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The current METAR for Stanstead in the UK says:

EGSS 031520Z AUTO 29011KT 9999 FEW021/// //////TCU 07/02 Q1022

I do not know how to interpret the /// and //////. From what I remember multiple slashes means that data is missing, however the data in this metar seems complete.

  • What does FEW021/// mean?
  • TCU means Towering Cumulous, which makes sense, but what do the leading ////// mean in this context?
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Complete, unless you want to know the height of base of those TCU's!

The bulk of the information you're after can be found in AIS-GEN 3.5 Section 10.17 - AUTO METAR coding

In describing the coding used by automated systems, it gives this example

NNNhhh/// Cloud detected, but it is unknown whether it is a convective cloud type (ie TCU or CB)

And this one

//////TCU Towering Cumulus detected, but no information on its height or cloud cover is available

Which more-or-less covers your two questions, I hope.

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It means that the report is from an automated weather station and information from sensors are not available for visibility etc.

From AIP Australia:

12.20.1 A report from a fully automated AWS that does not include information from sensors for visibility, weather or cloud will report ////, // or ////// respectively in lieu of these parameters.

For this METAR, it can be seen that there are a few clouds at 2100 (with further data missing) and there is towering cumulus, with further data (cloud amount and height missing). For example, decoding this METAR at flightutilities.com gives:

Location: EGSS

LONDON - STANSTED - ENGLAND

Report emitted the day: 03, time 15:20 UTC

Wednesday 3 February 2016 20:50 local time.

Report is fully automated, with no human intervention or oversight.

Wind: True direction = 290 degrees, speed: 11 knots (20 km/h) (6 m/s).

Minimum horizontal visibility: 10 km or more.

Clouds: A few (1-2 oktas), at 2100 feet above aerodrome level (640 meters), but it is unknown whether it is a convective cloud type.

Clouds: cloud type not reported, the cloud base is under the station level or not reported, towering cumulus.

Temperature: 7 degrees Celsius (45 Fahrenheit). Dewpoint: 2 degrees Celsius (36 Fahrenheit). Relative humidity 70.47 %. Wind Chill: 4 degrees Celsius (38 Fahrenheit).

QNH (Sea-level pressure): 1022 hPa (30.18 inches).

(I've removed some data and added emphasis).

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Ooh I like that Flight Utilities link - I'm terrible at deciphering METAR, that should help, thanks. (Edit: Oh, it's too clever and actually goes to look up the METAR rather than purely decoding it, damn - useful, but I was hoping to use it a little more literally. Still handy though) – Jon Story Feb 3 at 16:42
    
It looks like that reference for Australia doesn't match how the slashes are used in the U.K. (both instances of the slashes in the question are cloud-related and the triple slash isn't even listed as an option in that AIP quote, for example.) – reirab Feb 3 at 17:54

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