5.3.1 Actual Weather Reporting

Aerodrome Weather Reporting — METAR/SPECI Decoding Guide

5.3.0 Introduction

  • In order to facilitate a sophisticated decoding and interpretation of aerodrome weather reports around the world, the International Civil Aviation Organization (ICAO) and World Meteorological Organization (WMO) together with relevant organizations representing all users have designed a weather coding system intended to be used worldwide in all aerodrome weather reports.

  • Basically there are two types of weather reports in operational aviation:

    • METAR = Actual report
    • TAF = Forecast report.
  • In the next paragraphs relevant information needed for understanding and decoding, together with some examples are given. Full details regarding this matter are laid down in ICAO Annex 3 11th Edition and the WMO Manual on Codes Vol. I No. 306.

5.3.1 Actual Weather Reporting

1. GENERAL
There are two actual weather reports distinguished: Routine and Special Reports.

Routine reports are coded ‘METAR’ and are compiled each half hour or each hour at fixed times during the operational hours of the meteorological station issuing the airport.

Special reports (‘SPECI’) are supplementary to Routine reports and are only issued when improvements or deteriorations through certain criteria occur.

The actual weather report consists of several groups each containing specific details about the weather situation. Missing information is replaced by diagonals. Starting with an example of a report all groups will be discussed:

Example: METAR EHAM 0825Z 21016G28KT 1400SW 6000N R19R/P1500 SHRASN SCT005 SCT012CB OVC025 02/00 Q0998 RETS WS TKOF RWY24 NOSIG.

In plain language:
Actual weather report Amsterdam at 0825 UTC: Surface wind 210 degrees 16 knots gusting to 28 knots. Visibility 1400 meters to the Southwest and 6000 meters best visibility to the North. Runway 19R more than 1500 meters. Light showers with rain and snow (of less intensity). Clouds; scattered (1 to 4 oktas) at 500 feet, scattered (1 to 4 oktas) CB basis 1200 feet and overcast 8 oktas basis 2500 feet. Temperature 02°, dew point 00°. QNH 998 hPa. Recent thunderstorms. Windshear in take-off path (area) runway 24. No significant changes.

2. IDENTIFIER
There are three components in the report identifier:
  • Report type : ‘METAR’ or ‘SPECI’.
  • Station ident : ICAO four letter code.
  • Time : Time of observation in hours and minutes UTC followed by the letter Z.

Example: METAR EHAM 0825Z.

In meteorological bulletins containing several stations, there will be only one general identifier heading per bulletin. Here the code ‘SA’ (station actual) or ‘SP’ (special report) can be used instead of ‘METAR’ or ‘SPECI’.

3. SURFACE WIND
Wind is given as a mean wind, measured over a 10 minute period directly preceding the observation time. Wind direction is reported in degrees true, rounded to nearest 10 degrees. Wind speed can be given in kt, kmh or mps. Calm is indicated by ‘00000’, variable by ‘VRB’.

Example: 21016 kt (= 210° / 16 kt).

If during the 10 minutes preceding the observation time:

  • The wind direction has varied by 60 degrees or more, and with a mean speed greater than 3 kt. The two extreme directions will be indicated in clockwise order with the letter ‘V’ (varying) inserted between the two directions.

Example: 21016KT 180V250.

  • The maximum gust speed has exceeded the mean speed by 10 kt or more (20 kmh, 5mps), the gust will be reported by inserting the letter ‘G’ (gust) followed by gust speed.

Example: 21016G28KT.

  • There has been a marked discontinuity in the measured wind lasting at least two minutes, the mean wind value will be assessed over the period after the discontinuity.

4. HORIZONTAL VISIBILITY
The visibility is given in metres as a four figure group varying from ‘0000’ to ‘9999’. If there is no marked variation in visibility by direction. The minimum visibility is given, otherwise the reported minimum will be followed by one of the eight points of the compass to indicate the direction.

Example: 3000 or 3000NW.

NOTE: Marked variation means minimum visibility is less than 5000 metres and variations are at least 50% of the minimum visibility.

When the minimum visibility is less than 1500 metres and the visibility in another direction exceeds 5000 metres the maximum visibility will be mentioned additionally.

Example: 1400SW 6000N (= The visibility is 1400 m to the Southwest and 6000 m to the North).

5. RUNWAY VISUAL RANGE (RVR)
An RVR group starts with the prefix ‘R’ followed by a runway designator and a diagonal followed by the touchdown zone RVR in metres. When for more runways RVR is reported, each one will be coded individually.

Example: R19R/1100 R27/1250 (= RVR RW19R 1100 m; RW27 1250 m).

When the RVR exceeds 1500 metres it will be reported with a preceding ‘P’.

Example: R19R/P1500 (= Visibility RW19R is more than 1500 m).

When the RVR is below the minimum value which can be assessed it will be reported by the letter ‘M’ followed by the appropriate minimum value that can be assessed.

Example: R19R/M0050 (= Visibility RW19R is less than 50 m).

If it is possible to determine mean values they will be given over the 10 minute period directly preceding the observation time. Also trends and significant variations may be reported.

Trend:
If there is a distinct increasing or decreasing tendency during the ten minutes assessment period this can be indicated by subscripts ‘U’, ‘D’ or ‘N’ (increasing, decreasing or no change). (Distinct means that the mean value of the first 5 minutes differs 100 metres or more from the mean value of the latter 5 minutes).

Example: R19R/1100D (= Visibility RW19R is 1100 m decreasing).

Variations:
When the RVR varies significantly during the 10 minutes assessment time the 1 minute minimum and maximum values will be given in that order separated by a letter ‘V’ instead of the 10 minute mean value. (Significantly means that 1 minute extreme values differ more than 50 metres or 20% from the 10 minute mean value).

Example: R19R/0950V1150 (= Visibility RW19R varying between 950 and 1150 m).

NOTE: This RVR group may also be followed by a trend subscript if the 10 minute mean value shows a trend.

6. PRESENT WEATHER
A weather group consists of two to nine characters containing information about the intensity, proximity and weather phenomena. Not more than three weather groups will be given in a report. If there is a mixture of precipitation types they will be reported combined in one group with the dominant type first.

Example: –SHRASN means: Light rain and snow showers with rain being the dominant precipitation.

In the following table the available weather codes are listed.

QUALIFIER

BR, HZ, FU, IC, DU and SA will not be reported when the visibility is more than 5000 metres.
BR (mist) is reported when visibility is between 1000 and 5000 metres because of water or ice particles.
FG is reported when visibility is reduced to less than 1000 m due to water or ice particles.
GR is only used when the diameter of hailstone is observed as 5 mm and more.
GS is used in other cases.

Example: MIFG, VCBLSN, +SHGR, –DZFG.

7. CLOUDS AND VERTICAL VISIBILITY
Clouds will be reported in six character groups. A three letter code followed by three figures indicating the height of a cloud layer in hundreds of feet above aerodrome level.

FEW (Few) Indicates 1 to 2 OKTAS
SCT (Scattered) Indicates 3 to 4 OKTAS
BKN (Broken) Indicates 5 to 7 OKTAS (= Ceiling)
OVC (Overcast) Indicates 8 OKTAS.

NOTE: The cloud base is rounded down, e.g. 1850 ft will be encoded to 1800 ft.

Example: SCT005, BKN020, OVC045.

Types of clouds are not mentioned unless they are significant. Significant clouds are:
  • CB (Cumulonimbus)
  • TCU (Towering cumulus).

Example: SCT012CB.

Cloud groups are reported in ascending order in the following way:
  • First group : Lowest individual layer of any amount.
  • Second group : Next individual layer of more than 2 OKTAS.
  • Third group : Next higher layer of more than 4 OKTAS.
  • Additional group : Significant convective clouds (CB or TCU) if not already reported in one of the three groups above.

Example: FEW005, SCT012CB, OVC025 (= Clouds: 1 to 2 oktas basis 500 ft, 3 to 4 oktas cumulonimbus basis 1200 ft and 8 oktas basis 2500 ft).

In case there are no cloud layers or significant clouds to report:
  • CAVOK : Replaces visibility, weather and cloud groups in the following conditions:
    • Visibility 10 km or more.
    • No cloud below 5000 ft or below the highest MSA whichever is greater and no CB.
    • No precipitation, thunderstorm, shallow fog, low drifting snow or sand.
  • SKC : No clouds to report and CAVOK does not apply.
  • VV003 : Sky obscured followed by vertical visibility in hundreds of feet. If this cannot be assessed the group will read VV///.

8. AIR TEMPERATURE AND DEW POINT
Temperature and dew point are given in degrees Celsius rounded off to the next whole degree. Temperatures below zero are indicated by an ‘M’.

Example: 02/00, 01/M01.

9. AIR PRESSURE (QNH)
QNH is presented by a letter ‘Q’, followed by four digits representing Hectopascals (hPa) rounded off to the next whole value.

Example: Q0998 (= 998 hPa).

10. SUPPLEMENTARY INFORMATION
Recent weather: are ‘operationally significant’ weather phenomena observed in the period of the previous observation but not at the moment. The codes will be preceded by the indicator RE. No intensities will be given. Thunderstorms are assumed to include heavy rain and/or hail.

Example: RETS (= Recent thunderstorms).

Wind shear: This may be inserted if reported or measured along the take-off or approach paths in the lowest 1600 ft with reference to the runway. The group begins with the code ‘WS’. No indication is given about the type of wind shear.

Example: WS TKOF RWY 24, WS LDG RWY 19R (= Windshear in the take-off path RW24 and landing path RW19R).

Other: Any important operationally information may be added in plain language, as long as it is self-explanatory.

11. TREND
The trend is an integral part of the weather report (METAR or SPECI). The information contained in the trend is a forecast covering a period of two hours from the time of observation of the actual report. It is only issued at selected airports. It indicates expected significant changes in one of the following elements: surface wind, visibility, weather and clouds or any other element which has operational value for a station (by agreement between local authorities and operators involved).
Two types of indicators are used for significant changes:
  • BECMG : Becoming, used where changes are expected to reach or pass through specified values at a regular or irregular rate.
  • TEMPO : Temporary, fluctuations of less than one hour and in aggregate less than half the period indicated by the beginning and the end of the forecast period in hours UTC.

ETIME-GROUP

If no significant change is expected to occur during the next two hours the code ‘NOSIG’ replaces the trend group.

Example: BECMG FM 1200 23030G45 KT
TEMPO FM 0800 TL 0900 2000 SHRA
BE CMG 5000.

NOTE: FM = from; TL = until; AT = at.

Change criteria for the different elements:

Surface wind:
  • Direction change > 60 degrees with mean windspeed before and/or after change > 10 kt.
    NOTE: Amsterdam FIR ≥ 30 degrees.
  • Mean wind speed change > 10 kt.
  • Change through operational important values.

Visibility:
Change through following values: 150, 350, 600, 800, 1500, 3000 metres. In case there is substantial VFR traffic also 5000.
NOTE: Amsterdam FIR 5000 and 8000 m.
If there is a significant reduction, the phenomena causing the reduction are mentioned.

Weather:
Change of significant weather. If there is no significant weather reported the code ‘NSW’ will be used.

Cloud and vertical visibility:
  • Height of ceiling (= BKN or OVC) is below or expected to fall below 1500 ft and is forecast to reach or pass 100, 200, 300, 500, 1000 and 1500 ft.
  • Height of the base of a cloud layer is below or expected to fall below 1500 ft and the cloud amount is forecast to change from:
    • SCT, FEW or SKC increasing to BKN or OVC.
    • BKN or OVC decreasing to SCT, FEW or SKC.
  • The sky is expected to remain or become obscured while vertical visibility observations are available and forecasted to change to or pass 100, 200, 500 ft.
  • A change to clear sky is indicated by ‘SKC’, replacing cloud and vertical visibility groups.
  • When no significant change in the clouds is expected during the validity period of the trend, the cloud groups are not repeated.
  • To indicate a change to no clouds below 1500 m / 5000 ft (or the highest minimum sector altitude, whichever is the greater), the abbreviation NSC (nil significant clouds) replaces the cloud and vertical VIS group, when CAVOK or SKC are not appropriate.

12. EXAMPLES
a. METAR EHRD 0700Z 13006KT 0400E R23/0950D DZ BCFG VV/// 04/04 Q1018 BECMG FM 0800 0800.
b. METAR EHAM 1025Z 23015KT 190V300 9999–RA SCT012 BKN020 OVC045 13/10 Q0996 TEMPO 3000 RA BKN008 OVC015.
c. METAR EGLL 1200Z 30025G38KT 260V360 1100E 6000SW +SHSNRAGS SCT004 BKN010CB 02/M02 Q0999 RETS WS LDG RWY 27L BECMG AT 1300 9999 NSW SCT015 BKN025.

In plain language:

a. Weather report Rotterdam at 0700 UTC: Surface wind 130 degrees at 6 knots, visibility (lowest) 400 metres to the east, RVR runway 23 950 metres with decreasing tendency, drizzle, fog patches, sky obscured vertical visibility not available, temperature 4 degrees, dew point 4 degrees, QNH 1018 hPa. Trend: Improving to 800 metres between 0800 UTC until after 0900 UTC.

b. Weather report Amsterdam at 1025 UTC: Surface wind 230 degrees 15 knots variable between 190 and 300 degrees, visibility 10 km or more with light rain. Clouds: Scattered at 1200 ft, broken at 2000 ft, overcast at 4500 ft, temperature 13 degrees, dewpoint 10 degrees, QNH 996 hPa, temporary visibility 3000 metres in moderate rain, clouds broken at 800 feet, overcast at 1500 feet.

c. Weather report London-Heathrow at 1200 UTC: Surface wind 300 degrees 25 knots gusting to 38 knots, varying between 260 and 360 degrees, visibility minimum 1100 metres to the east, maximum 6000 metres to the southwest, heavy snow-, rain- and small hail-showers. Clouds scattered at 400 ft, broken with CBs at 1000 ft, temperature 02 degrees, dewpoint minus 02 degrees, QNH 999 hPa, recent thunderstorm(s) wind shear in approach path RWY 27 left, improving at 1300 UTC to visibility 10 km or more, nil significant weather, clouds scattered at 1500 ft, broken at 2500 ft.