Today’s lesson was at 9am, so after a bowl of cereals I set off for the airfield. Derek called while I was en route to say the weather wasn’t good enough for flying so the lesson would have to be cancelled. Disheartened I decided to continue to the flying school anyway, maybe I could meet other students or instructors and chat about the highs and lows of learning to fly.
Andrew, one of the owners of the club was in reception when I arrived. He offered me a cup of tea and a lesson in METAR and TAF weather reports as consolation for not being able to fly. Firstly, I realised I’d been pronouncing METAR wrong, it’s not [met·are], but [meat·are]. We went into one of the briefing rooms and looked up the latest weather reports. Today’s information was as follows:
METAR EGGD 120820Z 22010KT 9999 FEW008 BKN015 15/14 Q1016
TAF EGGD 120503Z 1206/1306 21010KT 9999 SCT010
BECMG 1206/1209 BKN006
TEMPO 1206/1221 5000 RA
PROB30 TEMPO 1206/1221 1200 RADZ BR BKN001
BECMG 1211/1214 27009KT
PROB30 1221/1303 BKN015
PROB30 TEMPO 1303/1306 3000 RADZ BR BKN003
Andrew said the best place to learn about the METAR information was on the Met Office website, you need to sign up for a free account to access their General Aviation section, once logged in you can access the METAR decode information. I’ve also found this METAR Tutorial helpful.
TAF uses the same codes as METAR, so it’s worth learning the general format even if you
need to lookup specific details from time-to-time. Andrew explained that all TAF reports
cover a certain period, so the first thing to check was the first line to ensure it is
relevant to the time you’re interested in. Today’s report started 120503Z 1206/1306
which means it was issued on the 12th at 05:03 (UTC/GMT/Z) e.g. 6:03am in the UK because
of daylight saving time, and covers the period from the 12th at 06:00 (the 1206 part)
through to the 13th at 06:00 (the 1306 part), which translates to 7am today ‘till 7am on
tomorrow.
The rest of the first line (e.g. 21010KT 9999 SCT010) indicates the base weather the
remainder of the report is based on. So in this instance it means the wind is coming from
210 degrees at 10 knots, visibility is 10 km or more (the 9999 part is the visibility in
meters, and it’s limited to 4 characters, so if it’s all nines then you round it up to 10
km and assume it is at least this visibility, if not more) and there are scattered clouds
at 1,000 feet. That doesn’t look too bad, why was my lesson cancelled?
Well, that’s the next line, the BECMG 1206/1209 BKN006. This means the original forecast
is “becoming” broken clouds at 600 feet sometime between 7am and 10am today. By the way,
all these heights are relative to airfield elevation, so broken clouds at 600 feet is 600
feet above the airport. Broken cloud is actually quite a lot of cloud, so having it at
just 600 feet above the runway is a sure sign no VFR (Visual Flight Rules) flying would be
happening. There was no mention of the visibility or wind in this line, so these values
remain the same as they were in the first line, e.g. the wind is still coming from 210
degrees at 10 knots and the visibility is still 10 km or more.
Next Andrew explained the line TEMPO 1206/1221 5000 RA. This is called a “temporary”
which means it is forecast to happen for at least one hour, but not more than half the
total time indicated. So, we are expecting a visibility of 5,000 metres and rain sometime
after 7am and before 10pm on the 12th for anything between 1 and 7½ hours. There
was no mention of cloud cover, but the previous “becoming” did change the cloud cover
between 7am and 10am. So if this “temporary” happens before 10am then the cloud cover is
expected to be “broken at 600 feet”, if it’s after 10am, it is expected to revert to the
original report and be “scattered at 1,000 feet”. There’s also no mention of wind in the
“temporary” so that stays as originally reported, e.g. 210 degrees at 10 knots.
The next line PROB30 TEMPO 1206/1221 1200 RADZ BR BKN001 has two modifiers, a
“probability 30” and a “temporary”. Apparently there are only two types of probability
that would be used in a TAF, PROB30 and PROB40. The former means that it might happen,
the latter means it almost certainly will. As to why they chose 30 to mean “maybe” and 40
to mean “certainly” I have no idea. So this line translates to something like “between 7am
and 10pm on the 12th the weather may change for between 1 and 7½ hours to a
visibility of 1,200 metres, a drizzle of rain, mist and broken clouds at 100 feet above
the airport”. There was no mention of wind, so it will remain coming from 210 degrees at
10 knots, as mentioned in the first line. This is very poor flying weather, clouds at just
100 feet, rain, mist, and visibility of just over a kilometer, no wonder my lesson was
cancelled.
The next line is another “becoming” BECMG 1211/1214 27009KT this only affects the wind
speed and direction and means that between midday and 3pm on the 12th the wind is forecast
to be coming from 270 degrees at 9 knots. The last two lines had times past sunset, so we
didn’t bother decoding them, but it’s easy now to see how they would be decoded if I could
fly at night.