An explanation by Irish Weather Online's Fergal Tierney:
METARs (METeorological Aviation Reports) and TAFs (Terminal Aerodrome Forecasts) can appear quite confusing at first glance, but after a little practice they can be decoded instantly and can be a useful tool for anybody who wants a quick idea of the present weather, and what it will do in the coming hours. They are primarily issued for aviation personnel (Pilots, Air Traffic Controllers, Airport Authorities, etc.) but can be used by anybody who knows how to decode them.
A METAR 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, current weather, the coverage and heights of cloud layers, temperature, dewpoint, atmospheric pressure, plus in some cases, a trend forecast for the next two hours. Let's take an example from Dublin Airport.
EIDW 072100Z 35008KT 8000 -RADZ SCT002 SCT006 BKN012 03/03 Q0989 NOSIG=
Come back, don't be afraid, it's all very easy! Let's break it down:
EIDW - This is the ICAO Code for Dublin Airport.
072100Z - The first 2 digits are the date (in this case the 7th), the other digits are the time of the report, in UTC/GMT (Z is a military code for UTC). This report was issued at 2100UTC (or 9pm GMT)
36008KT - This is the average wind measured over the previous 10 minutes. The first 3 digits represent the direction (in degrees true) from which the wind is blowing. Imagine a clockface, divided into 360 divisions (degrees true). 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 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 and drizzle. Other common descriptors are RA (RAin), DZ (DriZzle), SN (SNow), RASN (Sleet, i.e. Rain and Snow), GR (Hail), 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 for an aircraft)).
03/03 - The first 2 digits are the temperature, and the last two digits are the dewpoint, both in °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 989hPa.
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.
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. 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=
The codes are similar to the METAR codes above.
EICK - The ICAO Code for Cork Airport.
081100Z - TAF issued at 1100GMT on the 8th.
0812/0912 - TAF is valid from 1200GMT on the 8th to 1200GMT 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 1,800ft above ground level.
BECMG 0812/0814 - BECoMinG (ie. a permanent change is expected to occur) between 1200 GMT and 1400GMT on the 8th....
26015KT - Wind from 260 degrees at 15 knots
TEMPO 0812/0820 27017G28KT SCT017CB - Between 1200GMT and 2000GMT on the 8th, it will TEMPOrarily be Wind 27017G28KT and SCT017CB (Scattered Cumulonimbus clouds at 1,800ft). "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 1500GMT and 1900GMT on the 8th, visibility will reduce to 5000 metres in light rain showers, with scattered clouds at 1000ft, scattered Cumulonimbus at 1,700ft, and broken clouds at 2,000ft.
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 that the change would occur.
BECMG 0820/0823 28008KT - Wind changing to 28008KT some time between 2000GMT and 2300GMT on the 8th
BECMG 0909/0912 23007KT= - Wind changing to 23007KT some time between 0900GMT and 1200GMT on the 9th. End of TAF.