3-1 CHAPTER 3. OBSERVATIONS The first of three distinct types of weather (meteorological) information is observations.
Observations are weather data collected by one or more sensors, and are the basic information upon which forecasts and advisories are made in support of a wide range of weather-sensitive activities within the public and private sectors, including aviation. Chapter 3 describes and discusses surface observations, aircraft observations, radar observations, and satellite observations (e.g., imagery).
3.1 Aviation Routine Weather Reports (METAR) and Special Weather Reports (SPECI).
Surface weather observations are fundamental to all meteorological services. Aviators typically view surface observations through METARs and SPECIs. Although the METAR/SPECI code is used worldwide, there are some code differences among countries. Each country is allowed to make modifications to the code for use in their particular country, as long as they notify the International Civil Aviation Organization (ICAO). These paragraphs will focus on the METAR/SPECI code as used in the United States.
3.1.1 Aviation Routine Weather Report (METAR).
The METAR report has been adopted by the United States to provide surface observations in support of aviation for the terminal. A METAR report includes the airport identifier, time of observation, wind, visibility, Runway Visual Range (RVR), present weather phenomena, sky conditions, temperature, dewpoint, and altimeter setting.
Excluding the airport identifier and the time of observation, this information is collectively referred to as the “body” of the report. Coded and/or plain language information elaborating on data in the body may be appended to the end of the METAR as “remarks.” The contents of the remarks section varies with the type of reporting station. At some designated stations, the METAR may be abridged to include only a few of the mentioned elements.
3.1.2 Special Weather Report (SPECI).
A SPECI is an unscheduled report taken when any of the criteria given in Table 3-1, SPECI Criteria, are observed during the period between hourly reports. SPECIs contain al l data elements found in a METAR. All SPECIs are issued as soon as possible when relevant criteria are observed.
Whenever SPECI criteria are met at the time of the routine METAR, a METAR is issued.
3-2 Table 3-1. SPECI Criteria 1 Wind Shift Wind direction changes by 45 degrees or more, in less than 15 minutes and the wind speed is 10 knots or more throughout the wind shift.
2 Visibility Surface visibility, as reported in the body of the report, decreases to less than, or if below, increases to equal to or exceeding:
• miles
• mile s
• 1 mile
• The lowest standard instrument approach procedure minimum as published in the Nat ional Ocean Service (NOS) U.S. Instrument Procedures. If none published, use ½ mile 3 Runway Visual Range (RVR) The highest value from the designated RVR runway decreases to less than, or if below, increases to equal to or exceeding 2,400 feet during the preceding 10 minutes. U.S. military stations may not report a SPECI based on RVR.
4 Tornado, Funnel Cloud, or Waterspout • Is observed • Disappears from sight, or ends 5 Thunderstorm
• Begins (a SPECI is not required to report the beginning of a new thunderstorm if one is currently reported)
• Ends 6 Precipitation
• Hail begins or ends
• Freezing precipitation begins, ends or changes intensity
• Ice pellets begin, end or change intensity 7 Squalls When a squall occurs.
8 Ceiling The ceiling (rounded to reportable values) forms or dissipates below, decreases to less than, or if below, increases to equal to or exceeding:
• 3,000 feet
• 1,5 00 feet
• 1,000 feet
• 500 feet
• The lowest standard instrument approach procedure minimum as published in the Nat ional Ocean Service (NOS) U.S. Instrument Procedures. If none published, use 200 feet.
9 Sky Condition A layer of clouds or obscurations aloft is present below 1,000 feet and no layer aloft was reported below 1,000 feet in the preceding METAR or SPECI.
10 Volcanic Eruption When an eruption is first noted.
11 Aircraft Mishap Upon notification of an aircraft mishap, unless there has been an intervening observation.
12 Miscellaneous Any other meteorological situation designated by the responsible agency of which, in the opinion of the observer, is critical.
The information in Table 3-1 can be found in the Federal Meteorological Handbook No. 1 (FMH-1), Section 2.5.2, Aviation Selected Special Weather Report (SPECI).
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3.1.3 General Types of Observations.
There are three general types of surface observations:
• Manual Observation. Weather observations done by a human weather observer who is certified by the FAA.
• A utomated Observation. Automated observations are derived from instruments and algorithms without human input or oversight. In the United States, there are two main kinds of automated observing systems: the automated surface observing system (ASOS) and the Automated Weather Observing System (AWOS). Detailed information on ASOS and AWOS can be found in the Aeronautical Information Manual (AIM), Chapter 7, Safety of Flight, Section 1, Meteorology. Automated METARs and SPE CIs contain AUTO in the report (see paragraph 3.1.5.4).
• Augmented Observation. At select airports in the United States, the automated observing system will have input and oversight by human weather observers or tower controllers certified in weather observing. These are referred to as augmented stations. Human observers report weather elements that are beyond the capabilities of the automated system and/or are deemed operationally significant. The weather elements observed and reported by the human observer vary, depending on the selected airport. AUTO is not used in augmented reports.
Whereas manual stations were most common prior to the mid-1990s in the United States, the vast majority of today’s METARs and SPECIs are from fully-automated stations.
3.1.4 Recency of Observed Elements at Automated Stations.
For those elements that the human observer evaluates using spatial averaging techniques
(e.g., sky cover and visibility), the automated station substitutes time averaging of sensor data. Therefore, in an automated observation, sky condition is an evaluation of sensor data gathered during the 30-minute period ending at the actual time of the observation.
All other elements are based on sensor data that is within 10 minutes or less of the actual time of the observation.
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3.1.5 Format.
Figure 3-1. METAR/SPECI Coding Format
A U.S. METAR/SPECI has two major sections: the body (consisting of a maximum of 11 groups) and the remarks (consisting of 2 categories). When an element does not occur, or cannot be observed, the corresponding group is omitted from that particular report.
3.1.5.1 Type of Report.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 The type of report, METAR or SPECI, precedes the body of all reports, but may not be shown or displayed on all aviation weather websites.
3.1.5.2 Station Identifier.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 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.
3-5 Unlike the IATA codes, the ICAO codes have a regional structure. The first letter identifies the region and country (see Figure 3-2, ICAO Continental Codes). 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.
Figure 3-2. ICAO Continental Codes
In the continental United States (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.
For a list of all U.S. identifiers, refer to FAA Order 7350.9, Location Identifiers. For a complete worldwide listing, refer to ICAO Document 7910, Location Indicators. Both are available online.
3.1.5.3 Date and Time of Report.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132
3-6 The date and time are coded in all reports as follows: the day of the month is the first two digits (01), followed by the hour (19) and the minutes (55).
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)).
For example, METAR KOKC 011955Z would be disseminated as the 2000 hour routine report for station KOKC, taken on the 1st of the month at 1955 UTC.
3.1.5.4 Report Modifier (As Required).
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 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.
3.1.5.5 Wind Group.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 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 degrees are preceded with a 0. For example, a wind direction of 90 degrees 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 (kts) using the units, tens digits, and, when required, the hundreds digit. When wind speeds are less than 10 kts, a leading 0 is used to maintain at least a two-digit wind code. For example, a wind speed of 8 kts will be coded 08KT. The wind group is
3-7 always coded with a KT to indicate wind speeds are reported in knots. Other countries may use kilometers per hour (KPH) or meters per second (MPS) instead of knots.
Examples:
• 05008KT Wind 50 degrees at 8 kts
• 15014KT Wind 150 degrees at 14 kts
• 340112KT Wind 340 degrees at 112 kts
3.1.5.5.1 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 kts 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, if required, the hundreds digit. For example, a wind out of the west at 20 kts with gusts to 35 kts would be coded 27020G35KT.
3.1.5.5.2 Variable Wind Direction (speed 6 kts or less).
Wind direction may be considered variable when, during the previous 2-minute evaluation period, the wind speed was 6 kts 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 kts, it would be coded VRB03KT.
3.1.5.5.3 Variable Wind Direction (speed greater than 6 kts).
Wind direction may also be considered variable when, during the 2-minute evaluation period, it varies by 60 degrees or more and the speed is greater than 6 kts. 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 degrees to 240 degrees at 10 kts, it would be coded 21010KT 180V240.
3.1.5.5.4 Calm Wind.
When no motion of air is detected, the wind is reported as calm. A calm wind is coded as 00000KT.
3.1.5.6 Visibility Group.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132
3-8 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 (sm). 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 (m).
U.S. automated stations use an M to indicate “less than.” For example, M1/4SM means a visibility of less than ¼ sm.
3.1.5.7 Runway Visual Range (RVR) Group.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 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 feet (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 ft to 3,000 ft, and increments of 500 ft from 3,000 ft 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
3-9 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 will be shown with V between them, indicating variable conditions. For example, the 10-minute RVR for Runway 01L varying between 600 ft 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 will be coded R27/P6000FT.
3.1.5.8 Present Weather Group.
METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 Pr esent weather includes precipitation, obscurations, and other weather phenomena. The appropriate notations found in Table 3-2, METAR/SPECI Notations for Reporting Present Weather, are used to code present weather.
3-10 Table 3-2. METAR/SPECI Notations for Reporting Present Weather1
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 Moderate2 PR Partial RA Rain FG Fog SQ Squalls + Heavy BC Patches SN Snow FU Smoke FC Funnel Cloud, Tornado or Waterspout4 VC In the Vicinity3 DR Low Drifting SG Snow Grains VA Volcanic Ash SS Sandstorm BL Blowing IC Ice Crystals (Diamond Dust) DU Widespread Dust DS Duststorm SH Shower(s) PL Ice Pellets SA Sand TS Thunderstorms GR Hail HZ Haze FZ Freezing GS Snow Pellets PY Spray UP Unknown Precipitation
1. The weather groups are constructed by considering columns 1 through 5 in the table above in sequence, i.e., intensity followed by description, followed by weather phenomena, e.g., heavy rain shower(s) is coded as +SHRA.
2. To denote moderate intensity no entry or symbol is used.
3. See text for vicinity definitions.
4. Tornadoes and waterspouts are coded as +FC.
Separate groups are used for each type of present weather. Each group is separated from the other by a space. METAR/SPECI reports contain no more than three present weather groups.
3-11 When more than one type of present weather is reported at the same time, present weather is reported in the following order:
• Tornadic activity (tornado, funnel cloud, or waterspout).
• Thunderstorm(s) (with and without associated precipitation).
• Present weather in order of decreasing dominance (i.e., the most dominant type reported first).
• Left-to-right in T able 3-2 (columns 1 through 5).
Qualifiers may be used in various combinations to describe weather phenomena. Present weather qualifiers fall into two categories: intensity (see paragraph 3.1.5.8.1) or proximity (see paragraph 3.1.5.8.2) and descriptors (see paragraph 3.1.5.8.3).
3.1.5.8.1 Intensity Qualifier.
The intensity qualifiers are light, moderate, and heavy. They are coded with precipitation types, except ice crystals (IC) and hail (GR), including those associated with a thunderstorm (TS) and those of a showery nature (SH).
Tornadoes and waterspouts are coded as heavy (+FC). No intensity is ascribed to the obscurations of blowing dust (BLDU), blowing sand (BLSA), and blowing snow (BLSN). Only moderate or heavy intensity is ascribed to sandstorm (SS) and duststorm (DS).
When more than one form of precipitation is occurring at a time, or precipitation is occurring with an obscuration, the reported intensities are not cumulative. The reported intensity will not be greater than the intensity for each form of precipitation. For example, -FZRAPL is light freezing rain and light ice pellets, not light freezing rain and moderate ice pellets.
3.1.5.8.2 Proximity Qualifier.
Weather phenomena occurring beyond the point of observation (between 5 and 10 sm) are coded as in the vicinity (VC). VC can be coded in combination with thunderstorm (TS), fog (FG), shower(s) (SH), well-developed dust/sand whirls (PO), blowing dust (BLDU), blowing sand (BLSA), blowing snow (BLSN), sandstorm (SS), and duststorm (DS).
Intensity qualifiers are not coded in conjunction with VC.
For example, VCFG can be decoded as meaning some form of fog is between 5 and 10 sm of the point of observation. If VCSH is coded, showers are occurring between 5 and 10 sm of the point of observation.
Weat her phenomena occurring at the point of observation (at the station) or in the vicinity of the point of observation are coded in the body of the report.
Weather phenomena observed beyond 10 sm from the point of observation (at the station) is not coded in the body, but may be coded in the remarks section (see paragraph 3.1.5.12).
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3.1.5.8.3 Descriptor Qualifier.
Descriptors are qualifiers that further amplify weather phenomena and are used in conjunction with some types of precipitation and obscurations. The descriptor qualifiers are: shallow (MI), partial (PR), patches (BC), low drifting (DR), blowing (BL), shower(s) (SH), thunderstorm (TS), and freezing (FZ).
Only one descriptor is coded for each weather phenomena group
(e.g., FZDZ).
The descriptors shallow (MI), partial (PR), and patches (BC) are only coded with fog (FG) (e.g., MIFG). Mist (BR) is not coded with any descriptor.
The descriptors low drifting (DR) and blowing (BL) will only be coded with dust (DU), sand (SA), and snow (SN) (e.g., BLSN or DRSN). DR is coded with DU, SA, or SN for raised particles drifting less than 6 ft above the ground.
When blowing snow is observed with snow falling from clouds, both phenomena are reported (e.g., SN BLSN). If blowing snow is occurring and the observer cannot determine whether or not snow is also falling, then BLSN is reported. Spray (PY) is coded only with blowing (BL).
The descriptor for showery-type precipitation (SH) is coded only with one or more of the precipitation qualifiers for rain (RA), snow (SN), ice pellets (PL), or hail (GR). When any type of precipitation is coded with VC, the intensity and type of precipitation is not coded.
The descriptor for thunderstorm (TS) may be coded by itself when the thunderstorm is without associated precipitation. A thunderstorm may also be coded with the precipitation types of rain (RA), snow (SN), ice pellets (PL), snow pellets (GS), or hail (GR). For example, a thunderstorm with snow and/or GR hail and/or snow pellets would be coded as TSSNGRGS. TS is not coded with SH.
The descriptor freezing (FZ) is only coded in combination with fog (FG), drizzle (DZ), or rain (RA) (e.g., FZRA). FZ is not coded with SH.
3.1.5.8.4 Precipitation.
Precipitation is any form of water particle, whether liquid or solid, that falls from the atmosphere and reaches the ground. The precipitation types are:
drizzle (DZ), rain (RA), snow (SN), snow grains (SG), ice crystals (IC), ice pellets (PL), hail (GR), snow pellets (GS), and unknown precipitation (UP). UP is reported if an automated station detects the occurrence of precipitation, but the precipitation sensor cannot recognize the type.