Aviation Weather: A Guide To METARs And TAFs

Automatic Weather StationOf all the activities carried out by man, flying a plane obviously ranks right up there when it comes to the importance of knowing what the weather is doing, or is likely to do. Whether flying along alone on a light aircraft, or across the Atlantic with 250 others on a large airliner, there’s nowhere to go only down if you find yourself in weather that is beyond the limits of the aircraft. It’s highly important, therefore, that the pilot has a sound knowledge of what kind of weather that’s likely to be encountered along the route.

It is therefore not a coincidence that most of the routine-reporting weather stations around the world are located at airports. Even the smallest airfield will have a wind sock, probably the simplest type of weather instrument there is,but one that can give sufficient information for a pilot to make a decision on landing or takeoff. Obviously the equipment becomes more sophisticated at the larger airports, with both manned and automatic reports and forecasts available to the pilot and the general public. For reasons of efficiency and clarity these reports and forecasts are sent out in special coded format, but with a little practice you will not need to be a pilot or air traffic controller to be able to decipher them. To do so can give you so much more information than the standard public reports can ever give you. I have written this brief guide to decoding this information so you can be more informed on what to expect when you’re out and about, be it playing golf or just hanging out the washing!

METARs

A METAR (METeorological Aviation Report) is a coded text report of the current weather conditions observed at an airport, and is issued every 30 minutes in Ireland. It gives information on wind (speed and direction), visibility (in metres), current weather (rain, fog, etc.), the coverage and heights of cloud layers (in oktas and feet, respectively), temperature, dewpoint, atmospheric pressure, plus in some cases, a trend forecast for the next two hours. Should conditions change rapidly, special reports (SPECI) may be made more frequently than the standard 30 minutes.

Let’s take an example from Dublin Airport.

EIDW 072100Z 35008KT 8000 -RADZ SCT002 SCT006 BKN012 03/03 Q0989 NOSIG=

Let’s break it down:

EIDW – This is the ICAO Code for Dublin Airport.

072100Z – The first 2 digits (07) are the date (in this case the 7th), and the other digits (2100Z) are the time of the report, in UTC/GMT (Z is a military code for UTC). This report was issued at 2100 UTC (or 9pm GMT) on the 7th.

36008KT – This is the average wind measured over the previous 10 minutes. The first 3 digits (360) represent the direction (in degrees magnetic) from which the wind is blowing. Imagine a clockface, divided into 360 divisions (degrees). North is 0° or 360°, East is 90°, South is 180°, West is 270°, etc. A direction of say 350° means wind blowing from just west of due North. The direction will always be reported by 3 digits, so a wind from say 30° (North-northeast) would be written as 030 (and read “zero three zero”). The next 2 digits (08) are the speed, in knots (1 knot = 1.15 mph or 1.85 kph). So in the METAR above we have an 8kt wind blowing from roughly north (350°, “three five zero degrees”). Sometimes you will see something like 27014G29KT. The G29 part represents the highest gust recorded in the last 10 minutes, so the wind in this case would be read “Wind two seven zero degrees, one four, gusting two niner, knots”. You may also see something like 310V060 following the wind group. This shows the variation in wind direction, so in this case it would read “varying between three one zero and zero six zero degrees”.

8000 – This is the minimum visibility observed/measured, in metres. For visibilities of 10 km or more only the 4 digits 9999 will be used.

-RADZ – The current weather is light rain (-RA) and drizzle (DZ). Other common descriptors are SN (SNow), RASN (Sleet, i.e. Rain and Snow), GR (Hail, from the French “Grandines”), GS (Graupel), TSRA (ThunderStorm with RAin), FG, (FoG), BR (Mist), MIFG (Shallow FoG), FZFG (FreeZing FoG), HZ (HaZe). A – before it means “Light”, a + means “Heavy”, and no quantifier means “Moderate”. So +TSRA means “Thunderstorm with heavy rain”, RA means “Moderate Rain”, -SN means “Light Snow”, etc.

SCT006 BKN012 – These represent the coverage and heights of the cloud bases. SCT (Scattered) means 3-5 oktas (eighths) of the sky is covered, BKN (Broken) means 6-7 oktas. 006 and 012 are the cloud-base heights, in hundreds of feet above ground level (not sea level). Therefore the METAR reports Scattered clouds at 600ft and Broken clouds at 1,200ft above ground level (agl).

Other cloud descriptors are SKC (SKy Clear), FEW (1-2 oktas) and OVC (Overcast, 8 oktas). In good weather you may see CAVOK (Ceiling And Visibility OK) replacing the visibility and cloud groups, as it means that conditions are good for flying (no significant weather, visibility of 10km or more, no clouds below 5,000ft, no Cumulonimbus (Cb) or Towering Cumulus (TuCu) clouds (which would imply strong turbulence and possible lightning for an aircraft)).

03/03 – The first 2 digits are the temperature, and the last two digits are the dewpoint, both in °C, therefore the temperature and dewpoint are both 3 °C. Negative values are shown with an M in front, eg. M03/M06 (temperature -3 °C, dewpoint -6 °C).

Q0989 – This is the sea-level atmospheric pressure, in whole hectoPascals (or millibars). In this case it’s 989 hPa. By setting this pressure on the altimeter it will report the plane’s height above sea-level.

NOSIG – This means “NO SIGnificant change is expected within the next two hours”. In some cases, where the weather is changing rapidly, this will be replaced by a trend forecast, in the same format explained in the TAF section next.

= – This signifies the end of the report.


TAFs

While METARs report the actual current weather, a TAF is a coded text forecast of the weather conditions expected within a 5 nautical mile radius of the airport over the duration of the validity period indicated. As it is only a forecast for the local area, in some cases it may not be representative of the conditions further afield, due to local topography, lakes, etc. TAFs are issued every six hours, at 0500, 1100, 1700 and 2300Z, and are valid for 24 hours from the hour following the time of issue (or for 9 hours for the regional airports). In Ireland, TAFs are generated by meteorologists at Shannon Airport, and this human input ensures that they are as accurate as possible, but in some cases they may be amended to reflect changes different to those forecast.

Here is an example of a TAF issued for Cork Airport.

EICK 081100Z 0812/0912 25010KT 9999 SCT018
BECMG 0812/0814 26015KT
TEMPO 0813/0820 27017G28KT SCT017CB
PROB30 TEMPO 0815/0819 5000 -SHRA SCT010 SCT017CB BKN020
BECMG 0820/0823 28008KT
BECMG 0909/0912 23007KT=

Many of the codes are similar to the METAR codes above.

EICK – The ICAO Code for Cork Airport.

081100Z – TAF issued at 1100 GMT on the 8th.

0812/0912 – TAF is valid from 1200 GMT on the 8th to 1200 GMT on the 9th.

25010KT – Wind is expected to be from 250 degrees (Westerly) at 10 knots.

9999 – Visibility 10 km or more.

SCT018 – Scattered clouds at 1,800 ft above ground level.

BECMG 0812/0814 – BECoMinG (ie. a permanent change is expected to occur) between 1200 GMT and 1400 GMT on the 8th to …

26015KT – Wind from 260 degrees at 15 knots

TEMPO 0812/0820 27017G28KT SCT017CB – Between 1200 GMT and 2000 GMT on the 8th, it will TEMPOrarily be Wind 27017G28KT and SCT017CB (Scattered Cumulonimbus clouds at 1,800 ft). “Temporarily” means “for less than half of the time period specified”.

PROB30 TEMPO 0815/0819 5000 -SHRA SCT010 SCT017CB BKN020 – There is a 30% probability that temporarily between 1500 GMT and 1900 GMT on the 8th, visibility will reduce to 5000 metres in light rain showers, with scattered clouds at 1000 ft, scattered Cumulonimbus at 1,700 ft, and broken clouds at 2,000 ft.

PROB30 means a slight chance. PROB40 means a medium chance. These are ways for the forecaster to show his/her confidence in this part of the forecast. PROB30 means there is a possibility that warrants a mention, but it probably won’t happen. PROB40 means it’s more likely to happen, but it’s still not certain. If they wrote BECMG, they would be fully confident (or as confident as they can ever be!) that the change would occur.

BECMG 0820/0823 28008KT – Wind changing to 28008KT some time between 2000 GMT and 2300GMT on the 8th

BECMG 0909/0912 23007KT= – Wind changing to 23007KT some time between 0900 GMT and 1200 GMT on the 9th. = means End of TAF.


This is a good site for viewing the latest METARs and TAFs in both coded and decoded format. Click on each airport. With a little practice, you will be able to read through each report without needing to consult the decoded text on the right.

This site gives the latest METARs and TAFs (undecoded) for Europe and North Africa, overlayed on the latest IR satellite picture (hover or click on the airport, or for global METARs and TAFs use the menu on the left).

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3 thoughts on “Aviation Weather: A Guide To METARs And TAFs

    • We don’t usually get weather severe enough to include a sky almost overcast with Cbs. Most of our Cbs come in the form of individual showers, so SCT is usually as bad as it gets.

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