Visibility reports in METAR observations (also applies to TAF's etc.)

On the 25th November, there will be a significant change in the way the
visibility group(s) in a METAR is(are) determined. (Though I understand
that this will NOT apply, for the time being, to observations from UK
military airfields).

Outside of North America, it has always been the practice to report the
*lowest* visibility as the primary coded value in a METAR (and other
reports/forecasts for aviation, e.g. TREND, TAF), no matter what
proportion of the observing 'horizon' was affected.

This will change such that the value given will be the 'prevailing
visibility', defined as the value that it reached or exceeded within at
least half of the surface of the aerodrome (or the horizon circle).
These areas could be contiguous, or non-contiguous sectors.

For the lowest (worst) visibility on an airfield (or within the
immediate vicinity) to be reported under this new regime, it will have
to meet certain criteria - principally, the value would have to be below
1500 m, or less than half of the 'prevailing' value, as found above.
Such variations will produce an additional group after the prevailing
visibility entry, e.g. 7000 2000SW BR MIFG etc.

This means that whereas we've got used to assuming that a METAR will
*always* provide the worst visibility to be found, there may be
occasions when, although a single figure is given, there is poorer
visibility that does not meet the 'additional' reporting criteria. For
example, in the previous paragraph, if there had been no shallow fog and
the visibility to the SW was 4000, then only the '7000 BR' would appear.

I have updated the text within the uk.sci.weather FAQ suite of pages to
reflect these changes. Specifically, for the change as it affects the
METAR, find the decode page at:-

http://homepage.ntlworld.com/booty.w...TAR_decode.htm

and for background, see the note issued by the CAA at:-

http://www.caa.co.uk/docs/33/FOD200419.pdf
(a PDF file)

I have extracted the following from this latter for ease of reference:-

" 1.2.1 Operators are familiar with situations in which the actual
aerodrome visibility is equal to or greater than that given in the TAF,
TREND or METAR. With the advent of 'prevailing visibility' this may no
longer be the case and, in future, the visibility encountered could be
significantly less than that reported."

Martin.

FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm

"Martin Rowley" wrote in message
...

Outside of North America, it has always been the practice to report the
*lowest* visibility as the primary coded value in a METAR (and other
reports/forecasts for aviation, e.g. TREND, TAF), no matter what
proportion of the observing 'horizon' was affected.

Out of interest, the Austrians have used prevailing visibility for some
time, there's an example at Salzburg from today:

METAR LOWS 201250Z 23010KT 9999 4000SE -SHSN SCT025 BKN035 00/M02 Q1019
TEMPO 1200 SHSN SCT007 BKN012 16550195=

Mind you it looks like Exeter may have had a false start by a week...

METAR EGTE 190920Z 00000KT 9999 6000NE VCSH FEW015 SCT025 04/03 Q1023=

There are a number of states who already seem to be using prevailing vis
too:

ENBS 160720Z VRB02KT 9999 3000N VCSH FEW004 SCT0013 SCT050 M05/M06 Q0985=

Well today's the day. I've only had a quick look but interestingly haven't
seen any prevailing vis. examples yet.

However, Bristol are testing out NSC (nil sig. cloud) in readiness for it's
implementation next autumn..
EGGD 251350Z 17005KT 7000 NSC OVC058 10/08 Q1022 =

Also impressed with the accuracy of cloud reporting at Manchester :-

EGCC 251350Z 19005KT 9000 BKN074 10/07 Q1021 NOSIG =
EGCC 251320Z 16008KT 9000 BKN074 10/07 Q1021 NOSIG =
EGCC 251250Z 15008KT 9000 BKN075 10/07 Q1021 NOSIG =
EGCC 251220Z 16008KT 8000 BKN073 10/07 Q1021 NOSIG =
EGCC 251120Z 20005KT 6000 BKN072 09/07 Q1022 NOSIG =

Hohum.

Jon.

EGCC 251350Z 19005KT 9000 BKN074 10/07 Q1021 NOSIG =
EGCC 251320Z 16008KT 9000 BKN074 10/07 Q1021 NOSIG =
EGCC 251250Z 15008KT 9000 BKN075 10/07 Q1021 NOSIG =
EGCC 251220Z 16008KT 8000 BKN073 10/07 Q1021 NOSIG =
EGCC 251120Z 20005KT 6000 BKN072 09/07 Q1022 NOSIG =

that looks like international standard ;-))
LOWI 250250Z VRB01KT 9999 FEW016 BKN029 01/M01 Q1030 NOSIG=
LOWI 250320Z 00000KT 9999 FEW016 BKN029 00/M00 Q1030 NOSIG=
LOWI 250350Z 00000KT 9999 FEW016 SCT029 M00/M01 Q1030 NOSIG=
LOWI 250420Z 00000KT 9999 FEW016 SCT029 M01/M02 Q1030 NOSIG=
LOWI 250450Z VRB01KT 9999 FEW014 M01/M02 Q1030 NOSIG=
LOWI 250520Z 31002KT 9999 FEW014 M01/M02 Q1030 NOSIG=
LOWI 250650Z 29002KT 9999 FEW010 SCT300 M01/M02 Q1030 NOSIG=
LOWI 250720Z VRB01KT 9999 FEW010 SCT300 M01/M02 Q1030 NOSIG=
LOWI 250750Z VRB01KT 9999 FEW010 M01/M02 Q1030 NOSIG=
LOWI 250820Z VRB03KT 9999 FEW010 00/M01 Q1030 NOSIG=
LOWI 250850Z VRB02KT 9999 FEW012 01/M01 Q1030 NOSIG=
LOWI 250920Z VRB01KT 9999 FEW017 02/M01 Q1029 NOSIG=
LOWI 250950Z VRB02KT 9999 FEW020 02/M02 Q1029 NOSIG=
[INNSBRUCK / AUSTRIA]
regardswolfgang ex loww, vie