Meteorological Aerodrome Reports (METARs) — Page 480, Lesson 454

I want to walk you through the Meteorological Aerodrome Report — the METAR — and how it's structured, starting with a detail about how QNH is reported in North America versus the rest of the world. In North America, QNH — that's the barometric pressure adjusted to sea level — is reported in inches of mercury. The letter 'A' is used to indicate this. So if you see A2989 in a METAR, that means a QNH of 29.89 inches of mercury. That's a different unit from what we use in most other places, where QNH is given in hectopascals. Now let's decode a full METAR example. Here's one for Oxford/Kidlington: METAR EGTK 231020Z 26012G25KT 220V300 9999 –RA FEW060 SCT120 10/M02 Q0991 RETS Let me walk through each part. "METAR" tells us this is a routine meteorological aerodrome report. "EGTK" is the ICAO location indicator for Oxford/Kidlington. "231020Z" means the observation was taken on the 23rd day of the month at 1020 UTC — that's 10:20 in the morning, Coordinated Universal Time. Next comes the surface wind: "26012G25KT" means the wind is from 260 degrees true at 12 knots, gusting to 25 knots. Then "220V300" tells us the wind direction is variable between 220 degrees true and 300 degrees true. After that, "9999" means the prevailing visibility is 10 kilometres or more — that's the maximum code for visibility. Then we have "–RA" — the dash before RA means light rain. RA is the code for rain. Now the cloud groups: "FEW060" means there are 1 to 2 oktas of cloud at 6,000 feet. "SCT120" means 3 to 4 oktas at 12,000 feet. FEW and SCT are cloud amount codes — FEW is 1-2 oktas, SCT is scattered at 3-4 oktas. Then we have "10/M02" — that's the air temperature of +10 degrees Celsius and the dew point of -2 degrees Celsius. The slash separates them, and the M before 02 means minus. "Q0991" is the QNH of 991 hectopascals. And finally "RETS" — that's a recent weather code, which I'll explain in a moment. Let's talk about recent weather. If there has been significant weather in the past hour, or since the last METAR was issued, and that weather has now ceased or reduced in intensity, a METAR code group beginning with RE will appear. RE stands for recent. So in our example, RETS means there has been a thunderstorm during the past hour, but it has now abated, leaving only light rain. The present weather is reported as light rain — –RA — and the fact that there were thunderstorms in the past hour is reported by the code RETS. Now let's look at windshear reporting. Although it's not currently issued at United Kingdom airfields, windshear information may be reported in a METAR. It's denoted by the letters WS, followed by the necessary details. For example, WS ALL RWY means windshear on all runways. WS 30 means windshear present on Runway 30. So our full METAR could look like this: METAR EGTK 231020Z 26012G25KT 220V300 9999 –RA FEW060 SCT120 10/M02 Q0991 RETS WS ALL RWY Now we come to the TREND forecast. A TREND forecast is valid for 2 hours after the time of the observation of the METAR, and it constitutes the final section of the METAR. The change in weather conditions indicated by the code TREND can be further qualified by two codes: BECMG, meaning becoming, or TEMPO, meaning temporarily. BECMG indicates that the change in the present weather will be long-lasting. TEMPO, on the other hand, means that the change is temporary, and that the different conditions will prevail for periods of less than one hour only, and no more than half the time period in aggregate. The codes may be followed by a time period in hours and minutes. The time periods given may be preceded by FM meaning from, TL meaning until, or AT meaning at. Here's an example: TEMPO FM1020 TL1220 1000 +SHRA. This translates as: temporarily, from 1020Z to 1220Z, the visibility will reduce to 1000 metres, in heavy showers of rain. The + before SHRA means heavy, SH means showers, and RA is rain. If there is no expected change in the meteorological conditions being forecast by the METAR, the code NOSIG is used. NOSIG indicates that no significant change is expected in the next two hours. So a complete METAR with all these elements could look like this: METAR EGTK 231020Z 26012G25KT 220V300 9999 –RA FEW060 SCT120 10/M02 Q0991 RETS WS ALL RWY NOSIG That's the full picture — from the basic METAR structure through recent weather, windshear reporting, and the TREND forecast with BECMG, TEMPO, and NOSIG.