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| METARs and TAFs | |
James Nugent   14th Jul 2011 17:49:15 | METARs and TAFs:
METAR is a quasi-acronym. It stands for Meteorological Actual/Aerodrome Report. They are routine aerodrome reports. METARs are issued every 30 minutes during Aerodrome Operating hours. TAFs are Terminal Aerodrome Forecasts. TAFs describe the forecast prevailing conditions at an aerodrome. They usually cover periods of 9 to 24 hours. For aerodromes such as London Heathrow, the TAFs cover periods of up to 36 hours. These reports have a highly standardised format. The International Civil Aviation Organisation (ICAO) have standardised METARs and TAFs to a large degree, allowing them to be understood throughout most of the world. In the UK, METARs are issued at 20 and 50 minutes past each hour. In other parts of Europe, they are issued at 30 minutes past, and on the hour. These reports can be accessed on this website: http://euro.wx.propilots.net/ Open the menu at the top left. Select ‘METAR - TAF multiple in/out’. Typing in the ICAO codes of airports, separated by a space, will display the METARs and TAFs for that Airport. These reports can be used by both ATCs and Pilots to determine the current and forecast weather conditions. For ATCs, they will help determine the active runway. METARs start with the ICAO identifier of the airport. The Date and Time that the report was issued is next. These times are always in UTC (Universal Coordinated Time), more commonly known as GMT. Note that this time NEVER changes regardless of time zone, or daylight savings time. So, from around March to October, the time on the METAR report will be one hour behind our time. Aviation works to UTC time all over the world. Clocks in Aircraft and ATC Towers/Centres display UTC time, and not Local time. The reports then give details of things such as: Wind - its direction and speed, as well as, reports of any gusts. Visibility - always reported in four digits, “9999” meaning +10kilometres. (RVR - the Runway Visual Range is only reported when the visibility is less than 1500m) Weather - two letters that detail things from RA - rain - to FU - smoke! Cloud - for the purpose of reporting the cloud base, the sky is divided into 8 Oktas (eighths). This is followed by the HEIGHT of the clouds i.e. how high above the aerodrome the clouds are, and not their altitude (height above sea level) Temp/Dew Point - simply two numbers separated by a ‘/’. The dew point is the temperature at which water vapour in the air will condense, forming clouds or fog. QNH - this is the Sea Level Pressure, and is always given as four numbers. An Altimeter is simply a Barometer that is ‘calibrated’, or ‘set’, in Feet. As the pressure changes - with increasing or decreasing altitude - the pointer on the altimeter moves. Recent Weather - this may include things like “RERA”, meaning REcent RAin. Wind Shear - This is a sudden, violent change in wind direction and/or speed. As the wind direction and/or speed (i.e. its Velocity) changes, the Indicated Airspeed of an Aircraft can be affected. It is the speed that displays the rate at which the air is passing the aircraft, and therefore the wing. A drop of 20 knots (not uncommon) can adversely affect the Lift Production of a wing. Wind Shear is approached with extreme caution, and pilots may delay an approach if it is present on the approach or departure path of an airport. It is not to be confused with Turbulence, something that is less severe. Trend - Not reported at all airports. It notes changes expected in the next two hours, whether they be temporary or more permanent. This can also be ‘NOSIG’, meaning no significant changes. TAFs contain much of the same information. They do not include details of temperature/dewpoint or QNH. Now, to decode some METARs: EGLL 141620Z 31009KT 270V350 CAVOK 24/04 Q1017 ICAO - EGLL, London Heathrow Date/Time of Issue - 14th day of the month (July), at 1620Z - 20 past 4 in the afternoon. The local time is therefore 17:20BST (British Summer Time). Wind - 310degrees at 9knots, about 11mph. The wind is varying between 270 and 350 degrees. Vis, RVR, Weather and Cloud - All taken care of in this METAR with a simple Clouds And Visibility OK! CAVOK means that the visibility is +10km, the lowest cloud is not below 5,000ft, and there is no significant clouds i.e. clouds that could bring rain/showers. Temp/Dewpoint - 24 degrees Celsius and a dewpoint of 4degrees Celsius. QNH - is 1017 Recent weather, wind shear and Trend - not included as, at the time of writing this, the UK is under the influence of a ‘Ridge’ of High Pressure. This has a good influence on the weather. Due to this good weather, I am including a few older METARs, which will demonstrate worse conditions: EIDW 260900Z 24035G55KT 210V270 1700 +SHRA BKN007 OVC015CB 08/07 TEMPO 3500 ICAO - EIDW, Dublin Airport Date/Time - 26th day of the month, at 0900Z Wind - Blowing from 240degrees at a speed of 35knots (40mph), and gusting up to 55knots (64mph). It is varying from 210 to 270 degrees. Visibility…. - this is 1,700meres. The reason for the decrease in Visibility is “+SHRA”. + meaning Heavy. SH = Showers of RA = Rain. Cloud - Firstly, BKN007. As mentioned, the sky is divided into Oktas. BKN = Broken, means 5 to 7 Oktas. 007 = 700ft AAL (Above Aerodrome Level). Secondly, OVC = 8 Oktas - Overcast at “015”, 1,500ft AAL. The CB is the type of cloud. There are many types of cloud. The Cumulonimbus (CB) cloud is often the cause of Thunder and Lightening, Turbulence, Icing, Wind Shear, sometimes Showers of Hail Stones or Heavy Rain. This is the reason they are mentioned in the reports. Temp/Dewpoint - 8 and 7. Recent Weather and Wind Shear - nil Trend - TEMPO = temporary change, lasting 60minutes or less. 3500 = 3,500m visibility. This is due to the heavy showers and low clouds. EDDL 301550Z 26005KT 0550 R23L/0450 FZFG OVC002 M02/M02 Q0994 BECMG OVC005 ICAO - EDDL, Dusseldorf, Germany Time - 30th day at 15:50UTC W/V (Wind Velocity) - from 260degrees at 5knots. Visibility - 0550 = 550metres. RVR - The Runway Visual Range for Runway23L is 0450 = 450metres. Weather - Both Visibilities reported are due to FZFG = FReezing FoG and the Clouds. Cloud - OVC002 = Overcast (8 Oktas) at 200ft. Temp/Dewpoint - M02 = Minus 2 QNH - Q0994 = 994millibars/hectopascals Trend - BECMG = a more permanent change in the weather, as opposed to TEMPO. The weather will change to OVC005 = Overcast clouds at 500ft AAL. TAFs use similar codes for various weather conditions. There is a slight difference in the way the time is displayed. A few examples: EGLL 141655Z 1418/1524 33010KT 9999 SCT045 BECMG 1419/1422 27007KT BECMG 1509/1512 21010KT PROB40 TEMPO 1522/1524 8000 -RA BKN012 ICAO - EGLL, London Heathrow. Time of Issue - 14th day (today) at 1655Z - 1755BST/local. Since this is a forecast, the time is given at which the changes in the Forecast are expected. 1418/1524 - From 18Z on the 14th day, to 24Z on the 15th day. As mentioned, the TAF for EGLL is a 36-hour TAF - It forecasts from 1800Z (on the 14th) to 24Z (same as Midnight on the 16th). So, 1418/1524 33010KT 9999 SCT045 - From 1800Z on the 14th (July) to 24Z on the 15th/00Z on the 16th, Winds are forecast to be 330degrees at 10knots. The visibility is expected to be 9999 = +10km. The clouds will be Scattered (3 or 4 Oktas) at 4,500ft. BECMG 1419/1422 27007KT - BECMG = a more permanent change. From time 1900Z on the 14th to 2200Z on the 14th, the winds will blow from 270 degrees at 7knots. BECMG 1509/1512 21010KT - Becoming, from 0900Z (the minutes are not included, only the hour) on the 15th to 12Z on the 15th, the wind will be 210degrees at 10knots. PROB40 - This mean ‘Probability 40’. There is only PROB30 or PROB40 in TAFs. PROB40 is a high probability, PROB30 is less likely. So, PROB40 TEMPO 1522/1524 8000 -RA BKN012 - Probability 40 that TEMPOrarily between 2200Z and 2400Z on the 15th day, that the Visibility will be 8000metres in “-RA”. This means Light RAin. - = light, no symbol = moderate, + = heavy. The clouds will be BRoKen at 012, 1,200ft AAL. The best way to get used to these reports is to read them! There are a few other codes used in the METARs and TAFs that are less common. You can Google these or just ask me. I hope this helps Pilots and ATC :) |
Steff Missot 14th Jul 2011 19:57:41 | This is a link to a decent METAR decoder.
http://heras-gilsanz.com/manuel/METAR-Decoder.html |
Joe Clifford  14th Jul 2011 20:03:43 | :D decoder is sooooo cheating :P
Nicely Explained BTW :D |
| Steff Missot 14th Jul 2011 21:17:41 | I Know, I used it a lot on my vatsim sessions:P
Thanks for the great explanation James! |
| Laurence Woodside 15th Jul 2011 12:51:33 | Great Stuff James,
Another aviation mystery explained! |
Mohammad Freigoun  16th Jul 2011 11:37:14 | Well Explained James, True PPL stuff . |
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