Aviation Weather Services - Introduction - Federal Aviation Administration
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Chapter 13
Aviation Weather Services
Introduction
In aviation, weather service is a combined effort of the
National Weather Service (NWS), Federal Aviation
Administration (FAA), Department of Defense (DOD), other
aviation groups, and individuals. Because of the increasing
need for worldwide weather services, foreign weather
organizations also provide vital input.
While weather forecasts are not 100 percent accurate,
meteorologists, through careful scientific study and computer
modeling, have the ability to predict weather patterns, trends,
and characteristics with increasing accuracy. Through a
complex system of weather services, government agencies,
and independent weather observers, pilots and other aviation
professionals receive the benefit of this vast knowledge base
in the form of up-to-date weather reports and forecasts.
These reports and forecasts enable pilots to make informed
decisions regarding weather and flight safety before and
during a flight.
13-1
Observations radiosonde observations, pilot weather reports (PIREPs),
Aircraft Meteorological Data Relay (AMDAR) and the
The data gathered from surface and upper altitude
Meteorological Data Collection and Reporting System
observations form the basis of all weather forecasts,
(MDCRS). A radiosonde is a small cubic instrumentation
advisories, and briefings. There are four types of weather
package that is suspended below a six foot hydrogen- or
observations: surface, upper air, radar, and satellite.
helium-filled balloon. Once released, the balloon rises at a rate
of approximately 1,000 feet per minute (fpm). As it ascends,
Surface Aviation Weather Observations
the instrumentation gathers various pieces of data, such as air
Surface aviation weather observations (METARs) are a
temperature, moisture, and pressure, as well as wind speed
compilation of elements of the current weather at individual
and direction. Once the information is gathered, it is relayed
ground stations across the United States. The network is
to ground stations via a 300 milliwatt radio transmitter.
made up of government and privately contracted facilities
that provide continuous up-to-date weather information.
The balloon flight can last as long as 2 hours or more and
Automated weather sources, such as the Automated Weather
can ascend to altitudes as high as 115,000 feet and drift as
Observing Systems (AWOS), Automated Surface Observing
far as 125 miles. The temperatures and pressures experienced
Systems (ASOS), as well as other automated facilities, also
during the flight can be as low as -130 °F and pressures as
play a major role in the gathering of surface observations.
low as a few thousandths of what is experienced at sea level.
Surface observations provide local weather conditions
Since the pressure decreases as the balloon rises in the
and other relevant information for a specific airport. This
atmosphere, the balloon expands until it reaches the limits
information includes the type of report, station identifier,
of its elasticity. This point is reached when the diameter has
date and time, modifier (as required), wind, visibility,
increased to over 20 feet. At this point, the balloon pops and
runway visual range (RVR), weather phenomena, sky
the radiosonde falls back to Earth. The descent is slowed by
condition, temperature/dew point, altimeter reading, and
means of a parachute. The parachute aids in protecting people
applicable remarks. The information gathered for the surface
and objects on the ground. Each year over 75,000 balloons
observation may be from a person, an automated station, or
are launched. Of that number, 20 percent are recovered and
an automated station that is updated or enhanced by a weather
returned for reconditioning. Return instructions are printed
observer. In any form, the surface observation provides
on the side of each radiosonde.
valuable information about individual airports around the
country. Although the reports cover only a small radius, the
Pilots also provide vital information regarding upper air
pilot can generate a good picture of the weather over a wide
weather observations and remain the only real-time source
area when many reporting stations are viewed together.
of information regarding turbulence, icing, and cloud
heights. This information is gathered and filed by pilots
Air Route Traffic Control Center (ARTCC)
in flight. Together, PIREPs and radiosonde observations
The Air Route Traffic Control Center (ARTCC) facilities provide information on upper air conditions important for
are responsible for maintaining separation between flights flight planning. Many domestic and international airlines
conducted under instrument flight rules (IFR) in the en have equipped their aircraft with instrumentation that
route structure. Center radars (Air Route Surveillance Radar automatically transmits in flight weather observations
(ARSR)) acquire and track transponder returns using the same through the DataLink system.
basic technology as terminal radars. Earlier center radars
displayed weather as an area of slashes (light precipitation) The Aircraft Meteorological Data Relay (AMDAR) is
and Hs (moderate rainfall). Because the controller could not an international program utilizing commercial aircraft to
detect higher levels of precipitation, pilots had to be wary provide automated weather observations. The AMDAR
of areas showing moderate rainfall. Newer radar displays program provides approximately 220,000-230,000 aircraft
show weather as three shades of blue. Controllers can select observations per day on a worldwide basis utilizing aircraft
the level of weather to be displayed. Weather displays of onboard sensors and probes that measure wind, temperature,
higher levels of intensity make it difficult for controllers to humidity/water vapor, turbulence and icing data. AMDAR
see aircraft data blocks, so pilots should not expect air traffic vertical profiles and en route observations provide significant
control (ATC) to keep weather displayed continuously. benefits to the aviation community by enhancing aircraft
safety and operating efficiency through improved weather
Upper Air Observations analysis and forecasting. The AMDAR program also
Observations of upper air weather are more challenging contributes to improved short and medium term numerical
than surface observations. There are several methods by weather forecasts for a wide range of services including
which upper air weather phenomena can be observed:
13-2
severe weather, defense, marine, public weather and
environmental monitoring. The information is down linked
either via Very High Frequency (VHF) communications
through the Aircraft Communications Addressing and
Reporting System (ACARS) or via satellite link through the
Aircraft to Satellite Data Acquisition and Relay (ASDAR).
The Meteorological Data Collection and Reporting System
(MDCRS) is an automated airborne weather observation
program that is used in the U.S. This program collects and
disseminates real-time upper-air weather observations from
participating airlines. The weather elements are down linked
via ACARS and are managed by Aeronautical Radio, Inc.
(ARINC) who then forwards them in Binary Universal Form
for the Representation of Meteorological Data (BUFR)
format to the NWS and in raw data form to the Earth Science
Research Laboratory (ESRL) and the participating airline.
More than 1,500 aircraft report wind and temperature data
with some of these same aircraft also providing turbulence
and humidity/water vapor information. In conjunction with Figure 13-1. Example of a weather radar scope.
avionics manufacturers, each participating airline programs
their equipment to provide certain levels of meteorological
data. The monitoring and collection of climb, en route, and
descent data is accomplished through the aircraft’s Flight Data
Acquisition and Monitoring System (FDAMS) and is then
transmitted via ACARS. When aircraft are out of ACARS
range, reports can be relayed through ASDAR. However, in
most cases, the reports are buffered until the aircraft comes
within ACARS range, at which point they are downloaded.
Radar Observations
There are four types of radars which provide information
about precipitation and wind.
1. The WSR-88D NEXRAD radar, commonly called
Doppler radar, provides in-depth observations that
inform surrounding communities of impending
weather. Doppler radar has two operational modes:
clear air and precipitation. In clear air mode, the radar
is in its most sensitive operational mode because a Figure 13-2. WSR-88D Weather Radar Echo Intensity Legend.
slow antenna rotation allows the radar to sample the
atmosphere longer. Images are updated about every
Reflectivity (dBZ) Ranges Weather Radar Echo Intensity
10 minutes in this mode.
40–50 Heavy
mode when precipitation is present. A faster antenna 50+ dBZ Extreme
rotation in this mode allows images to update at Figure 13-3. WSR-88D Weather Radar Precipitation Intensity
a faster rate, approximately every 4 to 6 minutes. Terminology.
Intensity values in both modes are measured in
dBZ (decibels of Z) and are depicted in color on the
radar image. [Figure 13-1] Intensities are correlated
to intensity terminology (phraseology) for ATC
purposes. [Figures 13-2 and 13-3]
13-3
2. FAA terminal Doppler weather radar (TDWR), recordings of meteorological and aeronautical information.
installed at some major airports around the country, TIBS provides area and route briefings, airspace procedures,
also aids in providing severe weather alerts and and special announcements. The recordings are automatically
warnings to ATC. Terminal radar ensures pilots updated as changes occur. It is designed to be a preliminary
are aware of wind shear, gust fronts, and heavy briefing tool and is not intended to replace a standard briefing
precipitation, all of which are dangerous to arriving from a FSS specialist. The TIBS service can only be accessed
and departing aircraft. by a touchtone phone. The phone numbers for the TIBS
3. The third type of radar commonly used in the detection service are listed in the Chart Supplement U.S. (formerly
of precipitation is the FAA airport surveillance radar. Airport/Facility Directory).
This radar is used primarily to detect aircraft, but it
Hazardous Inflight Weather Advisory Service
also detects the location and intensity of precipitation,
(HIWAS)
which is used to route aircraft traffic around severe
weather in an airport environment. Hazardous Inflight Weather Advisory Service (HIWAS),
available in the 48 conterminous states, is an automated
4. Airborne radar is equipment carried by aircraft to continuous broadcast of hazardous weather information
locate weather disturbances. The airborne radars over selected VOR navigational aids (NAVAIDs). The
generally operate in the C or X bands (around 6 broadcasts include advisories such as AIRMETS, SIGMETS,
GHz or around 10 GHz, respectively) permitting convective SIGMETS, and urgent PIREPs. The broadcasts
both penetration of heavy precipitation, required for are automatically updated as changes occur. Pilots should
determining the extent of thunderstorms, and sufficient contact a FSS or EFAS for additional information. VORs that
reflection from less intense precipitation. have HIWAS capability are depicted on aeronautical charts
with an “H” in the upper right corner of the identification
Satellite box. [Figure 13-4]
Advancement in satellite technologies has recently allowed
for commercial use to include weather uplinks. Through the Transcribed Weather Broadcast (TWEB) (Alaska
use of satellite subscription services, individuals are now able Only)
to receive satellite transmitted signals that provide near real- A continuous automated broadcast of meteorological and
time weather information for the North American continent. aeronautical data over selected low or medium frequency (L/
MF) and very high frequency (VHF) omnidirectional range
Service Outlets (VOR) NAVAID facilities. The broadcasts are automatically
Service outlets are government, government contract, or updated as changes occur. The broadcast contains adverse
private facilities that provide aviation weather services. Several conditions, surface weather observations, PIREPS, and
different government agencies, including the FAA, National a density altitude statement (if applicable). Recordings
Oceanic and Atmospheric Administration (NOAA), and the may also include a synopsis, winds aloft forecast, en route
NWS work in conjunction with private aviation companies and terminal forecast data, and radar reports. At selected
to provide different means of accessing weather information. locations, telephone access to the TWEB has been provided
(TEL-TWEB). Telephone numbers for this service are found
Flight Service Station (FSS)
The FSS is the primary source for preflight weather
information. A preflight weather briefing from an FSS can be
obtained 24 hours a day by calling 1-800-WX BRIEF from
anywhere in the United States and Puerto Rico. Telephone
numbers for FSS can be found in the Chart Supplement U.S.
(formerly Airport/Facility Directory) or in the United States
Government section of the telephone book.
The FSS also provides inflight weather briefing services
and weather advisories to flights within the FSS area of
responsibility.
Telephone Information Briefing Service (TIBS) Symbol indicates HIWAS
The Telephone Information Briefing Service (TIBS),
provided by FSS, is a system of automated telephone Figure 13-4. HIWAS availability is shown on sectional chart.
13-4
in the Alaska Chart Supplement U.S. (formerly Airport/ 5. En route forecast—a summary of the weather forecast
Facility Directory). These broadcasts are made available for the proposed route of flight.
primarily for preflight and inflight planning, and as such, 6. Destination forecast—a summary of the expected
should not be considered as a substitute for specialist- weather for the destination airport at the estimated
provided preflight briefings. time of arrival (ETA).
Weather Briefings 7. Forecast winds and temperatures aloft—a forecast of
the winds at specific altitudes for the route of flight.
Prior to every flight, pilots should gather all information
The forecast temperature information aloft is provided
vital to the nature of the flight. This includes an appropriate
only upon request.
weather briefing obtained from a specialist at a FSS.
8. Notices to Airmen (NOTAM)—information pertinent
For weather specialists to provide an appropriate weather to the route of flight that has not been published in the
briefing, they need to know which of the three types of NOTAM publication. Published NOTAM information
briefings is needed—standard, abbreviated, or outlook. Other is provided during the briefing only when requested.
helpful information is whether the flight is visual flight rules 9. ATC delays—an advisory of any known ATC delays
(VFR) or IFR, aircraft identification and type, departure that may affect the flight.
point, estimated time of departure (ETD), flight altitude, route
10. Other information—at the end of the standard briefing,
of flight, destination, and estimated time en route (ETE).
the FSS specialist provides the radio frequencies
needed to open a flight plan and to contact EFAS. Any
This information is recorded in the flight plan system and a
additional information requested is also provided at
note is made regarding the type of weather briefing provided.
this time.
If necessary, it can be referenced later to file or amend a
flight plan. It is also used when an aircraft is overdue or is
Abbreviated Briefing
reported missing.
An abbreviated briefing is a shortened version of the standard
briefing. It should be requested when a departure has been
Standard Briefing
delayed or when weather information is needed to update
A standard briefing provides the most complete information
the previous briefing. When this is the case, the weather
and a more complete weather picture. This type of briefing
specialist needs to know the time and source of the previous
should be obtained prior to the departure of any flight and
briefing so the necessary weather information is not omitted
should be used during flight planning. A standard briefing
inadvertently. It is always a good idea for the pilot to update
provides the following information in sequential order if it
the weather information whenever he/she has additional time.
is applicable to the route of flight.
1. Adverse conditions—this includes information about Outlook Briefing
adverse conditions that may influence a decision to An outlook briefing should be requested when a planned
cancel or alter the route of flight. Adverse conditions departure is 6 hours or more away. It provides initial forecast
include significant weather, such as thunderstorms or information that is limited in scope due to the time frame
aircraft icing, or other important items such as airport of the planned flight. This type of briefing is a good source
closings. of flight planning information that can influence decisions
2. VFR flight not recommended—if the weather for regarding route of flight, altitude, and ultimately the go/no-go
the route of flight is below VFR minimums, or if decision. A prudent pilot requests a follow-up briefing prior
it is doubtful the flight could be made under VFR to departure since an outlook briefing generally only contains
conditions due to the forecast weather, the briefer may information based on weather trends and existing weather in
state “VFR flight not recommended.” It is the pilot’s geographical areas at or near the departure airport. A standard
decision whether or not to continue the flight under briefing near the time of departure ensures that the pilot has
VFR, but this advisory should be weighed carefully. the latest information available prior to his/her flight.
3. Synopsis—an overview of the larger weather picture.
Aviation Weather Reports
Fronts and major weather systems that affect the
general area are provided. Aviation weather reports are designed to give accurate
depictions of current weather conditions. Each report
4. Current conditions—the current ceilings, visibility, provides current information that is updated at different times.
winds, and temperatures. If the departure time is more Some typical reports are METARs and PIREPs.
than 2 hours away, current conditions are not included
in the briefing.
13-5
Aviation Routine Weather Report (METAR) 4. Modifier—denotes that the METAR/SPECI came from
A METAR is an observation of current surface weather an automated source or that the report was corrected. If
reported in a standard international format. While the the notation “AUTO” is listed in the METAR/SPECI,
METAR code has been adopted worldwide, each country is the report came from an automated source. It also lists
allowed to make modifications to the code. Normally, these “AO1” (for no precipitation discriminator) or “AO2”
differences are minor but necessary to accommodate local (with precipitation discriminator) in the “Remarks”
procedures or particular units of measure. This discussion of section to indicate the type of precipitation sensors
METAR covers elements used in the United States. employed at the automated station.
When the modifier “COR” is used, it identifies a
METARs are issued on a regularly scheduled basis unless corrected report sent out to replace an earlier report
significant weather changes have occurred. A special
that contained an error (for example: METAR KGGG
METAR (SPECI) can be issued at any time between routine
161753Z COR).
METAR reports.
5. Wind—reported with five digits (14021KT) unless
Example: the speed is greater than 99 knots, in which case the
METAR KGGG 161753Z AUTO 14021G26KT 3/4SM wind is reported with six digits. The first three digits
+TSRA BR BKN008 OVC012CB 18/17 A2970 RMK indicate the direction the true wind is blowing from in
PRESFR tens of degrees. If the wind is variable, it is reported
as “VRB.” The last two digits indicate the speed of
A typical METAR report contains the following information the wind in knots unless the wind is greater than 99
in sequential order: knots, in which case it is indicated by three digits. If
the winds are gusting, the letter “G” follows the wind
1. Type of report—there are two types of METAR
speed (G26KT). After the letter “G,” the peak gust
reports. The first is the routine METAR report that is
recorded is provided. If the wind direction varies more
transmitted on a regular time interval. The second is
than 60° and the wind speed is greater than six knots,
the aviation selected SPECI. This is a special report
a separate group of numbers, separated by a “V,” will
that can be given at any time to update the METAR for
indicate the extremes of the wind directions.
rapidly changing weather conditions, aircraft mishaps,
or other critical information. 6. Visibility—the prevailing visibility (¾ SM) is reported
in statute miles as denoted by the letters “SM.” It is
2. Station identifier—a four-letter code as established by
reported in both miles and fractions of miles. At times,
the International Civil Aviation Organization (ICAO).
runway visual range (RVR) is reported following the
In the 48 contiguous states, a unique three-letter
prevailing visibility. RVR is the distance a pilot can
identifier is preceded by the letter “K.” For example,
see down the runway in a moving aircraft. When RVR
Gregg County Airport in Longview, Texas, is
is reported, it is shown with an R, then the runway
identified by the letters “KGGG,” K being the country
number followed by a slant, then the visual range
designation and GGG being the airport identifier.
in feet. For example, when the RVR is reported as
In other regions of the world, including Alaska and
R17L/1400FT, it translates to a visual range of 1,400
Hawaii, the first two letters of the four-letter ICAO
feet on runway 17 left.
identifier indicate the region, country, or state. Alaska
identifiers always begin with the letters “PA” and 7. Weather—can be broken down into two different
Hawaii identifiers always begin with the letters “PH.” categories: qualifiers and weather phenomenon
Station identifiers can be found by calling the FSS, a (+TSRA BR). First, the qualifiers of intensity,
NWS office, or by searching various websites such proximity, and the descriptor of the weather are given.
as DUATS and NOAA's Aviation Weather Aviation The intensity may be light (–), moderate ( ), or heavy
Digital Data Services (ADDS). (+). Proximity only depicts weather phenomena that
are in the airport vicinity. The notation “VC” indicates
3. Date and time of report—depicted in a six-digit group
a specific weather phenomenon is in the vicinity
(161753Z). The first two digits are the date. The last
of five to ten miles from the airport. Descriptors
four digits are the time of the METAR/SPECI, which
are used to describe certain types of precipitation
is always given in coordinated universal time (UTC).
and obscurations. Weather phenomena may be
A “Z” is appended to the end of the time to denote
reported as being precipitation, obscurations, and
the time is given in Zulu time (UTC) as opposed to
other phenomena, such as squalls or funnel clouds.
local time.
13-6
Descriptions of weather phenomena as they begin or Sky Cover Contraction
end and hailstone size are also listed in the “Remarks”
Less than 1∕8 (Clear) SKC, CLR, FEW
sections of the report. [Figure 13-5] 1∕8–2∕8 (Few)
FEW
8. Sky condition—always reported in the sequence of ³∕8–4∕8 (Scattered) SCT
5∕8–7∕8 (Broken) BKN
amount, height, and type or indefinite ceiling/height 8∕8 or (Overcast)
OVC
(vertical visibility) (BKN008 OVC012CB, VV003).
The heights of the cloud bases are reported with a Figure 13-6. Reportable contractions for sky condition.
three-digit number in hundreds of feet AGL. Clouds
above 12,000 feet are not detected or reported by an
12. Remarks—the remarks section always begins with the
automated station. The types of clouds, specifically
letters “RMK.” Comments may or may not appear in
towering cumulus (TCU) or cumulonimbus (CB)
this section of the METAR. The information contained
clouds, are reported with their height. Contractions
in this section may include wind data, variable
are used to describe the amount of cloud coverage and
visibility, beginning and ending times of particular
obscuring phenomena. The amount of sky coverage is
phenomenon, pressure information, and various other
reported in eighths of the sky from horizon to horizon.
information deemed necessary. An example of a
[Figure 13-6]
remark regarding weather phenomenon that does not
9. Temperature and dew point—the air temperature and fit in any other category would be: OCNL LTGICCG.
dew point are always given in degrees Celsius (C) or This translates as occasional lightning in the clouds
(18/17). Temperatures below 0 °C are preceded by and from cloud to ground. Automated stations also use
the letter “M” to indicate minus. the remarks section to indicate the equipment needs
10. Altimeter setting—reported as inches of mercury maintenance.
("Hg) in a four-digit number group (A2970). It is
always preceded by the letter “A.” Rising or falling Example:
pressure may also be denoted in the “Remarks” METAR KGGG 161753Z AUTO 14021G26KT 3/4SM
sections as “PRESRR” or “PRESFR,” respectively. +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK
PRESFR
11. Zulu time—a term used in aviation for UTC, which
places the entire world on one time standard.
Qualifier Weather Phenomena
Intensity or Proximity 1 Descriptor 2 Precipitation 3 Obscuration 4 Other 5
– Light MI Shallow DZ Drizzle BR Mist PO Dust/sand whirls
Moderate (no qualifier) BC Patches RA Rain FG Fog SQ Squalls
+ Heavy DR Low drifting SN Snow FU Smoke FC Funnel cloud
VC in the vicinity BL Blowing SG Snow grains DU Dust +FC Tornado or waterspout
SH Showers IC Ice crystals (diamond dust) SA Sand SS Sandstorm
TS Thunderstorms PL Ice pellets HZ Haze DS Dust storm
FZ Freezing GR Hail PY Spray
PR Partial GS Small hail or snow pellets VA Volcanic ash
UP *Unknown precipitation
The weather groups are constructed by considering columns 1–5 in this table in sequence:
intensity, followed by descriptor, followed by weather phenomena (e.g., heavy rain showers(s) is coded as +SHRA).
* Automated stations only
Figure 13-5. Descriptors and weather phenomena used in a typical METAR.
13-7
Explanation: one weather phenomenon encountered. A PIREP is normally
Routine METAR for Gregg County Airport for the 16th transmitted as an individual report but may be appended to
day of the month at 1753Z automated source. Winds are a surface report. Pilot reports are easily decoded, and most
140 at 21 knots gusting to 26. Visibility is ¾ statute mile. contractions used in the reports are self-explanatory.
Thunderstorms with heavy rain and mist. Ceiling is broken
at 800 feet, overcast at 1,200 feet with cumulonimbus clouds. Example:
Temperature 18 °C and dew point 17 °C. Barometric pressure UA/OV GGG 090025/TM 1450/FL 060/TP C182/SK
is 29.70 "Hg and falling rapidly. 080 OVC/WX FV04SM RA/TA 05/WV 270030KT/TB
LGT/RM HVY RAIN
Pilot Weather Reports (PIREPs)
PIREPs provide valuable information regarding the Explanation:
conditions as they actually exist in the air, which cannot be Type: .................................Routine pilot report
gathered from any other source. Pilots can confirm the height Location: .......................... 25 NM out on the 090° radial,
of bases and tops of clouds, locations of wind shear and Gregg County VOR
turbulence, and the location of inflight icing. If the ceiling is Time: ................................ 1450 Zulu
below 5,000 feet, or visibility is at or below five miles, ATC Altitude or Flight Level: 6,000 feet
facilities are required to solicit PIREPs from pilots in the area. Aircraft Type: ................... Cessna 182
When unexpected weather conditions are encountered, pilots Sky Cover: ........................8,000 overcast
are encouraged to make a report to a FSS or ATC. When a Visibility/Weather: ........... 4 miles in rain
pilot weather report is filed, the ATC facility or FSS adds it Temperature: .....................5 °Celsius
to the distribution system to brief other pilots and provide Wind: ................................ 270° at 30 knots
inflight advisories. Turbulence: .......................Light
Icing: ................................ None reported
PIREPs are easy to file and a standard reporting form outlines Remarks: .......................... Rain is heavy
the manner in which they should be filed. Figure 13-7 shows
the elements of a PIREP form. Item numbers 1 through 5 are
required information when making a report, as well as at least
Encoding Pilot Weather Reports (PIREPS)
1 XXX 3-letter station identifier Nearest weather reporting location to the reported phenomenon
2 UA Routine PIREP, UUA-Urgent PIREP.
3 /OV Location Use 3-letter NAVAID idents only.
a. Fix: /OV ABC, /OV ABC 090025.
b. Fix: /OV ABC 045020-DEF, /OV ABC-DEF-GHI
4 /TM Time 4 digits in UTC: /TM 0915.
5 /FL Altitude/flight level 3 digits for hundreds of feet. If not known, use UNKN: /FL095, /FL310, /FLUNKN.
6 /TP Type aircraft 4 digits maximum. If not known, use UNKN: /TP L329, /TP B727, /TP UNKN.
7 /SK Sky cover/cloud layers Describe as follows:
a. Height of cloud base in hundreds of feet. If unknown, use UNKN.
b. Cloud cover symbol.
c. Height of cloud tops in hundreds of feet.
8 /WX Weather Flight visibility reported first:
Use standard weather symbols:
/WX FV02SM RA HZ, /WX FV01SM TSRA.
9 /TA Air temperature in celsius (C) If below zero, prefix with a hyphen: /TA 15, /TA M06.
10 /WV Wind Direction in degrees magnetic north and speed in six digits:
/WV270045KT, WV 280110KT.
11 /TB Turbulence Use standard contractions for intensity and type (use CAT or CHOP when
appropriate). Include altitude only if different from /FL, /TB EXTRM, /TB
LGT-MOD BLO 090.
12 /IC Icing Describe using standard intensity and type contractions. Include altitude only if
different than /FL: /IC LGT-MOD RIME, /IC SEV CLR 028-045.
13 /RM Remarks Use free form to clarify the report and type hazardous elements first:
/RM LLWS -15KT SFC-030 DURC RY22 JFK.
Figure 13-7. PIREP encoding and decoding.
13-8
Aviation Forecasts 7. Forecast significant weather—weather phenomena
Observed weather condition reports are often used in the are coded in the TAF reports in the same format as
creation of forecasts for the same area. A variety of different the METAR.
forecast products are produced and designed to be used in the 8. Forecast sky condition—given in the same format as
preflight planning stage. The printed forecasts that pilots need the METAR. Only cumulonimbus (CB) clouds are
to be familiar with are the terminal aerodrome forecast (TAF), forecast in this portion of the TAF report as opposed
aviation area forecast (FA), inflight weather advisories to CBs and towering cumulus in the METAR.
(SIGMET, AIRMET), and the winds and temperatures aloft
9. Forecast change group—for any significant weather
forecast (FB).
change forecast to occur during the TAF time period,
the expected conditions and time period are included
Terminal Aerodrome Forecasts (TAF)
in this group. This information may be shown as from
A TAF is a report established for the five statute mile
(FM), and temporary (TEMPO). “FM” is used when a
radius around an airport. TAF reports are usually given for
rapid and significant change, usually within an hour, is
larger airports. Each TAF is valid for a 24 or 30-hour time
expected. “TEMPO” is used for temporary fluctuations
period and is updated four times a day at 0000Z, 0600Z,
of weather, expected to last less than 1 hour.
1200Z, and 1800Z. The TAF utilizes the same descriptors
and abbreviations as used in the METAR report. The TAF 10. PROB30—a given percentage that describes the
includes the following information in sequential order: probability of thunderstorms and precipitation
occurring in the coming hours. This forecast is not
1. Type of report—a TAF can be either a routine forecast
used for the first 6 hours of the 24-hour forecast.
(TAF) or an amended forecast (TAF AMD).
2. ICAO station identifier—the station identifier is the Example:
same as that used in a METAR. TAF
3. Date and time of origin—time and date (081125Z) KPIR 111130Z 1112/1212
of TAF origination is given in the six-number code TEMPO 1112/1114 5SM BR
with the first two being the date, the last four being FM1500 16015G25KT P6SM SCT040 BKN250
the time. Time is always given in UTC as denoted by FM120000 14012KT P6SM BKN080 OVC150 PROB30
the Z following the time block. 1200/1204 3SM TSRA BKN030CB
FM120400 1408KT P6SM SCT040 OVC080
4. Valid period dates and times—The TAF valid period TEMPO 1204/1208 3SM TSRA OVC030CB
(0812/0912) follows the date/time of forecast origin
group. Scheduled 24 and 30 hour TAFs are issued Explanation:
four times per day, at 0000, 0600, 1200, and 1800Z. Routine TAF for Pierre, South Dakota…on the 11th day of
The first two digits (08) are the day of the month for the month, at 1130Z…valid for 24 hours from 1200Z on the
the start of the TAF. The next two digits (12) are the 11th to 1200Z on the 12th…wind from 150° at 12 knots…
starting hour (UTC). 09 is the day of the month for visibility greater than 6 SM…broken clouds at 9,000 feet…
the end of the TAF, and the last two digits (12) are temporarily, between 1200Z and 1400Z, visibility 5 SM in
the ending hour (UTC) of the valid period. A forecast mist…from 1500Z winds from 160° at 15 knots, gusting
period that begins at midnight UTC is annotated as 00. to 25 knots visibility greater than 6 SM…clouds scattered
If the end time of a valid period is at midnight UTC, at 4,000 feet and broken at 25,000 feet…from 0000Z wind
it is annotated as 24. For example, a 00Z TAF issued from 140° at 12 knots…visibility greater than 6 SM…clouds
on the 9th of the month and valid for 24 hours would broken at 8,000 feet, overcast at 15,000 feet…between 0000Z
have a valid period of 0900/0924. and 0400Z, there is 30 percent probability of visibility 3
5. Forecast wind—the wind direction and speed forecast SM…thunderstorm with moderate rain showers…clouds
are coded in a five-digit number group. An example broken at 3,000 feet with cumulonimbus clouds…from
would be 15011KT. The first three digits indicate the 0400Z…winds from 140° at 8 knots…visibility greater than
direction of the wind in reference to true north. The 6 miles…clouds at 4,000 scattered and overcast at 8,000…
last two digits state the windspeed in knots appended temporarily between 0400Z and 0800Z…visibility 3 miles…
with “KT.” Like the METAR, winds greater than 99 thunderstorms with moderate rain showers…clouds overcast
knots are given in three digits. at 3,000 feet with cumulonimbus clouds…end of report (=).
6. Forecast visibility—given in statute miles and may
be in whole numbers or fractions. If the forecast is
greater than six miles, it is coded as “P6SM.”
13-9
Area Forecasts (FA) and implies there may be occurrences of severe or greater
The FA gives a picture of clouds, general weather conditions, turbulence, severe icing, low-level wind shear, and IFR
and visual meteorological conditions (VMC) expected over conditions. The final line of the precautionary statement alerts
a large area encompassing several states. There are six areas the user that heights, for the most part, are MSL. Those that
for which area forecasts are published in the contiguous 48 are not MSL will state AGL or CIG.
states. Area forecasts are issued three times a day and are 3. Synopsis—gives a brief summary identifying the
valid for 18 hours. This type of forecast gives information location and movement of pressure systems, fronts,
vital to en route operations, as well as forecast information and circulation patterns.
for smaller airports that do not have terminal forecasts.
Example:
Area forecasts are typically disseminated in four sections and SYNOPSIS…LOW PRES TROF 10Z OK/TX PNHDL AREA
include the following information: FCST MOV EWD INTO CNTRL-SWRN OK BY 04Z.
1. Header—gives the location identifier of the source of WRMFNT 10Z CNTRL OK-SRN AR-NRN MS FCST LIFT
the FA, the date and time of issuance, the valid forecast NWD INTO NERN OK-NRN AR EXTRM NRN MS BY 04Z.
time, and the area of coverage.
Explanation:
Example: As of 1000Z, there is a low pressure trough over the Oklahoma
DFWC FA 120945 and Texas panhandle area, which is forecast to move eastward
SYNOPSIS AND VFR CLDS/WX into central to southwestern Oklahoma by 0400Z. A warm
SYNOPSIS VALID UNTIL 130400 front located over central Oklahoma, southern Arkansas, and
CLDS/WX VALID UNTIL 122200…OTLK VALID northern Mississippi at 1000Z is forecast to lift northwestward
122200-130400 into northeastern Oklahoma, northern Arkansas, and extreme
OK TX AR LA MS AL AND CSTL WTRS northern Mississippi by 0400Z.
4. VFR Clouds and Weather—This section lists expected
Explanation: sky conditions, visibility, and weather for the next 12
The area forecast shows information given by Dallas Fort hours and an outlook for the following 6 hours.
Worth, for the region of Oklahoma, Texas, Arkansas,
Louisiana, Mississippi, and Alabama, as well as a portion Example:
of the Gulf coastal waters. It was issued on the 12th day S CNTRL AND SERN TX
of the month at 0945. The synopsis is valid from the time AGL SCT-BKN010. TOPS 030. VIS 3-5SM BR. 14-16Z
of issuance until 0400 hours on the 13th. VFR clouds and BECMG AGL SCT030. 19Z AGL SCT050.
weather information on this area forecast are valid until 2200 OTLK…VFR
hours on the 12th and the outlook is valid from 2200Z on the OK
12th to 0400Z on the 13th. PNDLAND NW…AGL SCT030 SCT-BKN100.
2. Precautionary statements—IFR conditions, mountain TOPS FL200.
obscurations, and thunderstorm hazards are described 15Z AGL SCT040 SCT100. AFT 20Z SCT TSRA DVLPG..
in this section. Statements made here regarding height FEW POSS SEV. CB TOPS FL450.
are given in MSL, and if given otherwise, AGL or OTLK…VFR
ceiling (CIG) is noted.
Explanation:
Example: In south central and southeastern Texas, there is a scattered
SEE AIRMET SIERRA FOR IFR CONDS AND MTN to broken layer of clouds from 1,000 feet AGL with tops at
OBSCN. 3,000 feet, visibility is 3 to 5 SM in mist. Between 1400Z and
TS IMPLY SEV OR GTR TURB SEV ICE LLWS AND 1600Z, the cloud bases are expected to increase to 3,000 feet
IFR CONDS. AGL. After 1900Z, the cloud bases are expected to continue
NON MSL HGTS DENOTED BYAGL OR CIG. to increase to 5,000 feet AGL and the outlook is VFR.
Explanation: In northwestern Oklahoma and panhandle, the clouds are
The area forecast covers VFR clouds and weather, so the scattered at 3,000 feet with another scattered to broken layer
precautionary statement warns that AIRMET Sierra should at 10,000 feet AGL, with the tops at 20,000 feet. At 1500
be referenced for IFR conditions and mountain obscuration. Z, the lowest cloud base is expected to increase to 4,000
The code TS indicates the possibility of thunderstorms feet AGL with a scattered layer at 10,000 feet AGL. After
13-102000Z, the forecast calls for scattered thunderstorms with rain phenomena considered potentially hazardous to light aircraft
developing and a few becoming severe; the CB clouds have and aircraft with limited operational capabilities.
tops at flight level (FL) 450 or 45,000 feet MSL.
An AIRMET includes forecast of moderate icing, moderate
It should be noted that when information is given in the area turbulence, sustained surface winds of 30 knots or
forecast, locations may be given by states, regions, or specific greater, widespread areas of ceilings less than 1,000 feet
geological features such as mountain ranges. Figure 13-8 and/or visibilities less than three miles, and extensive
shows an area forecast chart with six regions of forecast, mountain obscurement.
states, regional areas, and common geographical features.
Each AIRMET bulletin has a fixed alphanumeric designator,
Inflight Weather Advisories numbered sequentially for easy identification, beginning with
Inflight weather advisories, which are provided to en route the first issuance of the day. Sierra is the AIRMET code used
aircraft, are forecasts that detail potentially hazardous to denote IFR and mountain obscuration; Tango is used to
weather. These advisories are also available to pilots prior denote turbulence, strong surface winds, and low-level wind
to departure for flight planning purposes. An inflight shear; and Zulu is used to denote icing and freezing levels.
weather advisory is issued in the form of either an AIRMET,
SIGMET, or convective SIGMET. Example:
BOSS WA 211945
AIRMET AIRMET SIERRA UPDT 3 FOR IFR AND MTN OBSCN
AIRMETs (WAs) are examples of inflight weather advisories VALID UNTIL 220200
that are issued every 6 hours with intermediate updates AIRMET IFT…ME NH VT MA CT RI NY NJ AND CSTL
issued as needed for a particular area forecast region. The WTRS FROM CAR TO YSJ TO 150E ACK TO EWR TO
information contained in an AIRMET is of operational YOW TO CAR OCNL CIG BLW 010/VIS BLW 3SM
interest to all aircraft, but the weather section concerns PCPN/BR. CONDS CONT BYD 02Z THRU 08Z
Stra
it of
Jua
nD
e Fu
ca
SLC Salt Lake City CHI Chicago BOS Boston
Mtns
s
sc oun et
tn
Olympic Mtns
ad d
S ug
NE WA
M
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P
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East NW
e
NE
t Ra ng e
Fort Peck
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Columbia
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Interior Slopes of Reservoir Riv Sou WRN Upper MI
sc
Basin
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ME
Ca
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Ca
Coast and Va
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Costal Valley
WA lle d S E t io n
of
Fl at he
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Bit ter roo
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wh rio ec
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ND Ar
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Su line ERN Upper MI Valley
lS
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Red River
Blu tns a ke re S
White Mtns
Upper
ta
Mtnsn
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MT Cntrl
a
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as
Mississippi
Wil ostal R
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Slope ac MI Adirondac
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NH
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Big Horn NW NE Thumb r Mohawk
Big Horn Mtns Hills v er B trl
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Coastal Mtns and Valleys
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ill
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CA Ridge Area Country
L e as
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La
Nebraska N
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NRN
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PA
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Sacramento
NRN Great e l L r i NW
Nevad n So Central rV Central Nebraska k ra
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Lake E SE
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Sierra Salt SE alley N Extrm S
Valley
a
Mtns Lake
NV
Mtns
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NECeIow NW N Third
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SW Cntrl
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SRN
Lake Tahoe SW SE W
OH MD
Wasatch
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Great U inta B
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Southern e IN
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N Colorado Re owL S
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Sie s Cen Va blic r SW r
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CO
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DC
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East
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CA
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Eastern Plains
KS e
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Death Lake M Sa
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M
Flint
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Mead r Va n Lu NE
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Coastal Fou ers Lower Arkansas SRN
sta
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Areas Ozarks Area IL Southern SE
e Grand Cornrea y
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Coa
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nt Ozarks
Channel ng Colorado
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Imperia R Northwest Southwest Hills Mtn
l im Mtns AZ Southeast Northern
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Valleys Texas Central of a
Color
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Mtn ento
Gulf of Eas l
Alab
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Santa Catalina ley
NM st Cntr
er Val MS
Mississ
We trl
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Gilariv
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SW SE Cn
Eastern AL
m
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North Central NE S
GA
Ch Mt
Lower
North
iric ns
SE Coastal
ah
West of thern Waters
ec Northeast
TX SW Extr m Sou
MIA Miami
ua
SFO
P
Pecos os R East of Georgia
Central
Pecos FL W 85
Extrm
ive
Mo rea
San Francisco LA rth
A
r
South No
bile
Southwest Southeast l Ap
Big South Central ta De al
as lta
B
Are end Are Baach
Co a y ee North
a p er lain
Up P
Lower al Centra
l
st
Rio
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oa n
i FL
Valley Mi P Sou th
Coastal Bend Coastal Waters–from Extrm
e am
DFW Lower coast outward to the
South
S tr
Upper
Coastal
Dallas/Fort Worth flight information Keys
f
Plain
ul
region border
G
Lower Keys Florida Straits
Figure 13-8. Area forecast region map.
13-11AIRMET MTN OBSCN…ME NH VT MA NY PA Washington, for a defined area from Seattle to Portland to
FROM CAR TO MLT TO CON TO SLT TO SYR TO CAR Eugene to Seattle. It calls for occasional severe clear air
MTNS OCNLY OBSCD BY CLDS/PCPN/BR. CONDS turbulence between FL280 and FL350 due to the location of
CONT BYD 02Z THRU 08Z the jet stream. These conditions will begin after 0200Z and
continue beyond the forecast scope of this SIGMET of 0530Z.
Explanation:
AIRMET SIERRA was issued for the Boston area at 1945Z Convective Significant Meteorological Information
on the 21st day of the month. SIERRA contains information (WST)
on IFR and/or mountain obscurations. This is the third
A Convective SIGMET (WST) is an inflight weather
updated issuance of this Boston AIRMET series as indicated
advisory issued for hazardous convective weather that affects
by “SIERRA UPDT 3” and is valid until 0200Z on the
the safety of every flight. Convective SIGMETs are issued
22nd. The affected states within the BOS area are: Maine,
for severe thunderstorms with surface winds greater than 50
New Hampshire, Vermont, Massachusetts, New York, and
knots, hail at the surface greater than or equal to ¾ inch in
Pennsylvania. Within an area bounded by: Caribou, ME;
diameter, or tornadoes. They are also issued to advise pilots
to Saint Johns, New Brunswick; to 150 nautical miles east
of embedded thunderstorms, lines of thunderstorms, or
of Nantucket, MA; to Newark, NJ; to Ottawa, Ontario;
thunderstorms with heavy or greater precipitation that affect
to Caribou, ME. The effected states within Caribou, ME
40 percent or more of a 3,000 square mile or greater region.
to Millinocket, ME to Concord, NH to Slate Run, PA to
Syracuse, NY to Caribou, ME will experience ceilings
Convective SIGMETs are issued for each area of the
below 1,000 feet/visibility below 3 SM, precipitation/mist.
contiguous 48 states but not Alaska or Hawaii. Convective
Conditions will continue beyond 0200Z through 0800Z.
SIGMETs are issued for the eastern (E), western (W), and
central (C) United States. Each report is issued at 55 minutes
SIGMET past the hour, but special Convective SIGMETs can be
SIGMETs (WSs) are inflight advisories concerning non- issued during the interim for any reason. Each forecast is
convective weather that is potentially hazardous to all valid for 2 hours. They are numbered sequentially each day
aircraft. They report weather forecasts that include severe from 1–99, beginning at 00Z time. If no hazardous weather
icing not associated with thunderstorms, severe or extreme exists, the convective SIGMET is still issued; however, it
turbulence or clear air turbulence (CAT) not associated with states “CONVECTIVE SIGMET…NONE.”
thunderstorms, dust storms or sandstorms that lower surface
or inflight visibilities to below three miles, and volcanic ash. Example:
SIGMETs are unscheduled forecasts that are valid for 4 hours MKCC WST 221855
unless the SIGMET relates to a hurricane, in which case it CONVECTIVE SIGMET 20C
is valid for 6 hours. VALID UNTIL 2055Z
ND SD
A SIGMET is issued under an alphabetic identifier, from FROM 90W MOT-GFK-ABR-90W MOT
November through Yankee. The first issuance of a SIGMET INTSFYG AREA SEV TS MOVG FROM 24045KT. TOPS
is designated as an Urgent Weather SIGMET (UWS). ABV FL450. WIND GUSTS TO 60KTS RPRTD.
Reissued SIGMETs for the same weather phenomenon are TORNADOES…HAIL TO 2 IN… WIND GUSTS TO
sequentially numbered until the weather phenomenon ends. 65KTS
POSS ND PTN
Example:
SFOR WS 100130 Explanation:
SIGMET ROME02 VALID UNTIL 100530 Convective SIGMET was issued for the central portion of
OR WA the United States on the 22nd at 1855Z. This is the 20th
FROM SEA TO PDT TO EUG TO SEA Convective SIGMET issued on the 22nd for the central
OCNL SEV CAT BTN FL280 AND FL350 EXPCD United States as indicated by “20C” and is valid until 2055Z.
DUE TO JTSTR. The affected states are North and South Dakota, from 90
CONDS BGNG AFT 0200Z CONTG BYD 0530Z . nautical miles west of Minot, ND; to Grand Forks, ND; to
Aberdeen, SD; to 90 nautical miles west of Minot, ND. An
Explanation: intensifying area of severe thunderstorms moving from 240
This is SIGMET Romeo 2, the second issuance for this degrees at 45 knots (to the northeast). Thunderstorm tops will
weather phenomenon. It is valid until the 10th day of the be above FL 450. Wind gusts up to 60 knots were reported.
month at 0530Z time. This SIGMET is for Oregon and
13-12Also reported were tornadoes, hail to 2 inches in diameter, and altitude is 6,000 feet for the forecast winds. In this case,
wind gusts to 65 knots possible in the North Dakota portion. “2714” means the wind is forecast to be from 270° at a speed
of 14 knots.
Winds and Temperature Aloft Forecast (FB)
Winds and temperatures aloft forecasts (FB) provide wind A six-digit group includes the forecast temperature aloft.
and temperature forecasts for specific locations throughout The elevation at Denver (DEN) is 5,431 feet, so the lowest
the United States, including network locations in Hawaii reportable altitude is 9,000 feet for the winds and temperature
and Alaska. The forecasts are made twice a day based on the forecast. In this case, “2321-04” indicates the wind is forecast to
radiosonde upper air observations taken at 0000Z and 1200Z. be from 230° at a speed of 21 knots with a temperature of –4 °C.
Altitudes through 12,000 feet are classified as true altitudes, Weather Charts
while altitudes 18,000 feet and above are classified as Weather charts are graphic charts that depict current or
altitudes and are termed flight levels. Wind direction is forecast weather. They provide an overall picture of the
always in reference to true north, and wind speed is given in United States and should be used in the beginning stages of
knots. The temperature is given in degrees Celsius. No winds flight planning. Typically, weather charts show the movement
are forecast when a given level is within 1,500 feet of the of major weather systems and fronts. Surface analysis,
station elevation. Similarly, temperatures are not forecast for weather depiction, and significant weather prognostic charts
any station within 2,500 feet of the station elevation. are sources of current weather information. Significant
weather prognostic charts provide an overall forecast weather
If the wind speed is forecast to be greater than 99 knots but picture.
less than 199 knots, the computer adds 50 to the direction
and subtracts 100 from the speed. To decode this type of data Surface Analysis Chart
group, the reverse must be accomplished. For example, when The surface analysis chart depicts an analysis of the current
the data appears as “731960,” subtract 50 from the 73 and surface weather. [Figure 13-10] This chart is transmitted
add 100 to the 19, and the wind would be 230° at 119 knots every 3 hours and covers the contiguous 48 states and
with a temperature of –60 °C. If the wind speed is forecast to adjacent areas. A surface analysis chart shows the areas of
be 200 knots or greater, the wind group is coded as 99 knots. high and low pressure, fronts, temperatures, dew points, wind
For example, when the data appears as “7799,” subtract 50 directions and speeds, local weather, and visual obstructions.
from 77 and add 100 to 99, and the wind is 270° at 199 knots Surface weather observations for reporting points across
or greater. When the forecast wind speed is calm, or less than the United States are also depicted on this chart. Each of
5 knots, the data group is coded “9900,” which means light these reporting points is illustrated by a station model.
and variable. [Figure 13-9] [Figure 13-11] A station model includes:
Explanation of Figure 13-9: • Sky cover—the station model depicts total sky cover
The heading indicates that this FB was transmitted on the and is shown as clear, scattered, broken, overcast, or
15th of the month at 1640Z and is based on the 1200Z upper obscured/partially obscured.
air data. The valid time is 1800Z on the same day and should • Sea level pressure—given in three digits to the nearest
be used for the period between 1400Z and 2100Z. The tenth of a millibar (mb). For 1,000 mbs or greater,
heading also indicates that the temperatures above FL240 prefix a 10 to the three digits. For less than 1,000 mbs,
are negative. Therefore, the minus sign will be omitted for prefix a 9 to the three digits.
all forecast temperatures above FL240. • Pressure change/tendency—pressure change in tenths
of mb over the past 3 hours. This is depicted directly
A four-digit data group shows the wind direction in reference below the sea level pressure.
to true north and the wind speed in knots. The elevation at
Amarillo, Texas (AMA) is 3,605 feet, so the lowest reportable • Dew point—given in degrees Fahrenheit.
• Present weather—over 100 different standard weather
FB KWBC 151640 symbols are used to describe the current weather.
DATA BASED ON 151200Z
VALID 151800Z FOR USE 1400-2100Z • Temperature—given in degrees Fahrenheit.
TEMPS NEGATIVE ABV 24000
FB 3000 6000 9000 12000 18000 24000 30000
• Wind—true direction of wind is given by the wind
AMA 2714 2725+00 2625-04 2531-15 2542-27 265842 pointer line, indicating the direction from which the
DEN 2321-04 2532-08 2434-19 2441-31 235347 wind is blowing. A short barb is equal to 5 knots of
wind, a long barb is equal to 10 knots of wind, and a
Figure 13-9. Winds and temperature aloft forecast. pennant is equal to 50 knots.
13-13Figure 13-10. Surface analysis chart.
Wind speed Total sky cover
Wind direction
Temperature Sea level pressure
Present weather Pressure change in past 3 hours
34 147
Dew point **
32
28 / Pressure tendency
1. Total sky cover: Overcast
2. Temperature/dew point: 34 °F/32 °F
3. Wind: From the northwest at 20 knots (relative to true north)
Examples of Wind Speed and Direction Plots
Calm NE/ 5 kts SW/ 10 kts N / 15 kts W/ 50 kts S / 60 kts
4. Present weather: Continuous light snow
5. Sea level pressure: 1014.7 millibars (mb)
Note: Pressure is always shown in 3 digits to the nearest tenth of a millibar.
For 1,000 mb or greater, prefix a “10” to the 3 digits
For less than 1,000 mb, prefix a “9” to the 3 digits
6. Pressure change in past 3 hours: Increased steadily or unsteadily by 2.8 mb
Figure 13-11. Sample station model and weather chart symbols.
13-14Weather Depiction Chart ceiling height, weather, and obstructions to visibility, but
A weather depiction chart details surface conditions as does not include winds or pressure readings like the surface
derived from METAR and other surface observations. The analysis chart. A bracket ( ] ) symbol to the right of the station
weather depiction chart is prepared and transmitted by indicates the observation was made by an automated station.
computer every 3 hours beginning at 0100Z time and is valid
data for the forecast period. It is designed to be used for flight Significant Weather Prognostic Charts
planning by giving an overall picture of the weather across Significant weather prognostic charts are available for low-
the United States. [Figure 13-12] level significant weather from the surface to FL 240 (24,000
feet), also referred to as the 400 mb level and high-level
The weather depiction chart also provides a graphic display significant weather from FL 250 to FL 630 (25,000 to 63,000
of IFR, VFR, and marginal VFR (MVFR) weather. Areas of feet). The primary concern of this discussion is the low-level
IFR conditions (ceilings less than 1,000 feet and visibility significant weather prognostic chart.
less than three miles) are shown by a hatched area outlined
by a smooth line. MVFR regions (ceilings 1,000 to 3,000 The low-level chart is is a forecast of aviation weather
feet, visibility 3 to 5 miles) are shown by a nonhatched area hazards, primarily intended to be used as a guidance product
outlined by a smooth line. Areas of VFR (no ceiling or ceiling for briefing the VFR pilot. The forecast domain covers the 48
greater than 3,000 feet and visibility greater than five miles) contiguous states, southern Canada and the coastal waters for
are not outlined. Also plotted are fronts, troughs, and squall altitudes below 24,000 ft. Low altitude Significant Weather
lines from the previous hours surface analysis chart. charts are issued four times daily and are valid at fixed times:
0000, 0600, 1200, and 1800 UTC. Each chart is divided on
Weather depiction charts show a modified station model the left and right into 12 and 24 hour forecast intervals (based
that provides sky conditions in the form of total sky cover, on the current NAM model available).
Figure 13-12. Weather depiction chart.
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