in the
  forecasting process data collection, data
  processing and forecasting
• Collecting Weather Data
• The World Meteorological Organization (WMO), a
  United Nations agency is responsible for the
  standardization of observations and the
  international exchange of weather data

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• Processing Weather Data
• The National Weather Service (NWS), a part of the
  National Oceanic and Atmospheric Administration
  (NOAA), is the U.S. government agency that is
  primarily responsible for gathering and
  processing meteorological data and for providing
  weather forecasts and warnings

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• These continuing and complicated tasks are
  conducted by several NWS offices including the
  National Centers for Environmental Protection
  (NCEP) and local Weather Forecast Offices (WFO)
• One of the components of the NCEP is the Aviation
  Weather Center (AWC) which specializes in the
  preparation of aviation weather information

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• Forecasting Methods
• The most comprehensive forecast technique today
  is numerical weather prediction (NWP) which
  solves a set of mathematical equations (a
  numerical model) to predict the weather

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• Forecasting Accuracy
• In comparison to persistence forecasts, the
  accuracy of meteorological forecasts, which are
  based on scientific knowledge as used by NWP and
  weather forecasters, is much better and decreases
  much more slowly as the forecast period increases

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• However, for the longest forecast periods, the
  accuracy of meteorological forecasts is no better
  than that of climatological forecasts, which are
  based purely on past averages and are typically
  not very accurate at all

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• Forecasting the weather is somewhat chaotic,
  which is not too far from the mark. As applied
  to weather forecasting, chaos theory describes
  the potential difficulty in making forecasts for
  long periods of time
• The science of chaos began in the early 1960s
  with a mathematician and atmospheric scientist
  named Edward Lorenz and his exploration of
  computer forecasts

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• Section B Aviation Weather Forecast Products
• Decoding keys for aviation weather forecast
  products are included in Appendix D
• In addition, FAA Advisory Circular 00-45 Aviation
  Weather Services contains further information

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• Selectivity requires that you know exactly what
  you need, what is available and where and when it
  is available
• Visualization forming a mental image of current
  and forecast weather conditions

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• Forecast Products in Text Format
• Forecast products in text format include
• Terminal Aerodrome Forecasts (TAF) describes
  weather conditions that are expected to occur
  within a 5 nm radius of an airport over a 24 hour
  period.

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• In the US routine TAFs are issued 4 times daily
  at 0000Z, 0600Z, 1200Z, and 1800Z.
• The TAF is one of the most valuable sources for
  the predicted weather at a specific airport.
• Predicted sky condition, visibility, weather and
  obstructions to vision, wind direction and speed,
  and expected changes during the forecast period
  can be derived from TAFs.

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• Area Forecasts (FA) expected VMC, clouds and
  general weather conditions over an area the size
  of several states are described in an Area
  Forecast (FA).
• An FA is used to determine en-route weather,
  including conditions at airports that do not have
  terminal forecasts.

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• An FA should always be used in conjunction with
  the most recent in-flight weather advisories.
• Area forecasts are normally issued three times
  daily and are valid for 18 hours.

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• An FA has four sections
• Header describes the source of the FA, the date
  and time of issue, the valid times and the areas
  the FA covers
• Precautionary Statements describe IFR and
  mountain obscurations, thunderstorm hazards, and
  states that all heights are MSL unless otherwise
  noted

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• Synopsis a brief summary identifying the
  location and movement of pressure systems,
  fronts, and circulation patterns for the 18 hour
  forecast period.
• VFR Clouds and Weather this section lists
  expected sky condition, visibility and weather
  for the next 12 hours and an outlook for the
  following 6 hours

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• Area forecasts are issued by the Aviation Weather
  Center (AWC) for the six regions in the
  contiguous US.
• The Alaskan Aviation Weather Unit issues an FA
  for the state of Alaska while the Honolulu WFO
  handles Hawaii.
• Special FAs are also issued for the Gulf of
  Mexico and international airspace

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• Within its prescribed area, an FA describes
  weather features and conditions relative to
  common geographical regions and features

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• The contiguous US is divided into 6 forecast
  areas
• SFO San Francisco
• SLC Salt Lake City
• DFW Dallas / Fort Worth
• CHI Chicago
• BOS Boston
• MIA Miami

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• The area forecast covers an area of several
  states and can be used to determine en-route
  weather and conditions at your destination if no
  TAF has been issued

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• The geographical area designator map can be
  useful in determining the specific area described
  by an area forecast
• The contiguous US is divided into six forecast
  areas (colored area)
• You may want to keep a copy of this map with your
  flight planning materials so that you have access
  to it when you obtain weather information.

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• The accuracy of weather forecasts as a function
  of the forecast period and the forecasting method
• Comparison of the accuracy of persistence,
  meteorological, and climatological forecasts for
  forecast periods from a few hours to several days
• Very general

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• The actual rate of deterioration of the accuracy
  of forecasts for a given airport depends on its
  location, the season, and the forecast variable.

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• In-flight Weather Advisories (WS, WST, WA)
• consist of either an observation and a forecast,
  or just a forecast for the development of
  potentially hazardous weather

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• SIGMET (WS) describes conditions which can pose
  hazards to all aircraft.
• SIGMETs are valid for up to four hours.

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• If the following phenomena are observed or
  expected to occur, a SIGMET is issued
• 1. Severe icing not associated with thunderstorms
• 2. Severe or extreme turbulence or clear air
  turbulence not associated with thunderstorms
• 3. Dust storms or sandstorms lowering surface or
  in-flight visibilities to below three miles
• 4. Volcanic ash

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• SIGMETs are issued as warnings of hazardous
  weather, such as severe icing, which is of
  operational interest to all aircraft

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• Convective SIGMETs (WST) describes convective
  activity that is potentially hazardous to all
  categories of aircraft

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• Convective SIGMETs (WST) describes convective
  activity that is potentially hazardous to all
  categories of aircraft
• Bulletins are issued hourly with special
  advisories issued as required
• The forecast period for a WST is two hours or
  less
• Criteria for issuance are any of the following
  conditions

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• Severe thunderstorms (surface winds greater than
  or equal to 50 knots and/or hail at the surface
  greater than or equal to ¾ inches in diameter
  and/or tornadoes)
• Embedded thunderstorms
• A line of thunderstorms

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• Thunderstorms producing precipitation with an
  intensity greater than or equal to heavy and
  affecting 40 or more of an area at least 3,000
  square miles

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• Convective SIGMET bulletins are issued for the
  Eastern (E), Central (C), and Western (W) United
  States
• Convective SIGMET text is presented in figure 16-7

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• AIRMET (WA) issued for significant weather at
  intensities lower than those required for the
  issuance of a SIGMET
• Although AIRMETs are of operational interest to
  all aircraft, the weather conditions specified
  are particularly hazardous to light aircraft
  having limited capability or minimal equipment
  and instrumentation

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• There are three different AIRMETs
• AIRMET Sierra describes IFR conditions and/or
  extensive mountain obscurations
• AIRMET Tango describes areas of moderate
  turbulence, sustained surface winds in excess of
  30 knots and areas of non-convective low-level
  wind shear
• AIRMET Zulu describes moderate icing and
  provides freezing level heights

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• Pilots with limited experience or qualifications
  should pay special attention to these advisories

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• AIRMETs are issued every six hours with
  unscheduled updates and corrections issued as
  necessary
• Each bulletin contains any current AIRMETs that
  are in effect, an outlook for weather that is
  expected after the AIRMET valid period and any
  significant conditions that do not meet AIRMET
  criteria
• Examples of AIRMETs Sierra, Tango and Zulu are
  given in figure 16-8

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• Weather conditions that are particularly
  hazardous to small, single-engine aircraft are
  contained in an AIRMET

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• AIRMETs and SIGMETs are issued for the 6 regions
  corresponding to the FA areas
• These widespread advisories must be either
  affecting or forecasted to affect at least 3,000
  square miles at any one time
• International SIGMETs are issued worldwide by
  ICAO Meteorological Watch Offices (MWOs) see
  figure 16-9

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• The criteria for International SIGMETs are
  broader than the criteria for either WS or WST
  for the US
• Thunderstorms in lines, embedded in clouds, or in
  large areas producing tornadoes or large hail
• Tropical cyclones
• Severe icing
• Severe or extreme turbulence
• Dust storms or sand storms lowering visibility to
  less than 3 miles (5 km)
• Volcanic Ash

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• Transcribed Weather Broadcasts (TWEB)
• A Transcribed Weather Broadcasts (TWEB) is a
  synopsis and forecast for a 50-mile-wide corridor
  along a specific flight route or within 50 nm of
  the FSS
• TWEBs are prepared for more than 200 flight
  routes and local vicinities around the US

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• TWEB forecasts are valid for 12 hours
• They are updated 4 times daily
• TWEBs do not include areas of icing and
  turbulence
• TWEB example given in figure 16-10

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• Winds and Temperatures Aloft Forecasts (FD)
• FDs furnish a prediction of wind speed (knots),
  wind direction (degrees True), and temperature
  (degrees C) for selected altitudes at specific
  locations across the US, including Alaska, Hawaii
  and over some US coastal waters
• Figure 16-11 gives the distribution of forecast
  stations
• Winds and temperatures aloft contain wind
  direction in relation to true north, wind speed
  in knots and temperature in degrees Celsius for a
  range of altitudes

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• FDs wind speeds between 100 and 199 knots are
  encoded so direction and speed can be represented
  by four digits
• These winds are decoded by subtracting 50 from
  the two digit wind direction and adding 100 to
  the wind speed
• The negative sign for temperatures above 24,000
  feet MSL is not included
• A wind code of 9900 is interpreted as light and
  variable
• Forecast winds greater than or equal to 199 knots
  are reported as 199 knots

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• Other Advisories, Watches, and Warnings
• Center Weather Service Unit (CWSU) operated by
  NWS meteorologists within the confines of each
  ARTCC
• Meteorological impact statements (MIS) / center
  weather advisories (CWA) produced by the CWSU
  of each ARTCC
• Hurricane advisories (WH) issued to alert the
  aviation community to the presence of a hurricane
  located at least 300 nm offshore and threatening
  the coastline

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• Severe watch bulletin (WW) an unscheduled
  message that defines areas of possible severe
  thunderstorms or tornado activity
• Severe watch alert (AWW) a preliminary notice
  to alert forecasters, briefers and pilots that WW
  is being issued
• Severe thunderstorm warnings / tornado warnings
  public notifications that those phenomena have
  been sighted visually or by radar
• Convective outlook (AC) SPC also produces a
  convective outlook (AC) for the occurrence of
  thunderstorms (non-severe and severe) five times
  a day for the next 24 hours (day 1 convective
  outlook) and twice a day for the following 24
  hours (day 2 convective outlook)

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• Forecast Products in Graphic Format
• Descriptions of examples of all common forecast
  and analysis graphics are given in Appendix D

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• Significant Weather Prognostic Chart
• The significant weather prognostic chart can be
  used to determine areas to avoid, such as
  forecast locations of low visibilities or
  turbulence
• See figure 16-13
• Lower left hand corner of each panel gives the
  valid times for the forecast
• The surface prog in the lower panel uses standard
  symbols to depict fronts, isobars, pressure
  centers and areas of forecast precipitation

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• Regions of continuous precipitation and unstable
  showery precipitation are within a solid green
  line with hatching
• Intermittent precipitation is only enclosed with
  a solid green line
• The upper panel portrays forecast areas of IFR
  (solid red lines)

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• MVFR (scalloped green lines)
• VFR conditions
• Areas and layers of expected moderate or greater
  turbulence are enclosed with dashed yellow lines
• The highest freezing level (dashed green lines)
  and the intersection of the freezing level with
  the surface are also shown

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• High-level significant weather prog chart
• Covers the airspace from 25,000 feet to 60,000
  feet pressure altitude
• Charts from some world area forecast centers
  (WAFC) cover the layer from FL240 to FL630
• A wide range of information can be interpreted
  from this chart including forecast of
  thunderstorm areas, tropical cyclones, surface
  positions of well-defined convergence zones,
  movement of frontal systems and the locations and
  speeds of jetstreams see figure 16-14
• The positions of jet streams with speeds greater
  than 80 knots are indicated by long, heavy lines
  with arrowheads showing the direction of flow

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• Each jet is labeled with altitude speed and
  direction of the maximum wind in the jet core is
  shown with conventional wind barbs
• Heights of the tropopause are indicated in boxes
  relatively high and low tropopause heights are
  indicated with H and L respectively
• Areas of significant CB (thunderstorm) activity
  are enclosed in scallooed lines with heights of
  tops and bases indicated

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• If the base of the layer is below 25,000 feet it
  is indicated by xxx
• These areas include CB embedded in clouds, haze,
  or dust
• Areas of moderate and greater turbulence are
  enclosed in dashed lines
• Predicted intensities and heights of bases and
  tops of the turbulent layers are also given
• The high-level significant weather prog chart
  also includes positions of surface fronts, squall
  lines and the location of volcanic eruptions see
  Appendix D for more details

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• Forecast Winds and Temperatures Aloft Chart
• The third forecast graphic that is useful for
  flight planning is the forecast winds and
  temperatures aloft chart
• This is simply a graphical presentation of the
  information given in FD forecast bulletins

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• See figure 16-15 for a sample FD chart for
  international flights
• Wind and temperature predictions are produced at
  regularly spaced latitude and longitude positions
  on international charts
• Other useful forecast charts include the
  convective outlook chart and the volcanic ash
  forecast transport and dispersion (VAFTAD) chart

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• Section C Aviation Weather Information Sources

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• FAA Flight Service Stations (FSS)
• The flight service station (FSS) is one of the
  most common sources of weather information for
  pilots
• Automated flight service stations (AFSS) about
  one per state these are the result of
  consolidating older, manual stations
• More aviation weather briefing services are
  provided by FAA flight service stations than any
  other government service

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• A standard briefing provides you with the most
  complete weather picture tailored to your
  specific flight figure 16-16 lists the items
  included in a standard briefing
• For a telephone briefing it is helpful to have
  examined these data ahead of time
• An abbreviated briefing enables you to
  supplement mass disseminated data, update a
  previous briefing, or request specific
  information
• If your proposed departure time is six or more
  hours in the future an outlook briefing provides
  a general overview of forecasted weather

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• The most common method of obtaining weather
  information from an FSS is a briefing over the
  telephone
• You can obtain one of three types of briefings
  standard, abbreviated and outlook
• An FSS briefing can be obtained by dialing
  1-800-WX-BRIEF

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• You should request a standard briefing if you
  have received no preliminary weather information
  and are departing within the hour.
• To supplement mass disseminated data, an
  abbreviated briefing should be requested.
• The FSS provides an outlook briefing 6 or more
  hours in advance of your proposed departure time

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• Transcribed information briefing service (TIBS)
  an AFSS service that provides continuous
  telephone recordings of meteorological and/or
  aeronautical information 24 hours a day
• TIBS provides route briefings and depending on
  user demand aviation weather observations,
  forecasts and wind and temperature aloft
  forecasts
• TIBS information is frequently updated to ensure
  current and accurate weather data

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• Local numbers for TIBS are available in the AFD
  (Airport Facility Directory)
• En route flight advisory service (EFAS)
  probably the most familiar inflight service to
  pilots
• To use this service contact the specific EFAS by
  using the words Flight Watch

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• The frequency for flight watch below 18,000 feet
  MSL is 122.0 MHz
• Upon your request, the flight watch specialist
  can provide aviation weather information and
  time-critical enroute assistance
• If you are facing hazardous or unknown weather
  conditions, EFAS may recommend alternate or
  diversionary routes
• The receipt and rapid dissemination of pilot
  weather reports is a primary responsibility of
  EFAS

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• At altitudes below 18,000 feet, you can
  contact Flight Watch on 122.0 MHz for information
  regarding current weather along your proposed
  route of flight

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• Continuous Broadcasts of Weather Information
  (TWEB, HIWAS)
• TWEB is aired continuously
• over selected low and medium frequency NDBs
  (nondirectional beacons)
• 190 535 kHz
• and over VORs (very high frequency
  omni-directional ranges)
• 108.0 117.95 MHz

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• TWEBs contain in-flight cross-country weather
  information including winds and temperatures
  aloft forecasts

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• Hazardous inflight weather advisory service
  (HIWAS)
• another in-flight service that provides a
  continuous broadcast over selected VORs to inform
  you of hazardous flying conditions such as
  turbulence, icing, IFR conditions and high winds

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• Other Weather Information Sources
• Automatic terminal information service (ATIS)
  available at most major airports that have
  operational control towers
• ATIS is a pre-recorded report, broadcast on a
  dedicated frequency which includes information
  regarding current weather and pertinent local
  airport conditions
• ATIS is normally recorded every hour but may be
  updated any time

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• ATIS frequencies are listed in the AFD (Airport
  Facility Directory)
• Direct user access terminal service (DUATS) The
  FAA supports the DUATS service
• This computer-based program provides NWS and FAA
  products that are normally used in pilot weather
  briefings
• By using a personal computer and modem you can
  access weather information prior to flight

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• Flight plans can also be filed and amended
  through DUATS
• Other weather sources include weather forecasts
  by local news broadcasts, the Weather Channel,
  airing on cable television and Jeppesen DataPlan

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• Weather on the Internet
• The internet is probably one of the fastest
  growing sources of aviation weather
• Appendix F gives a large number of US and
  international agencies that provide both aviation
  weather information and general weather
  information

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• Summary
• This chapter has provided an understanding of the
  procedures used to collect and process raw
  weather data, and to make weather forecasts.
• The discussion of forecast methods has given you
  some insight into the accuracy of forecasts

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• This knowledge should assist you in interpreting
  and effectively using weather data for flight
  planning.
• In addition, you are now familiar with the
  essential aviation weather forecast products in
  both text and graphic formats
• You know of a variety of sources from which you
  can obtain observations and forecasts appropriate
  for your flying needs

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• The standard briefing is a useful guide in
  selecting pertinent information
• Your knowledge of the three-dimensional structure
  and behavior of weather systems allows you to
  integrate available information into a
  comprehensive picture
• This new information will help you practice
  selectivity and visualization in dealing with
  current and forecast flight weather.

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• A practical application of your knowledge of
  aviation weather resources is explored in the
  final chapter