Pilot's Handbook of Aeronautical Knowledge

Chapter 13: Aviation Weather Services

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Pilots must use several weather reports to get an overall picture of any weather that can affect the safe completion of a flight. Meteorologists have the ability to predict weather patterns, trends, and characteristics with increasing accuracy. Pilots and other aviation professionals benefit from a wide array of up-to-date weather reports and forecasts.

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Observations

There are four types of weather observations: surface, upper air, radar, and satellite. These observations form the basis of all weather forecasts, advisories, and briefings.

Surface Aviation Weather Observations

METARs, or surface aviation weather observations, are compiled from individual ground stations across the United States. Information may be from a person, an automated station, or an automated station that is updated or enhanced by a weather observer. Automated weather sources contribute a substantial amount of data. These include Automated Weather Observing Systems (AWOS) and Automated Surface Observing Systems (ASOS).

Surface observations provide local weather conditions and other relevant information for a specific airport. This information includes:

• The type of report
• Station identifier
• Date and time
• Modifier (as required)
• Wind
• Visibility
• Runway visual range (RVR)
• Weather phenomena
• Sky condition
• Temperature/dew point
• Altimeter reading
• Applicable remarks.

Air Route Traffic Control Centers (ARTCC) maintain aircraft separation for IFR flights and provide traffic advisories for VFR flights. Air Route Surveillance Radar (ARSR) acquires and tracks transponder returns using the same basic technology as terminal radars. Using ARSR, controllers can provide weather insights. However, high-intensity weather makes it difficult for controllers to see aircraft data blocks. Pilots should not expect ATC to provide continuous weather information.

Upper Air Observations

A radiosonde is a small cubic instrumentation package that is suspended below a small balloon. As it ascends, it gathers data, including air temperature, moisture, pressure, wind speed, and wind direction. Data is relayed to ground stations via radio.

Pilot Reports (PIREPs) are in-flight upper air weather observations offered by pilots to ATC. They are the only real-time source of information regarding turbulence, icing, and cloud heights.

Many airlines have equipped their aircraft with instrumentation that automatically transmits in-flight weather observations through the DataLink system. The Aircraft Meteorological Data Relay system (AMDAR) is an international program utilizing commercial aircraft to provide automated weather observations, providing thousands of reports per day worldwide.

The Meteorological Data Collection and Reporting System (MDCRS) is an automated airborne weather observation program that is used in the United States. MDCRS collects and disseminates real-time upper-air weather observations from participating airlines.

Radar Observations

There are four types of radars which provide information about precipitation and wind.

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 antenna rotates more slowly compared to precipitation mode, with a longer time to sample the atmosphere, as well as a longer time delay between pictures (about 10 minutes, compared to 4-6 minutes in precipitation mode.)

Intensity values in both modes are measured in decibels of Z (dBZ) and are depicted in color on the radar image. ATC will refer to precipitation as "light," "moderate," "heavy," or "extreme" based on the dBZ measure.

Terminal Doppler Weather Radar (TDWR) is installed at some major airports around the country. TDWR provides severe weather alerts and warnings to ATC. Terminal radar ensures pilots are aware of wind shear, gust fronts, and heavy precipitation.

Airport Surveillance Radar is used primarily to detect aircraft. However, it also detects the location and intensity of precipitation. This can be used to route aircraft traffic around severe weather in an airport environment.

Airborne radar is carried by aircraft to locate weather disturbances. It is capable of penetrating heavy precipitation, which is necessary to determine the extent of thunderstorms. It also can detect less intense precipitation.

Satellite

Through the use of satellite subscription services, individuals are now able to receive satellite transmitted signals that provide near real- time weather information for the North American continent.

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Service Outlets

Service outlets are government, government contract, or private facilities that provide aviation weather services.

The Flight Service Station (FSS) is the primary source for preflight weather information. A preflight weather briefing can be obtained by calling 1-800-WX BRIEF. Telephone numbers for Flight Service Stations also can be found in the Chart Supplement.

The FSS also provides inflight weather briefing services and weather advisories to flights within the FSS area of responsibility.

The Telephone Information Briefing Service (TIBS), provided by FSS, is a system of automated telephone recordings. TIBS provides area and route briefings, airspace procedures, and special announcements. It is designed to be a preliminary briefing tool and is not intended to replace a standard briefing from a FSS specialist. Phone numbers are listed in the Chart Supplement.

Transcribed Weather Broadcasts (TWEB) are only available in Alaska. They are continuous automated broadcasts of meteorological and aeronautical data over selected VOR navigational aids. Telephone numbers are found in the Alaska Chart Supplement.

The Hazardous Inflight Weather Advisory Service (HIWAS) was deactivated on January 8, 2020. It provided a broadcast of hazardous weather information over selected VOR navigational aids.

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Weather Briefings

A weather briefing, obtained from a specialist at an FSS, can be provided in three formats: standard, abbreviated, or outlook. Briefers will need to know:

• If the planned flight is VFR or IFR
• Aircraft identification and type
• Departure point
• Estimated time of departure (ETD)
• Flight altitude
• Route of flight
• Destination
• Estimated time en route (ETE)

A standard briefing provides the most complete information. This type of briefing should be obtained prior to the departure of any flight and should be used during flight planning.

A standard briefing provides the following information:

• Adverse conditions
• VFR flight not recommended
• Synopsis (an overview of the larger weather picture)
• Current conditions
• En route forecast
• Destination forecast
• Forecast winds and temperatures aloft
• Notices to Airmen (NOTAMs)
• ATC delays
• Other information

An abbreviated briefing is a shortened version of the standard briefing. It should be requested when a departure has been delayed or when weather information is needed to update the previous briefing.

An outlook briefing should be requested when a planned departure is six (6) hours or more away. It provides initial forecast information that is limited in scope due to the time frame of the planned flight. This briefing can influence the route of flight, altitude, and the go/no-go decision. It should be followed by a standard briefing prior to departure.

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Aviation Weather Reports

Aviation weather reports are designed to give accurate depictions of current weather conditions.

A METAR is an observation of current surface weather reported in a standard international format. The acronym stands for "Meteorological Terminal Aviation Routine" weather report. However, the Aeronautical Information Manual (AIM) refers to METAR as "Aviation Routine Weather Report."

A typical METAR report contains the following information in sequential order:

• Type of report
• Station identifier
• Date and time of report
• Modifier
• Wind
• Visibility
• Weather
• Sky condition
• Temperature and dew point
• Altimeter setting
• Zulu time
• Remarks

A routine METAR report is transmitted on a regular time interval. A SPECI can be given at any time to update the METAR for rapidly changing weather conditions, aircraft mishaps, or other critical information.

The station identifier is a four-letter code. In the 48 contiguous states, a three-letter identifier is preceded by the letter "K". Alaska identifiers begin with "PA". Hawaii identifiers begin with "PH".

Date and time is a six-digit group. The first two digits are the date. The last four digits are the time of the report, which is always in coordinated universal time (UTC), also referred to as "Zulu" time. A "Z" is appended to denote this.

The modifier denotes if the source is automated, or if the report has been corrected. "AUTO" the indicates that the report came from an automated source. "COR" identifies a corrected report that replaces an earlier report that contained an error. If the report is automated, the precipitation sensor type is designated by "AO1" or "AO2" in the "Remarks" section.

Wind is reported with five digits. The first three digits indicate the direction the true wind is blowing from in tens of degrees. The last two digits indicate wind velocity.

• Variable winds are denoted as "VRB".
• If winds are gusty, wind is followed by a "G" with the reported peak gust.
• If the wind direction varies more than 60° and wind velocity is greater than six knots, a separate group of numbers separated by a "V" will indicate the extremes of the wind directions.
• If wind velocity is greater than 99 knots, it will be represented with three digits.

Prevailing visibility is reported in statute miles, denoted by the letters "SM". This can include fractions of miles. Runway Visual Range (RVR), also may be reported. This is the distance a pilot can see down the runway in a moving aircraft.

• When RVR is reported, it is shown with an "R", followed by then the runway number and a slant, and then the visual range in feet.
• Example: R17L/1400FT is visual range of 1,400 feet on runway 17 left.

Weather includes qualifiers and weather phenomena. Qualifiers include intensity, proximity, and/or descriptors. Phenomena include precipitation, obscuration, or other types. The weather is then constructed in sequence — intensity, followed by descriptor, followed by weather phenomena — using symbols and abbreviations, per the following table:

Qualifier		Weather Phenomena
Intensity or Proximity	Descriptor	Precipitation	Obscuration	Other
– 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

For example, heavy rain showers is coded as +SHRA. Light drizzle is –DZ.

Descriptions of weather phenomena as they begin or end and hailstone size are also listed in the "Remarks" sections of the report

Sky condition is reported as amount, height (AGL), and type. There also can be an indefinite ceiling/height. Vertical visibility may also be reported.

• BKN008 is a broken ceiling at 800 feet. OVC012CB is an overcast ceiling of cumulonimbus clouds at 1,200 feet. VV003 is a vertical visibility of 300 feet.
• Clouds above 12,000 feet are not detected or reported by an automated station.
• The types of clouds, specifically towering cumulus (TCU) or cumulonimbus (CB) clouds, are reported with their height.
• The amount of sky coverage is reported in eighths of the sky from horizon to horizon.

Air temperature and dew point are always given in degrees Celsius (C) or (18/17). Temperatures below 0°C are preceded by the letter "M" to indicate minus.

Altimeter setting is reported as inches of mercury as a four-digit number group always preceded by the letter "A". (e.g., A2992). Rising or falling pressure may be denoted in the Remarks sections as "PRESRR" or "PRESFR".

The Remarks section always begins with "RMK". Information may include wind data, variable visibility, beginning and ending times of particular phenomenon, pressure information, and anything deemed necessary.

Example METAR:

• Routine METAR
• for Aurora State Airport
• for the 16th day of the month at 1753Z
• automated source
• Winds are 140 at 21 knots gusting to 26
• Visibility is 3/4 statute mile
• Thunderstorms with heavy rain and mist
• Ceiling is broken at 800 feet, overcast at 1,200 feet with cumulonimbus clouds
• Temperature 18°C and dew point 17°C
• Barometric pressure is 29.70 "Hg
• Barometric pressure is falling rapidly

Pilot Weather Reports (PIREPs)

Pilot Weather Reports (PIREPs) provide observations as they actually exist. These observations cannot be gathered from any other source.

If the ceiling is below 5,000 feet, or visibility is at or below five miles, ATC facilities are required to solicit PIREPs from pilots in the area. Pilots are encouraged to make a report to an FSS or ATC when unexpected weather is encountered.

The elements of a PIREP form are shown in the associated table. Items 1-5 are required when making a report, in addition to at least one weather phenomenon. PIREPS are easily decoded, and most contractions used in the reports are self-explanatory

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. Fix: /OV ABC, /OV ABC 090025. 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: Height of cloud base in hundreds of feet. If unknown, use UNKN. Cloud cover symbol. 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.

Example PIREP:

• Type: Routine pilot report
• Location: 25 nautical miles out on the 090° radial, Newberg VOR
• Time: 1450 Zulu
• Altitude or Flight Level: 6,000 feet
• Aircraft Type: Cessna 182
• Sky Cover: 8,000 overcast
• Visibility/Weather: 4 miles in rain
• Temperature: 5° Celsius
• Wind: 270° at 30 knots
• Turbulence: Light
• Icing: None reported
• Remarks: Rain is heavy.

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Aviation Forecasts

A variety of forecast products are used in preflight planning. Printed forecasts include the Terminal Aerodrome Forecast (TAF), Aviation Area Forecast (FA), Inflight Weather Advisories (SIGMET, AIRMET), and the Winds and Temperatures Aloft Forecast (FB).

Terminal Aerodrome Forecasts (TAF)

A Terminal Area Forecast (TAF) is established for the five statute mile radius around a large or busy airport.

Each TAF is valid for a 24 or 30-hour time period. It is updated four times a day at 0000Z, 0600Z, 1200Z, and 1800Z.

The TAF utilizes the same descriptors and abbreviations as used in the METAR report. The TAF includes the following information in sequential order.

1. Type of report
2. ICAO station identifier
3. Date and time of origin
4. Valid period dates and times
5. Forecast wind
6. Forecast visibility
7. Forecast significant weather
8. Forecast sky condition
9. Forecast change group
10. PROB30

A TAF can be either a routine forecast (TAF) or an amended forecast (TAF AMD).

The TAF valid period follows the date/time of forecast origin group. The first two digits are the day of the month for the start of the TAF, followed by two digits for the starting hour (UTC). A 12Z TAF issued on the 11th of the month and valid for 24 hours would have a valid period of 1112/1212. A 00Z TAF issued on the 9th of the month and valid for 24 hours would have a valid period of 0900/0924.

As with a METAR, the TAF forecast wind and speed forecast are coded in a five-digit number group. The wind direction is in reference to true north.

Forecast visibility is given in statute miles and may be in whole numbers or fractions. If the forecast visibility is greater than six miles, it is coded as "P6SM".

Forecast significant weather phenomena are coded in the TAF reports in the same format as the METAR.

Forecast sky condition is given in the same format as the METAR. Only cumulonimbus (CB) clouds are forecast in this portion of the TAF report. (METARs provide observations of both cumulonimbus and towering cumulus clouds).

If any significant weather change is forecast to occur during the TAF time period, the expected conditions and time period are included in the forecast change group. "FM" is used when a rapid and significant change, usually within an hour, is expected. "TEMPO" is used for temporary fluctuations of weather that expected to last less than one hour.

A given percentage that describes the probability of thunderstorms and precipitation occurring in the coming hours may be expressed with PROB with an associated percentage, e.g. "PROB30". This forecast is not used for the first six hours of the 24-hour forecast.

Example TAF:

• Routine TAF
• for Aurora, Oregon
• on the 11th day of the month, at 1130 Zulu
• valid for 24 hours from 1200Z on the 11th to 1200Z on the 12th
   
• temporarily, between 1200Z and 1400Z
• visibility 5 SM in mist
   
• from 1500Z
• winds from 160° at 15 knots, gusting to 25 knots
• visibility greater than 6 statute miles
• clouds scattered at 4,000 feet and broken at 25,000 feet
   
• from 0000Z
• wind from 140° at 12 knots
• visibility greater than 6 statute miles
• clouds broken at 8,000 feet, overcast at 15,000 feet
• between 0000Z and 0400Z, there is 30 percent probability of visibility 3 SM
• thunderstorm with moderate rain showers
• clouds broken at 3,000 feet with cumulonimbus clouds
   
• from 0400Z
• winds from 140° at 8 knots
• visibility greater than 6 statute miles
• clouds at 4,000 scattered and overcast at 8,000
   
• temporarily between 0400Z and 0800Z
• visibility 3 statute miles
• thunderstorms with moderate rain showers
• clouds overcast at 3,000 feet with cumulonimbus clouds

Area Forecast (FA)

An Area Forecast (FA) gives a picture of clouds, general weather conditions, and visual meteorological conditions (VMC) expected over a large area encompassing several states. Forecasts are published for six areas in the contiguous 48 states.

Area forecasts are issued three times a day and are valid for 18 hours.

Area forecasts provide vital information to en route operations. FAs also provide information for smaller airports that do not have Terminal Area Forecasts (TAFs).

When information is given in the area forecast, locations may be given by states, regions, or specific geological features such as mountain ranges.

Area forecasts are typically disseminated in four sections and include the following information:

1. Header
2. Precautionary statements
3. Synopsis
4. VFR Clouds and Weather

The header gives the location identifier of the source of the FA, the date and time of issuance, the valid forecast time, and the area of coverage.

• The area forecast shows information given by Dallas Fort Worth for the region of Oklahoma, Texas, Arkansas, Louisiana, Mississippi, and Alabama, as well as a portion of the Gulf coastal waters.
• It was issued on the 12th day of the month at 0945.
• The synopsis is valid from the time of issuance until 0400 hours on the 13th.
• VFR clouds and weather information on this area forecast are valid until 2200 hours on the 12th
• and the outlook is valid from 2200Z on the 12th to 0400Z on the 13th.

Precautionary statements include IFR conditions, mountain obscurations, and thunderstorm hazards. Heights are in MSL. If given otherwise, AGL or CIG (ceiling) is noted.

• The area forecast covers VFR clouds and weather, so the precautionary statement warns that AIRMET Sierra should be referenced for IFR conditions and mountain obscuration.
• The code TS indicates the possibility of thunderstorms and implies there may be occurrences of severe or greater turbulence, severe icing, low-level wind shear, and IFR conditions.
• The final line of the precautionary statement alerts the user that heights, for the most part, are MSL. Those that are not MSL will state AGL or CIG.

The synopsis gives a brief summary identifying the location and movement of pressure systems, fronts, and circulation patterns.

• there is a low pressure trough
• As of 1000Z
• over the Oklahoma and Texas panhandle area
• which is forecast to move eastward into central to southwestern Oklahoma
• by 0400Z.
• A warm front
• at 1000Z
• located over central Oklahoma, southern Arkansas, and northern Mississippi
• is forecast to lift northwestward into northeastern Oklahoma, northern Arkansas, and extreme northern Mississippi
• by 0400Z.

The VFR Clouds and Weather section lists expected sky conditions, visibility, and weather for the next 12 hours and an outlook for the following 6 hours.

• In south central and southeastern Texas
• there is a scattered to broken layer of clouds from 1,000 feet AGL
• with tops at 3,000 feet
• visibility is 3 to 5 SM in mist.
• Between 1400Z and 1600Z
• the cloud bases are expected to increase to 3,000 feet AGL.
• After 1900Z
• the cloud bases are expected to continue to increase to 5,000 feet AGL
• and the outlook is VFR.

• In northwestern Oklahoma and panhandle
• the clouds are scattered at 3,000 feet
• with another scattered to broken layer at 10,000 feet AGL
• with the tops at 20,000 feet.
• At 1500 Zulu
• the lowest cloud base is expected to increase to 4,000 feet AGL with a scattered layer at 10,000 feet AGL.
• After 2000 Zulu
• the forecast calls for scattered thunderstorms with rain developing and a few becoming severe…
• the cumulonimbus clouds have tops at flight level (FL) 450 or 45,000 feet MSL
• and the outlook is VFR.

Inflight Weather Advisories

Inflight weather advisories, which are provided to en route aircraft, are forecasts that detail potentially hazardous weather. They also are available to pilots prior to departure for flight planning purposes. An inflight weather advisory is issued in the form of either an AIRMET, SIGMET, or convective SIGMET.

AIRMETs (WAs) are inflight weather advisories issued every six (6) hours, with intermediate updates issued as needed for a particular area forecast region.

The information contained in an AIRMET is of operational interest to all aircraft, but the weather section concerns phenomena considered potentially hazardous to light aircraft and aircraft with limited operational capabilities.

An AIRMET includes forecast of moderate icing, moderate turbulence, sustained surface winds of 30 knots or greater, widespread areas of ceilings less than 1,000 feet and/or visibilities less than three miles, and extensive mountain obscurement.

Each AIRMET bulletin has a fixed alphanumeric designator, numbered sequentially for easy identification, beginning with the first issuance of the day.

• "Sierra" is the AIRMET code used to denote IFR and mountain obscuration.
• "Tango" is used to denote turbulence, strong surface winds, and low-level wind shear.
• "Zulu" is used to denote icing and freezing levels.

• Issued for the Boston area
• AIRMET weather advisory
• on the 21st day of the month at 1945Z
• The AIRMET type is SIERRA (IFR and mountain obscuration)
• the third updated issuance
• type is SIERRA is for IFR and mountain obscuration
• valid until 0200 Zulu on the 22nd day of the month.
• The affected states within the BOS area are: Maine, New Hampshire, Vermont, Massachusetts, Connecticut, Rhode Island, New York, and New Jersey…
• And coastal waters
• An area bounded by
• Caribou, Maine, to Saint Johns, New Brunswick…
• to 150 nautical miles east of Nantucket, Mass.…
• to Newark, N.J.…
• to Ottawa, Ontario…
• to Caribou, Maine.
• The affected states within Caribou, Maine to Millinocket, Maine to Concord, N.H. to Slate Run, Penn. to Syracuse, N.Y. to Caribou, Maine…
• will experience occasional ceilings below 1,000 feet…
• visibility below three statute miles…
• precipitation and mist.
• Conditions will continue beyond 0200 Zulu, and through 0800 Zulu.

• AIRMET issued for mountain obscuration…
• For these states, and an area bounded by these locations…
• Mountains occasionally obscured by clouds, precipitation, mist.
• Conditions will continue beyond 0200 Zulu, and through 0800 Zulu.

A SIGMET (WS) is an inflight advisory concerning non-convective weather that is potentially hazardous to all aircraft. This includes include severe icing, severe or extreme turbulence or clear air turbulence (CAT), dust storms or sandstorms that lower surface or inflight visibilities to below three miles, and volcanic ash. To be reported in a SIGMET, none of these phenomena would be associated with thunderstorms.

A SIGMET is issued under an alphabetic identifier, from November through Yankee. The first issuance of a SIGMET is designated as an Urgent Weather SIGMET (UWS). Reissued SIGMETs for the same weather phenomenon are sequentially numbered until the weather phenomenon ends.

• Issued for the San Francisco area
• SIGMET weather advisory
• on the 10th day of the month at 0130 Zulu.
• SIGMET Romeo 2, the second issuance for this weather phenomenon.
• Valid until the 10th day of the month at 0530 Zulu.
• For Oregon and Washington…
• for a defined area from Seattle to Portland to Eugene to Seattle.
• Occasional severe clear air turbulence between FL280 and FL350 is expected…
• due to the location of the jet stream.
• These conditions will begin after 0200Z&…
• continuing beyond the forecast scope of this SIGMET of 0530Z.

A Convective SIGMET (WST) is an inflight weather advisory issued for hazardous convective weather that affects the safety of every flight. Convective SIGMETs are issued for severe thunderstorms with surface winds greater than 50 knots, hail at the surface greater than or equal to 3/4 inch in diameter, or tornadoes.

Convective SIGMETs also are issued to advise pilots of embedded thunderstorms, lines of thunderstorms, or thunderstorms with heavy or greater precipitation that affect 40 percent or more of a 3,000 square mile or greater region.

Convective SIGMETs are issued for the eastern (E), western (W), and central (C) United States, but not Alaska or Hawaii.

Each report is issued at 55 minutes past the hour, but special Convective SIGMETs can be issued during the interim for any reason. Each forecast is valid for 2 hours.

Convective SIGMETs are numbered sequentially each day from 1-99, beginning at 00Z time. If no hazardous weather exists, the convective SIGMET is issued with the statement "CONVECTIVE SIGMET...NONE".

• Issued for the central portion of the United States
• Convective SIGMET weather advisory
• on the 22nd at 1855Z.
• This is the 20th Convective SIGMET issued on the 22nd for the central United States as indicated by "20C"…
• and is valid until 2055Z.
• The affected states are North and South Dakota
• from 90 nautical miles west of Minot, ND…
• to Grand Forks, ND…
• to Aberdeen, SD…
• to 90 nautical miles west of Minot, ND.
• An intensifying area of severe thunderstorms is moving from 240 degrees at 45 knots (to the northeast).
• Thunderstorm tops will be above FL 450.
• Wind gusts up to 60 knots were reported.
• Also reported were tornadoes, hail to 2 inches in diameter…
• and wind gusts to 65 knots possible in the North Dakota portion.

Winds and Temperature Aloft Forecast (FB)

Winds and Temperatures Aloft forecasts (FB) provide wind and temperature forecasts for specific locations throughout the United States, including network locations in Hawaii and Alaska.

FBs are made twice a day based on the radiosonde upper air observations taken at 0000Z and 1200Z.

Wind direction is always in reference to true north, and wind speed is given in knots. The temperature is given in degrees Celsius.

No winds are forecast when a given level is within 1,500 feet of the station elevation. Similarly, temperatures are not forecast for any station within 2,500 feet of the station elevation.

• If the wind speed is forecast to be greater than 99 knots but less than 199 knots, the computer adds 50 to the direction and subtracts 100 from the speed.
• For example, when the data appears as "731960", subtract 50 from the 73 and add 100 to the 19, and the wind would be 230° at 119 knots with a temperature of –60°C.

• If the wind speed is forecast to be 200 knots or greater, the wind group is coded as 99 knots.

• For example, when the data appears as "7799", subtract 50 from 77 and add 100 to 99, and the wind is 270° at 199 knots or greater.

• When the forecast wind speed is calm, or less than 5 knots, the data group is coded "9900", which means light and variable.

FB KWBC 151640 DATA BASED ON 151200Z VALID 151800Z FOR USE 1400-2100Z TEMPS NEGATIVE ABV 24000
FB	3000	6000	9000	12000	18000	24000	30000
AMA		2714	2725+00	2625-04	2531-15	2542-27	265842
DEN			2321-04	2532-08	2434-19	2441-31	235347

• This FB was transmitted on the 15th of the month at 1640Z and is based on the 1200Z upper air data.
• The valid time is 1800Z on the same day and should be used for the period between 1400Z and 2100Z.
• Temperatures above FL240 are negative. Therefore, the minus sign will be omitted for all forecast temperatures above FL240.
• A four-digit data group shows the wind direction in reference to true north and the wind speed in knots.
• The elevation at Amarillo, Texas (AMA) is 3,605 feet, so the lowest reportable altitude is 6,000 feet for the forecast winds.
• At 6,000 feet, wind is forecast to be from 270° at a speed of 14 knots.
• A six-digit group includes the forecast temperature aloft.
• The elevation at Denver (DEN) is 5,431 feet, so the lowest reportable altitude is 9,000 feet.
• At 9,000 feet, the wind is forecast to be from 230° at a speed of 21 knots with a temperature of –4°C.

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Weather Charts

Weather charts are graphic charts that depict current or forecast weather. They provide an overall picture of the United States, depicting the movement of major weather systems and fronts.

Surface analysis, weather depiction, and significant weather prognostic charts are sources of current weather information.

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Surface Analysis Chart

The surface analysis chart depicts an analysis of the current surface weather. A surface analysis chart shows the areas of high and low pressure, fronts, temperatures, dew points, wind directions and speeds, local weather, and visual obstructions.

The surface analysis chart is transmitted every three (3) hours and covers the contiguous 48 states and adjacent areas.

Surface weather observations for reporting points across the United States are also depicted on this chart. Each of these reporting points is illustrated by a station model, which includes:

• Sky cover
• Sea level pressure
• Pressure change/tendency
• Dew point
• Present weather
• Temperature
• Wind

 

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Weather Depiction Chart

A weather depiction chart details surface conditions as derived from METAR and other surface observations.

• Areas of IFR conditions (ceilings less than 1,000 feet and visibility less than three miles) are shown by a hatched area outlined by a smooth line.
• MVFR regions (ceilings 1,000 to 3,000 feet, visibility 3 to 5 miles) are shown by a nonhatched area outlined by a smooth line.
• Areas of VFR (no ceiling or ceiling greater than 3,000 feet and visibility greater than five miles) are not outlined.
• Also plotted are fronts, troughs, and squall lines from the previous hours surface analysis chart.

A bracket ( ] ) symbol to the right of the station indicates the observation was made by an automated station.

Significant Weather Prognostic Charts

Significant Weather Prognostic Charts are available for low-level significant weather from the surface to FL 240. It is a forecast of aviation weather hazards, primarily intended for briefing VFR pilots.

Each chart is divided on the left and right into 12 and 24 hour forecast intervals (based on the current NAM model available).

The two panels depict freezing levels, turbulence, and low cloud ceilings and/or restrictions to visibility (shown as contoured areas of MVFR and IFR conditions). These charts enable the pilot to pictorially evaluate existing and potential weather hazards they may encounter.

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Commercial Pilot & Flight Instructor Test Questions

Instrument Pilot Test Questions