Aviation Weather Data

Overview

Information on various datasets displayed on AviationWeather.gov. For information on retrieving raw data, please see the Data API.

Other references:

METARs

METARs are a format for encoding reported weather observations. The format is standardized through the International Civil Aviation Organization and regulated by the World Meteorological Organization. In the United States the format is further standardized through the Federal Meteorological Handbook.

From the Federal Meteorological Handbook volume one:

METAR contains a report of wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting collectively referred to as "the body of the report". In addition, coded and/or plain language information which elaborates on data in the body of the report may be appended to the METAR. This significant information can be found in the section referred to as "Remarks". The contents of the remarks will vary according to the type of weather station. At designated stations, the METAR may be abridged to include one or more of the above elements.

SPECI is an unscheduled report taken when any of the criteria given in paragraph 2.5.2.a have been observed [(significant wind shift; visibility, runway visual range, ceiling, sky condition change; tornado; volcano; or mishap)]. SPECI shall contain all data elements found in a METAR plus additional plain language information which elaborates on data in the body of the report. All SPECIs shall be made as soon as possible after the relevant criteria are observed.

Report elements, adapted from the FAA Aviation Weather Handbook:

Station identifier

The station identifier, in ICAO format, is included in all reports to identify the station to which the coded report applies.

The ICAO airport code is a four-letter alphanumeric code designating each airport around the world. The ICAO codes are used for flight planning by pilots and airline operation departments. These codes are not the same as the International Air Transport Association (IATA) codes encountered by the general public used for reservations, baggage handling, and in airline timetables.

Unlike the IATA codes, the ICAO codes have a regional structure. The first letter identifies the region and country (see Figure 24-2). In some regions, the second letter identifies the country. ICAO station identifiers in Alaska begin with “PA,” Hawaii begins with “PH,” Guam begins with “PG,” and Puerto Rico begins with “TS.” For example, the San Juan, Puerto Rico, IATA identifier “SJU” becomes the ICAO identifier “TSJU.” The remaining letters are used to identify each airport.

In the CONUS, ICAO station identifiers are coded K, followed by the three-letter IATA identifier. For example, the Seattle, WA, IATA identifier “SEA” becomes the ICAO identifier “KSEA.” ICAO station identifiers in Alaska, Hawaii, and Guam begin with the continent code P.

Date and Time of Report

The date and time are coded in all reports as follows: the day of the month is the first two digits, followed by the hour, and the minutes.

The coded time of observations is the actual time of the report, or when the criteria for a SPECI is met or noted.

If the report is a correction to a previously disseminated report, the time of the corrected report is the same time used in the report being corrected.

The date and time group always ends with a Z, indicating Zulu time (or Coordinated Universal Time (UTC)).

Report Modifier

The report modifier AUTO identifies the METAR/SPECI as a fully automated report with no human intervention or oversight. In the event of a corrected METAR or SPECI, the report modifier “COR” is substituted for “AUTO.”

Winds

Wind is the horizontal motion of air past a given point. It is measured in terms of velocity, which is a vector that includes direction and speed. It indicates the direction the wind is coming from. In the wind group, the wind direction is coded as the first three digits (220) and is determined by averaging the recorded wind direction over a 2-minute period. It is coded in tens of degrees relative to true north using three figures. Directions less than 100° are preceded with a 0. For example, a wind direction of 90° is coded as 090. A wind from the north is coded as 360.

Immediately following the wind direction is the wind speed coded in two or three digits (15). Wind speed is determined by averaging the speed over a 2-minute period and is coded in whole knots using the units, tens digits, and, when appropriate, the hundreds digit. When wind speeds are less than 10 kt, a leading 0 is used to maintain at least a two-digit wind code. For example, a wind speed of 8 kt is coded 08KT. The wind group is always coded with a KT to indicate wind speeds are reported in knots. Other countries may use km/h or meters per second (m/s) instead of knots.

Wind gust

Wind speed data for the most recent 10 minutes is examined to evaluate the occurrence of gusts. Gusts are defined as rapid fluctuations in wind speed with a variation of 10 kt or more between peaks and lulls. The coded speed of the gust is the maximum instantaneous wind speed.

Wind gusts are coded in two or three digits immediately following the wind speed. Wind gusts are coded in whole knots using the units, tens, and, when appropriate, the hundreds digit. For example, a wind out of the west at 20 kt with gusts to 35 kt would be coded 27020G35KT.

Wind direction

Wind direction may be considered variable when, during the previous 2-minute evaluation period, the wind speed was 6 kt or less. In this case, the wind may be coded as VRB in place of the three-digit wind direction. For example, if the wind speed was recorded as 3 kt, it would be coded VRB03KT.

Wind direction may also be considered variable when, during the 2-minute evaluation period, it varies by 60° or more and the speed is greater than 6 kt. In this case, a variable wind direction group immediately follows the wind group. The directional variability is coded in a clockwise direction and consists of the extremes of the wind directions separated by a V. For example, if the wind is variable from 180 to 240° at 10 kt, it would be coded 21010KT 180V240.

When no motion of air is detected, the wind is reported as calm. A calm wind is coded as 00000KT.

Visibility

Visibility is a measure of the opacity of the atmosphere. It is defined as the greatest horizontal distance at which selected objects can be seen and identified, or its equivalent derived from instrumental measurements.

Prevailing visibility is the reported visibility considered representative of recorded visibility conditions at the manual station during the time of observation. It is the greatest distance that can be seen throughout at least half of the horizon circle, not necessarily continuous.

Surface visibility is the prevailing visibility from the surface at manual stations or the visibility derived from sensors at automated stations.

The visibility group is coded as the surface visibility in statute miles. A space is coded between whole numbers and fractions of reportable visibility values. The visibility group ends with SM to indicate that the visibility is in statute miles. For example, a visibility of 1½ sm is coded 1 1/2SM. Most other countries use meters.

U.S. automated stations use an M to indicate “less than.” For example, M1/4SM means a visibility of less than ¼ sm.

Runway Visual Range

The RVR is an instrument-derived value representing the horizontal distance a pilot may see down the runway.

RVR is reported whenever the station has RVR equipment and prevailing visibility is 1 sm or less, and/or the RVR for the designated instrument runway is 6,000 ft or less. Otherwise, the RVR group is omitted. RVR is coded in the following format: The initial R is code for runway and is followed by the runway number. When more than one runway is defined with the same runway number, a directional letter is coded on the end of the runway number. Next is a solidus (/) followed by the visual range in feet, and then FT completes the RVR report. For example, an RVR value for Runway 01L of 800 ft would be coded R01L/0800FT. Most other countries use meters.

In the United States, RVR values are coded in increments of 100 ft up to 1,000 ft, increments of 200 ft from 1,000 to 3,000 ft, and increments of 500 ft from 3,000 to 6,000 ft. Manual RVR is not reported below 600 ft.

For U.S. airports only, the touchdown zone's (TDZ) RVR is reported. For U.S. airports with multiple runways, the operating runway with the lowest touchdown RVR is reported. RVR may be reported for up to four designated runways in other countries.

When the RVR varies by more than one reportable value, the lowest and highest values are shown with V between them, indicating variable conditions. For example, the 10-minute RVR for Runway 01L varying between 600 and 1,000 ft would be coded R01L/0600V1000FT.

If RVR is less than its lowest reportable value, the visual range group is preceded by M. For example, an RVR for Runway 01L of less than 600 ft is coded R01L/M0600FT.

If RVR is greater than its highest reportable value, the visual range group is preceded by a P. For example, an RVR for Runway 27 of greater than 6,000 ft is coded R27/P6000FT.

Weather

Separate groups are used for each type of present weather. Each group is separated from the other by a space. METARs/SPECIs contain no more than three present weather groups.

QualifierWeather Phenomena
Intensity or ProximityDescriptorPrecipitationObscurationOther
- Light
+ Heavy
VC Vicinity		MI Shallow
PR Partial
BC Patches
DR Low
Drifting
BL Blowing
SH Shower(s)
TS Thunderstorms
FZ Freezing		DZ Drizzle
RA Rain
SN Snow
SG Snow
Grains
IC Ice Crystals (Diamond Dust)
PL Ice
Pellets
GR Hail
GS Snow Pellets
UP Unknown Precipitation		BR Mist
FG Fog
FU Smoke
VA Volcanic
Ash
DU Widespread
Dust
SA Sand
HZ Haze
PY Spray		PO Dust/Sand
Whirls
SQ Squalls
FC Funnel Cloud, Tornado, or Waterspout
SS Sandstorm
DS Dust Storm

Sky condition

Sky condition is a description of the appearance of the sky. It includes cloud cover, vertical visibility, or clear skies.

The sky condition group is based on the amount of cloud cover (the first three letters) followed by the height of the base of the cloud cover (final three digits). No space is between the amount of cloud cover and the height of the layer. The height of the layer is recorded in feet AGL.

Sky condition is coded in ascending order and ends at the first overcast layer. At mountain stations, if the layer is below station level, the height of the layer is coded with three slashes (///).

Vertical visibility is coded as VV, followed by the vertical visibility into the indefinite ceiling. An “indefinite ceiling” is a ceiling classification applied when the reported ceiling value represents the vertical visibility upward into surface-based obscuration. No space is between the group identifier and the vertical visibility.

Clear skies are coded in the format SKC or CLR. When SKC is used, an observer indicates no layers are present; CLR is used by automated stations to indicate no layers are detected at or below 12,000 ft. Each coded layer is separated from the others by a space. A report of clear skies (SKC or CLR) is a complete layer report within itself. The abbreviations FEW, SCT, BKN, and OVC are followed by the height of the layer.

Temperature & dewpoint

Temperature is the degree of hotness or coldness of the ambient air, as measured by a suitable instrument. Dewpoint is the temperature to which a given parcel of air must be cooled at constant pressure and constant water vapor content for the air to become fully saturated.

Temperature and dewpoint are coded as two digits rounded to the nearest whole degree Celsius. For example, a temperature of 0.3 ºC would be coded at 00. Sub-zero temperatures and dewpoints are prefixed with an M. For example, a temperature of 4 ºC with a dewpoint of -2 ºC would be coded as 04/M02; a temperature of -2 ºC would be coded as M02.

If temperature is not available, the entire temperature/dewpoint group is not coded. If dewpoint is not available, temperature is coded followed by a solidus (/) and no entry is made for dewpoint. For example, a temperature of 1.5 ºC and a missing dewpoint would be coded as 02/.

Altimeter

The altimeter setting group codes the current pressure at elevation. This setting is then used by aircraft altimeters to determine the true altitude above a fixed plane of MSL.

The altimeter group always starts with an A and is followed by the four-digit group representing the pressure in tens, units, tenths, and hundredths of inches of mercury. The decimal point is not coded. For example, an altimeter setting of 29.92 inHg would be coded as A2992.

Remarks

Remarks are included in METAR and SPECI, when appropriate. Remarks are separated from the body of the report by the contraction RMK. When no remarks are necessary, the contraction RMK is not used.

Automated Station

AO1 or AO2 is coded in METAR/SPECI from automated stations. Automated stations without a precipitation discriminator are identified as AO1; automated station with a precipitation discriminator are identified as AO2.

Wind

The peak wind is coded in the format, PK WND dddff(f)/(hh)mm of the next METAR, where PK_WND is the remark identifier, ddd is the direction of the peak wind, ff(f) is the peak wind speed since the last METAR, and (hh)mm is the time of occurrence.

A wind shift is coded in the format, WSHFT (hh)mm, where WSHFT is the remark identifier and (hh)mm is the time the wind shift began. The contraction FROPA may be entered following the time if it is reasonably certain that the wind shift was the result of a frontal passage.

Visibility

Tower visibility or surface visibility are coded in the formats, TWR VIS vvvvv or SFC VIS vvvvv, respectively, where vvvvv is the observed tower/surface visibility value.

Variable prevailing visibility sis coded in the format VIS vnvnvnvnvnVvxvxvxvxvx, where VIS is the remark identifier, vnvnvnvnvnvnvnvnvnvn is the lowest visibility evaluated, V denotes variability between two values, and vxvxvxvxvx is the highest visibility evaluated.

The sector visibility is coded in the format, VIS [DIR] vvvvv, where VIS is the remark identifier, [DIR] defines the sector to 8 points of the compass, and vvvvv is the sector visibility in statute miles.

At designated automated stations, the visibility at a second location is coded in the format VIS vvvvv [Location], where VIS is the remark identifier, vvvvv is the measured visibility value, and [Location] is the specific location of the visibility sensor(s) at the station. This remark shall only be generated when the condition is lower than that contained in the body of the report.

Lightning

When lightning is observed at a manual station, the frequency, type of lightning, and location are reported. The remark is coded in the format [Frequency] LTG [Location].

When lightning is detected by an automated system, it is reported as TS in the body of the report with no remark; between 5 and 10 nautical miles, it is reported as VCTS in the body of the report with no remark; beyond 10 but less than 30 nautical miles, it is reported in remarks only as LTG DSNT followed by the direction.

Precipitation Timing

At designated stations, the beginning and ending of precipitation is coded in the format, w'w'B(hh)mmE(hh)mm, where w'w' is the type of precipitation, B denotes the beginning, E denotes the ending, and (hh)mm is the time of occurrence, For example, if rain began at 0005, ended at 0030, and snow began at 0020, and ended at 0055, the remarks would be coded "RAB05E30SNB20E55".

The beginning and ending of thunderstorm(s) is coded in the format, TSB(hh)mmE(hh)mm, where TS indicates thunderstorm, B denotes the beginning, E denotes the ending, and (hh)mm is the time of occurrence. For example, if a thunderstorm began at 0159 and ended at 0230, the remark would be coded "TSB0159E30".

Thunderstorms

Thunderstorms are coded in the format, TS LOC (MOV_DIR), where TS identifies the thunderstorm activity, LOC is the location of the thunderstorm(s) from the station, and MOV_DIR is the movement with direction. For example, a thunderstorm southeast of the station and moving toward the northeast would be coded "TS SE MOV NE".

Hailstone size is coded in the format, GR [size], where GR is the remark identifier and [size] is the diameter of the largest hailstone. The hailstone size is coded in 1/4 inch increments. For example, "GR 1 3/4" would indicate that the largest hailstones were 1 3/4 inches in diameter.

Snow pellet intensity is coded in the format, GS [intensity], where GS is the remark identifier and [intensity] is the observed intensity. The snow pellet intensity is coded as LGT, MOD, or HVY indicating light, moderate, or heavy intensity

Clouds

Virga is coded in the format, VIRGA_(DIR), where VIRGA is the remark identifier and DIR is the direction from the station.

Variable ceiling height is coded in the format, CIG hnhnhnVhxhxhx where CIG is the remark identifier, hnhnhn is the lowest ceiling height evaluated, V denotes variability between two values, and hxhxhx is the highest ceiling height evaluated.

Obscurations are coded in the format, w'w' [NsNsNs]hshshs, where w'w' is the weather causing the obscuration at the surface or aloft, NsNsNs is the applicable sky cover amount of the obscuration aloft (FEW, SCT, BKN, OVC) or at the surface (FEW, SCT, BKN), and hshshs is the applicable height. Surface-based obscurations have a height of “000".

Variable sky condition remark is coded in the format, NsNsNs (hshshs) V NsNsNs, where NsNsNs (hshshs) and NsNsNs identifies the two operationally significant sky conditions and V denotes the variability between the two ranges.

Cumulonimbus are coded in the format, CB or CBMAM LOC (MOV_DIR), where CB or CBMAM is the cloud type, LOC is the direction from the station, and MOV_DIR is the movement with direction.

Towering cumulus clouds is coded in the format, TCU [DIR], where TCU is the cloud type and DIR is the direction from the station.

Altocumulus castellanus are coded in the format, ACC [DIR], where ACC is the cloud type and DIR is the direction from the station. Stratocumulus (SCSL), altocumulus (ACSL), or cirrocumulus (CCSL), or rotor clouds are in the format, CLD [DIR], where CLD is the cloud type and DIR is the direction from the station.

Ceiling height at a second location is coded in the format, CIG hhh [LOC], where CIG is the remark identifier, hhh is the measured height of the ceiling, and [LOC] is the specific location of the ceilometer(s) at the station

Pressure

When the pressure is rising or falling rapidly at the time of observation, the remark PRESRR (pressure rising rapidly) or PRESFR (pressure falling rapidly) is included in the report.

Sea-level pressure is coded in the format SLPppp, where SLP is the remark identifier and ppp is the sea-level pressure in hectopascals. If sea-level pressure is not available, it is coded as SLPNO.

Precipitation

Hourly precipitation amount is coded in the format, Prrrr, where P is the group indicator and rrrr is the water equivalent of all precipitation that has occurred since the last METAR. The amount is coded in hundredths of an inch. The group is omitted if no precipitation occurred since the last METAR.

6RRRR, where 6 is the group indicator and RRRR is the amount of precipitation. The amount of precipitation (water equivalent) accumulated in the past 3 hours is reported in the 3-hourly report; the amount accumulated in the past 6 hours is reported in the 6-hourly report. The amount of precipitation is coded in inches, using the tens, units, tenths and hundredths digits of the amount. When an indeterminable amount of precipitation has occurred during the period, RRRR is coded 6////.

7R24R24R24R24, where 7 is the group indicator and R24R24R24R24 is the 24- hour precipitation amount. The 24-hour precipitation amount may be included in the 1200 UTC (or other agency designated time) report whenever more than a trace of precipitation (water equivalent) has fallen in the preceding 24 hours. The amount of precipitation is coded by using the tens, units, tenths, and hundredths of inches (water equivalent) for the 24-hour period. If more than a trace (water equivalent) has occurred and the amount cannot be determined, it is coded as 7////.

Snow

Snow increasing rapidly remark may be reported when the snow depth increases by 1 inch or more in the past hour. The remark is coded in the format, SNINCR [inches-hour/inches on ground], where SNINCR is the remark indicator, inches-hour is the depth increase in the past hour, and inches on ground is the total depth of snow on the ground at the time of the report. The depth increase in the past hour and the total depth on the ground are separated from each other by a slash "/".

Total snow depth on the ground group may be coded in the 0000, 0600, 1200, and 1800 UTC observations whenever there is more than a trace of snow on the ground. The remark is coded in the format, 4/sss, where 4/ is the group indicator and sss is the snow depth in whole inches using three digits.

Water equivalent of snow on the ground may be coded each day, in the 1800 UTC report, if the average snow depth is 2 inches or more. The remark is coded in the format, 933RRR, where 933 is the group indicator and RRR is the water equivalent of snow, i.e., snow, snow pellets, snow grains, ice pellets, ice crystals, and hail, on the ground. The water equivalent is coded in tens, units, and tenths of inches, using three digits. If the water equivalent of snow consists entirely of hail, the group is not coded.

Temperature

Hourly temperature and dew point group are coded to the tenth of a degree Celsius in the format, TsnT'T'T'snT'dT'dT'd, where T is the group indicator, sn is the sign of the temperature, T'T'T' is the temperature, and T'dT'dT'd is the dew point. The sign of the temperature and dew point is coded as 1 if the value is below 0°C and 0 if the value is 0°C or higher. The temperature and dew point is reported in tens, units, and tenths of degrees Celsius.

6-hourly maximum temperature group is coded in the format, 1snTxTxTx, where 1 is the group indicator, sn is the sign of the temperature, TxTxTx is the maximum temperature in tenths of degrees Celsius using three digits. The sign of the maximum temperature is coded as 1 if the maximum temperature is below 0°C and 0 if the maximum temperature is 0°C or higher

6-hourly minimum temperature group is coded in the format, 2snTnTnTn, where 2 is the group indicator, sn is the sign of the temperature, and TnTnTn is the minimum temperature in tenths of degrees Celsius using three digits.

24-hour maximum temperature and the 24-hour minimum temperature is coded in the format, 4snTxTxTxsnTnTnTn, where 4 is the group indicator, sn is the sign of the temperature, TxTxTx is the maximum 24-hour temperature, and TnTnTn is the 24-hour minimum temperature.

Ice

Hourly ice accretion amount is coded in the format I1nnn, where I is the ice accretion group indicator, 1 indicates an hourly amount, and nnn is the accretion amount since the last METAR. The amount is coded in hundredths of an inch.

3- and 6-hourly ice accretion group is coded in the format I3nnn and I6nnn, respectively, where I is the group indicator, 3 and 6 indicate 3- and 6- hourly groups, respectively, and nnn is the accretion amount. The amount of ice accretion in the past 3 hours is reported in the 3-hourly report; the amount of ice accretion in the past 6 hours is reported in the 6-hourly report. The amount is coded in hundredths of an inch. For example, I6023z would indicate 23/100 of an inch of ice accretion in the past 6 hours.

Sensor Status

The following flags are used to indicate information about the status of reporting equipment:

  • RVRNO: Runway Visual Range should be reported but is missing
  • PWINO: equipped with a present weather identifier and that sensor is not operating
  • PNO: equipped with a tipping bucket rain gauge and that sensor is not operating,
  • FZRANO: equipped with a freezing rain sensor and that sensor is not operating
  • TSNO: equipped with a lightning detection system and that sensor is not operating

Maintenance Indicator

A maintenance indicator ($) is coded when an automated system detects that maintenance is needed on the system.

TAFs

TAF is a concise statement of the expected meteorological conditions significant to aviation for a specified time period within 5 sm of the center of the airport's runway complex (terminal). TAFs use the same weather codes found in METARs.

In the United States, TAFs are issued by NWS Weather Forecast Offices for nearly 700 U.S. airports. The majority of TAFs provide a 24-hour forecast for the airport, while TAFs for some major airports provide a 30-hour forecast.

Report elements, adapted from the FAA Aviation Weather Handbook:

Issuance

Scheduled TAFs prepared by NWS offices are issued at least four times a day, every 6 hours. Some locations have amendments routinely issued 3 hours after the initial issuance.

Date/Time InitYYGGggZ

The date/time follows the terminal's location identifier. It contains the day of the month in two digits (YY) and the time in four digits (GGgg in hours and minutes) in which the forecast is completed and ready for transmission, with a Z appended to denote UTC. This time is entered by the forecaster. A routine forecast, TAF, is issued 20 to 40 minutes before the beginning of its valid period.

Valid Period

Y1Y1G1G1/Y2Y2G2G2

The TAF valid period follows the date/time of the forecast origin group. Scheduled 24- and 30-hour TAFs are issued four times per day, at 0000, 0600, 1200, and 1800Z. The first two digits (Y1Y1) are the day of the month for the start of the TAF. The next two digits (G1G1) are the starting hour (UTC). Y2Y2 is the day of the month for the end of the TAF, and the last two digits (G2G2) are the ending hour (UTC) of the valid period. A forecast period that begins at midnight UTC is annotated as 00. If the end time of a valid period is at midnight UTC, it is annotated as 24. For example, a 00Z TAF issued on the 9th of the month and valid for 24 hours would have a valid period of 0900/0924.

Whenever an amended TAF (TAF AMD) is issued, it supersedes and cancels the previous TAF. That is, users should not wait until the start of the valid period indicated within the TAF AMD to begin using it.

Wind

dddffGfmfmKT

The initial time period and any subsequent “from” (FM) groups begin with a mean surface wind forecast for that period. Wind forecasts are expressed as the mean three-digit direction (ddd, relative to true north) from which the wind is blowing, rounded to the nearest 10°, and the mean wind speed in knots (ff) for the time period. If wind gusts are forecast (gusts are defined as rapid fluctuations in wind speeds with a variation of 10 kt or more between peaks and lulls), they are indicated immediately after the mean wind speed by the letter G, followed by the peak gust speed expected. KT is appended to the end of the wind forecast group. Any wind speed of 100 kt or more will be encoded in three digits. Calm winds are encoded as 00000KT.

The prevailing wind direction is forecast for any speed greater than or equal to 7 kt. When the prevailing surface wind direction is variable (variations in wind direction of 30° or more), the forecast wind direction is encoded as VRBffKT. Two conditions where this can occur are very light winds and convective activity. Variable wind direction for very light winds must have a wind speed of 1 to 6 kt inclusive. For convective activity, the wind group may be encoded as VRBffGfmfmKT, where Gfmfm is the maximum expected wind gusts. VRB is not used in the non-convective LLWS group.

Squalls are forecast in the wind group as gusts (G) but must be identified in the significant weather group with the code SQ.

Visibility

VVVV

The initial time period and any subsequent FM groups include a visibility forecast in statute miles appended by the contraction SM. When the prevailing visibility is forecast to be less than or equal to 6 sm, one or more significant weather groups are included in the TAF. However, drifting dust (DRDU), drifting sand (DRSA), drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG) may be forecast with prevailing visibility greater than or equal to 7 sm.

When a whole number and a fraction are used to forecast visibility, a space is included between them (e.g., 1 1/2SM). Visibility greater than 6 sm is encoded as P6SM.

If the visibility is not expected to be the same in different directions, prevailing visibility is used. When volcanic ash (VA) is forecast in the significant weather group, visibility is included in the forecast, even if it is unrestricted (P6SM). For example, an expected reduction of visibility to 10 sm by volcanic ash is encoded in the forecast as P6SM VA.

Although not used by the NWS in U.S. domestic TAFs, the contraction CAVOK (ceiling and visibility OK) may replace the visibility, weather, and sky condition groups if all of the following conditions are forecast: visibility of 10 km (6 sm) or more, no clouds below 1500 m (5,000 ft) or below the highest minimum sector altitude (whichever is greater), no cumulonimbus, and no significant weather phenomena.

Significant Weather

w'w' or NSW

The significant weather group consists of the appropriate qualifier(s) and weather phenomenon contraction(s) or NSW (no significant weather).

If the initial forecast period and subsequent FM groups are not forecast to have explicit significant weather, the significant weather group is omitted. NSW is not used in the initial forecast time period or FM groups. One or more significant weather group(s) is (are) included when the visibility is forecast to be 6 sm or less.

The exceptions are: volcanic ash (VA), low drifting dust (DRDU), low drifting sand (DRSA), low drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG). Obstructions to vision are only forecast when the prevailing visibility is less than 7 sm or, in the opinion of the forecaster, is considered operationally significant.

Volcanic ash (VA) is always forecast when expected. When VA is included in the significant weather group, visibility is included in the forecast as well, even if the visibility is unrestricted (P6SM). NSW is used in place of significant weather only in a temporary (TEMPO) group to indicate when significant weather (including in the vicinity (VC)) included in a previous subdivided group is expected to end.

Multiple precipitation elements are encoded in a single group (e.g., -TSRASN). If more than one type of precipitation is forecast, up to three appropriate precipitation contractions can be combined in a single group (with no spaces) with the predominant type of precipitation being first. In this single group, the intensity refers to the total precipitation and can be used with either one or no intensity qualifier, as appropriate. In TAFs, the intensity qualifiers (light, moderate, and heavy) refer to the intensity of the precipitation and not to the intensity of any thunderstorms associated with the precipitation.

Intensity is coded with precipitation types (except ice crystals and hail), including those associated with thunderstorms and those of a showery nature (SH). No intensity is ascribed to blowing dust (BLDU), blowing sand (BLSA), or blowing snow (BLSN). Only moderate or heavy intensity is ascribed to a sandstorm (SS) and dust storm (DS).

Clouds

NsNsNshshshs or SKC

The initial time period and any subsequent FM groups include a cloud or obscuration group, used as appropriate to indicate the cumulative amount (NsNsNs) of all cloud layers in ascending order and height (hshshs), to indicate vertical visibility (VVhshshs) into a surface-based obstructing medium, or to indicate a clear sky (SKC). All cloud layers and obscurations are considered opaque.

Vertical Obscuration

VVhshshs

The vertical obscuration group is used to forecast, in hundreds of feet AGL, the vertical visibility (VV) into a surface-based total obscuration. VVhshshs is this ceiling at the height indicated in the forecast. TAFs do not include forecasts of partial obscurations (i.e., FEW000, SCT000, or BKN000).

Cloud Type

CB

The only cloud type included in the TAF is CB. CB follows cloud or obscuration height (hshshs) without a space whenever thunderstorms are included in the significant weather group (w'w'), even if thunderstorms are only forecast in the vicinity (VCTS). CB can be included in the cloud group (NsNsNshshshs) or the vertical obscuration group (VVhshshs) without mentioning a thunderstorm in the significant weather group (w'w'). Therefore, situations may occur where nearly identical NsNsNshshshs or VVhshshs appear in consecutive time periods, with the only change being the addition or elimination of CB in the forecast cloud type.

Low-Level Wind Shear (LLWS)

WShwshwshws/dddffKT

Wind shear (WS) is defined as a rapid change in horizontal wind speed and/or direction, with distance and/or a change in vertical wind speed and/or direction with height. A sufficient difference in wind speed, wind direction, or both can severely impact airplanes, especially within 2,000 ft AGL because of limited vertical airspace for recovery.

Forecasts of LLWS in the TAF refer only to non-convective LLWS from the surface up to and including 2,000 ft AGL. LLWS is always assumed to be present in convective activity. LLWS is included in TAFs on an “as-needed” basis to focus the aircrew's attention on LLWS problems that currently exist or are expected. Non-convective LLWS may be associated with the following: frontal passage, inversion, low-level jet, lee-side mountain effect, sea breeze front, Santa Ana winds, etc.

When LLWS conditions are expected, the non-convective LLWS code WS is included in the TAF as the last group (after cloud forecast). Once in the TAF, the WS group remains the prevailing condition until the next FM change group or the end of the TAF valid period if there are no subsequent FM groups. Forecasts of non-convective LLWS are not included in TEMPO or PROB groups.

Forecast Change Indicator

Forecast change indicator groups are contractions that are used to subdivide the forecast period (24 hours for scheduled TAFs; less for amended or delayed forecasts) according to significant changes in the weather. The forecast change indicators FM, TEMPO, and PROB are used when a change in any or all of the forecast elements is expected.

From (FM)
FMYYGGgg

The change group is used to indicate when prevailing conditions are expected to change significantly over a period of less than 1 hour. In these instances, the forecast is subdivided into time periods using the contraction FM, followed, without a space, by six digits, the first two of which indicate the day of the month and the final four indicate the time (in hours and minutes Z) the change is expected to occur. While the use of a four-digit time in whole hours (e.g., 2100Z) is acceptable, if a forecaster can predict changes and/or events with higher resolution, then more precise timing of the change to the minute will be indicated. All forecast elements following FMYYGGgg relate to the period of time from the indicated date and time (YYGGgg) to the end of the valid period of the terminal forecast, or to the next FM if the terminal forecast valid period is divided into additional periods.

The FM group will be followed by a complete description of the weather (i.e., self-contained), and all forecast conditions given before the FM group are superseded by those following the group. All elements of the TAF (e.g., surface wind, visibility, significant weather, clouds, obscurations, and when expected, non-convective LLWS) will be included in each FM group, regardless if they are forecast to change or not.

Temporary (TEMPO)
TEMPO YYGG/YeYeGeGe

The change-indicator group is used to indicate temporary fluctuations to forecast meteorological conditions that are expected to:

  • Have a high percentage (greater than 50 percent) probability of occurrence
  • Last for one hour or less in each instance; and
  • In the aggregate, cover less than half of the period YYGG to YeYeGeGe.

The first two digits (YY) are the day of the month for the start of the TEMPO. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the TEMPO period.

Each TEMPO group is placed on a new line in the TAF. The TEMPO identifier is followed by a description of all the elements in which a temporary change is forecast. A previously forecast element that has not changed during the TEMPO period is understood to remain the same and will not be included in the TEMPO group. Only those weather elements forecast to temporarily change are included in the TEMPO group.

TEMPO groups will not include forecasts of either significant weather in the vicinity (VC) or non-convective LLWS.

Probability (PROB)
PROB30 YYGG/YeYeGeGe

The probability group is only used by NWS forecasters to forecast a low-probability occurrence (30 percent chance) of a thunderstorm or precipitation event and its associated weather and obscuration elements (e.g., wind, visibility, and/or sky condition) at an airport.

The PROB30 group is the forecaster's assessment of probability of occurrence of the weather event that follows it. The first two digits (YY) are the day of the month for the start of the PROB30. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the PROB30 period. PROB30 is the only PROB group used in NWS' TAFs.

PIREPs

An Aircraft Report is a report of actual weather conditions encountered by an aircraft while in flight. There are two types of reports. An AIREP is a routine, often automated report of in-flight weather conditions such as wind and temperature. A PIREP is reported by a pilot to indicate encounters of hazardous weather such as icing or turbulence. Both are transmitted in real-time via radio to a ground station.

PIREPs and AIREPs are encoded differently. AIREP format is more common outside the contiguous U.S. even though there are some AIREPs over the CONUS. The location is specified by latitude and longitude which is better for international routes. PIREPs are preferred over the CONUS where the location is based on distance and direction to a known navaid such as a VOR.

Data is searchable using an airport identifier for a center point, radial distance, and a time interval.

PIREP elements, adapted from FAA 7110:

Location

/OV

Location is ented as one of:

  • Reference to a VHF NAVAID or an airport. Three or four alphanumeric identifier. If appropriate, the identifier is followed by three digits to define a radial and three digits to define the distance in nautical miles.
  • Route segment. Two or more fixes to describe a route.
  • Latitude and longitude. Latitude and longitude pair (in degrees and minutes) for cases where it is impracticable for the pilot to report using distance and direction from a NAVAID or airport.

Time

/TM

Time that the reported phenomenon occurred or was encountered. Report time in four digits UTC.

Altitude/flight level

/FL

Altitude in hundreds of feet (MSL) where the phenomenon was first encountered, UNKN if unknown. If the aircraft was climbing or descending, the appropriate contraction (DURC or DURD) should be in the remarks. If the condition was encountered within a layer, enter the altitude range within the appropriate TEI describing the condition.

Type of aircraft

/TP

ICAO aircraft type code, UNKN if unknown. Icing and turbulence reports must always include the aircraft type.

Sky condition

/SK

Height of cloud bases, tops, and cloud coverage. Height of the base of a layer of clouds in hundreds of feet (MSL) using three digits.Top of a layer in hundreds of feet (MSL) preceded by the word “TOP.”

Cloud cover amount ranges are entered with a hyphen and no spaces separating the amounts (for example, BKN-OVC). In cloud is coded as IMC. Multiple layers are separated by a slash (/).

Weather

/WX

Flight visibility, if reported, will be the first entry in the /WX field. FV followed by a two-digit visibility value rounded down, if necessary, to the nearest whole statute mile and append “SM” (FV03SM). If visibility is reported as unrestricted, enter FV99SM.

Flight weather types are reported using one or more of the standard surface weather reporting codes. Intensity of precipitation (- for light, no qualifier for moderate, and + for heavy) is indicated with precipitation types, except ice crystals and hail. Intensity of obscurations must be ascribed as moderate or + heavy for dust and sand storms only. No intensity for blowing dust, blowing sand, or blowing snow. When more than one form of precipitation is combined in the report, the dominant type is reported first.

Air Temperature

/TA

Air temperature using two digits in degrees Celsius. Prefix negative temperatures with an M (for example, /TA 08 or /TA M08).

Wind

/WV

Wind direction from which the wind is blowing using three figures. Directions less than 100 degrees must be preceded by a “0”. For example, a wind direction of 90 degrees is coded as 090. The wind speed as a two or three digit group immediately following the wind direction. The speed must be coded in whole knots using the hundreds digit (if not zero) and the tens and units digits. The wind group always ends with “KT” to indicate that winds are reported in knots.

Turbulence

/TB

Duration (INTMT, OCNL, CONS) and intensity using contractions LGT, MOD, SEV, or EXTRM. A range or variation of intensity with a hyphen (for example, MOD-SEV). If turbulence was not encountered, enter NEG. Optionally type CAT or CHOP. Altitude reported only if it differs from value reported in /FL. For ranges height values are separated with a hyphen. If lower or upper limits are not defined, BLO or ABV are used.

Icing

/IC

Intensity first using contractions TRACE, LGT, MOD, or SEV (or NEG for no icing). Optionally type RIME, CLR, or MX. Altitude reported only if different from the value reported in the /FL. For ranges height values are separated with a hyphen. If lower or upper limits are not defined, BLO or ABV are used.

Remarks

/RM

Used to report a phenomena which is considered important but do not fit in any of the other groups. This includes, but is not limited to, wind shear (including low-level wind shear) reports, thunderstorm lines, coverage and movement, size of hail (¼" increments), lightning, clouds observed but not encountered, geographical or local description of where the phenomenon occurred, International Standard Atmospheric (ISA) reports and contrails. Report hazardous weather first.

SIGMETs

Domestic SIGMETs

A U.S. SIGMET advises of weather, other than convective activity, that is potentially hazardous to all aircraft. SIGMETs are issued (for the lower 48 states and adjacent coastal waters) for the following weather-impacted reasons:

  • Severe Icing
  • Severe or Extreme Turbulence
  • Dust storms and/or sand storms lowering visibilities to less than three (3) miles
  • Volcanic Ash

If the total area affected during the forecast period is very large, only a small portion of this total area may be affected at any one time. SIGMETs are issued for 6 hour periods for conditions associated with hurricanes and 4 hours for all other events. If conditions persist beyond the forecast period, the SIGMET is updated and reissued. Convective SIGMETs are issued hourly for thunderstorm-related aviation hazards.

Convective SIGMETs

Convective SIGMETs are issued in the conterminous U.S. if these conditions are occurring or expected to occur:

  • Line of thunderstorms at least 60 miles long with thunderstorms affecting 40% of its length.
  • Area of thunderstorms covering at least 40% of the area concerned and exhibiting a very strong radar reflectivity or a significant satellite or lightning signature.
  • Embedded or severe thunderstorms expected to occur for more than 30 minutes.

Special issuance criteria include:

  • tornado
  • hail greater than or equal to 3/4 inches in diameter
  • wind gusts greater than or equal to 50 knots

Any convective SIGMET implies severe or greater turbulence, severe icing, and low level wind shear. A convective SIGMET may be issued for any convective situation which the forecaster feels is hazardous to all categories of aircraft. Bulletins are issued hourly at Hour+55. The text of the bulletin consists of either an observation and a forecast or just a forecast. The forecast is valid for up to 2 hours.

International SIGMETs

US SIGMETs cover the contiguous United States and follow US coding standards. Any SIGMET issued outside of the CONUS follows the international coding standard. The aviation hazards are similar to the US SIGMETs except convective SIGMETs are treated the same as other hazards. Here is the list of hazards:

  • Thunderstorms
  • Tropical cyclones
  • Turbulence
  • Icing
  • Volcanic ash
  • Dust and sand storms
  • Radiological cloud

International SIGMETs are defined within a specific FIR. Most FIRs are aligned with a country's airspace or a subset of that airspace. Ocean regions also have FIRs where SIGMETs are issued by adjacent countries. The US issues international SIGMETs for Alaska and for oceanic areas off the east coast of the US, Gulf of Mexico and a large part of the central northern Pacific.

G-AIRMETs

A G-AIRMET is a graphical advisory of weather that may be hazardous to aircraft, but are less severe than SIGMETs. They are only valid at specific time "snapshots". Forecasters create graphical objects depicting the areas and attributes of AIRMET hazards, which are distributed in BUFR file format. G-AIRMETs are issued at discrete times 3 hours apart for a period of up to 12 hours into the future (00, 03, 06, 09, and 12 hours). They are issued at 03:00, 09:00, 15:00 and 21:00 UTC (with updates issued as necessary). AIRMET are issued by the AWC for the lower 48 states and adjacent coastal waters.

The aviation hazards depicted in the G-AIRMET are:

Turbulence

Areas of moderate turbulence, other than convectively induced, including the vertical extent.

Low Level Wind Shear

LLWS is defined as wind shear below 2000 feet AGL, other than convectively induced, resulting in an air speed loss or gain of 20 knots or more. LLWS potential information is included after AIRMETs for moderate turbulence and/or sustained surface winds greater than 30 knots or statements indicating no significant turbulence is expected.

Strong Surface Winds

Areas of sustained surface winds greater than 30 knots. The direction and speed of the wind are not depicted; only the area where sustained surface winds greater than 30 knots will occur.

Freezing Level

Freezing level is defined as the lowest freezing level above the ground or at the SFC as appropriate. Freezing level information is included after AIRMETs for moderate icing or statements indicating that no significant icing is expected. Freezing levels are delineated using high altitude VOR locations describing the location of the lowest freezing level above the ground or SFC as appropriate. Freezing levels above the ground are delineated at 4000 feet intervals AMSL. Multiple freezing levels above the ground are delineated by high altitude and low altitude VOR locations. The range of freezing levels across the forecast area is also included.

IFR

Areas of cloud ceilings with bases less than 1000 feet AGL and/or areas of surface visibilities below 3 statute miles, including the weather causing the visibility restriction. The cause of the visibility restriction includes only widespread sand/dust storm, PCPN, FU HZ,BR,FG, and/or BLSN.

Mountain Obscuration

Areas of widespread mountain obscuration where VMC cannot be maintained, including the weather causing the obscuration. The weather causing the obscuration includes only CLDS, PCPN, FU, HZ, BR, and/or FG.

Center Weather Advisories

The CWA is an aviation weather warning for conditions meeting or approaching national in-flight advisory (AIRMET, SIGMET or SIGMET for convection) criteria. The CWA is primarily used by air crews to anticipate and avoid adverse weather conditions in the en route and terminal environments. It is not a flight planning product because of its short lead time and duration. Additionally, the CWA should be meteorologically consistent with other products and reflect conditions at the time of issuance and/or in the near future. If a CWA has been issued prior to coordination, notification to the appropriate offices, national center, or WFO should follow as soon as higher priority duties permit.

CWAs are valid for up to two (2) hours and may include forecasts of conditions expected to begin within two (2) hours of issuance. If conditions are expected to persist after the advisory's valid period, a statement to that effect should be included in the last line of the text. Follow-up CWAs should be issued as appropriate. Notice of significant changes in the phenomenon described in a CWA should be provided by a new CWA issuance for that phenomenon. If the forecaster deems it necessary, CWAs may be issued hourly for convective activity.

CWAs are be issued for any of the following events when they are expected to occur within two hours and have not been previously forecast by AWC or AAWU products, or to supplement the AWC and AAWU products.

Conditions meeting convective SIGMET criteria:

  • Icing - moderate or greater
  • Turbulence - moderate or greater
  • Heavy precipitation
  • Freezing precipitation
  • Conditions at or approaching Low IFR
  • Surface winds/gusts >30 knots
  • Low Level Wind Shear (surface - 2,000 feet)
  • Volcanic ash, dust storms, or sandstorms

Wind/temp

The Winds/Temps text data are the official FB Winds product from NCEP. This provides winds from 3000 to 53,000 feet out to 6, 12 and 24 hours. The data is extracted from model output for individual airports represented by a 3 letter IATA identifier. The data are grouped by region.

Here is a sample:

                DATA BASED ON 061800Z
                VALID 070000Z   FOR USE 2000-0300Z. TEMPS NEG ABV 24000
                
                FT  3000    6000    9000   12000   18000   24000  30000  34000  39000
                BRL 2313 2512+02 2811-02 3116-09 3336-22 3546-33 355949 355659 312855
                DBQ 2122 2413+02 2715-02 3021-09 3334-21 3646-33 345849 345459 323256
                DSM 2128 2319+03 2817-01 3020-06 3230-18 3333-32 334649 346958 333660
                MCW 2136 2427+04 2627-01 2821-07 3131-19 3237-32 325050 337359 324359

The data block shows ddffttt where dd is the wind direction in 10s of degrees, ff is the win speed in knots and ttt is the temperature in Celsius. At higher levels, the sign for the temperature is not needed since it's assumed all values are negative. If the first digit of the wind direction is greater than 4, then the wind speed is greater than 100. For example, 810550 would be 310 degrees (80-50) at 105 knots and the temperature is -50C.

Map of available sites

ITWS winds

Airport wind data from the FAA Integrated Terminal Weather System via SWIM. Data is only available for certain airports equipped with airport wind sensors and ITWS systems.

Multiple station identifiers can be entered, separated by commas. Graphs will display in one or two columns.

Wind speed in knots is displayed with a dark blue line, wind direction with a light blue line, and wind gusts in knots with orange dots.