Gday!

Referring to the June 2010 met book, page 7.14 has a TAF decode for YPAD in which the visibility is stated as '9999'.

In the decode it says "the least visibility forecast is 10km or more"

Just curious as to why it is "the least" visibility, as according to AIP GEN3.5-para12.7.1:
"In TAF the prevailing visibility (the greatest visibility covering more than half the aerodrome) is always given.

Maybe I am overthinking this but it would I would love some clarification as to weather (sic) TAF/ARFOR/METAR etc visibility is stipulated as being least/average/greatest vis.


Cheers!

Ok

answered my own question about the first bit, it says minimum visibility because 9999 = vis 10km or greater.

Still interested about further classification of visibility for the TAF/ARFOR/METAR though, yes I am splitting hairs!

It's all about 10km which is used as the standard below which visibility is considered to be an issue. If the visibility is LESS THAN 10km the greatest visibility is the one that matters. It is telling you that the visibility restriction may have operational significance, and it could be worse than reported.

9999 is used when the visibililty is AT LEAST 10km or greater. That means it wont be a significant operational issue. [in other words 'no worries mate!].

Bob

Thanks for the reply Bob,

Just to clarify, what would be the answer to this hypothetical question:

TAF ........... 5000 ...... (only visibility shown)

The visibility forecast for this aerodrome is

A)5000m
B)Less than 5000m
C)More than 5000m

Gday,

Vis would be 5km according to any (if any) conditions applicable. Usually a visibility like 5000 would be accompanied by 'SHRA' or similar.

Sorry if I have cut anyone's grass..

Cheers

Sorry to bring this one back, but I still have a little misunderstanding.

In excercise OPM1 Page 7.6 it states:

7000

The minimum visibility reported is 7000m or 7km.

However I note that GEN3.5 para 12.7 "In TAF the prevailing visibility the greatest visibility covering more than half the aerodrome) is always given"

I still dont get it

Well that's a surprise! The AIP did say [up to about a year ago] that in a TAF the minimum visibility is always given. You are correct, that has now been changed to the maximum visibility prevailing over more than half the horizon is always given.

Note that in a report such as a METAR or SPECI, if the visibility is not the same in all directions and the minimum visibility applies to more than half the horizon, then the minimum visibility will be reported.

Thanks for pointing that out. It will be changed in the next print of the book.