AMOSSG/5-SN No. 9

11/3/05

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

FIFTH MEETING

Montreal, 11 to 14 April 2005

Agenda Item	5:	Aerodrome observation requirements

PREVAILING VISIBILITY

(Presented by A. Wells)

SUMMARY
Prevailing visibility has been successfully introduced in UK civil aeronautical meteorological observations. During the implementation process, several issues have been identified on the practical interpretation of prevailing visibility. This paper outlines these issues and suggests minor changes to some of the definitions and terms used in ICAO Annex 3.

1.	INTRODUCTION

1.1	The concept of prevailing visibility became incorporated into ICAO Annex 3 during November 2004 as part of Amendment 73 to Annex 3. This method of reporting visibility in METARs and SPECIs replaced the previous methodology of reporting minimum visibility.

1.2	UK airports have successfully adopted this new practice into their METAR reporting regime.

1.3	Feedback has been positive, however, several issues have been identified with the operational use of prevailing visibility. These issues are outlined below, together with suggestions for minor changes to definitions and terms used in Annex 3.

2.	DISCUSSION

2.1	The definition of prevailing visibility, as defined in Annex 3, Chapter 1 is as follows:

"Prevailing visibility is the visibility value, observed in accordance with the definition of "visibility", which is reached or exceeded within at least half the horizon circle or within at least half of the surface of the aerodrome. These areas could comprise contiguous or non-contiguous sectors."

2.1.1	It is possible to draw an incorrect conclusion from this definition. Namely, that the "visibility value,…., which is reached of exceeded within half the horizon circle….", could in essence define any visibility that covers at least 180° of the circle.

2.1.2	By inserting the word "greatest" into the definition, the definition becomes less likely to be misinterpreted, i.e.:

Prevailing visibility is the greatest visibility value, observed in accordance with the definition of "visibility", which is reached or exceeded within at least half the horizon circle or within at least half of the surface of the aerodrome. These areas could comprise contiguous or non-contiguous sectors.

2.2	During the implementation of prevailing visibility, a number of requests for clarification on the practical interpretation of prevailing visibility were received. The most common issues raised were:

a)	reporting of minimum visibility in the METAR. Consider the situation where a prevailing visibility of 25km and a minimum visibility of 12km to the West are observed. One interpretation of how this should be coded in the METAR is to report "9999 9999W". However, the benefit of such a report is not clear. The UK has stated in its Aeronautical Information Publication (AIP) that,"There is no requirement to report the minimum visibility if it is 10km or more".

b)	use of CAVOK with prevailing visibility. Consider the situation where a prevailing visibility of 12km and a minimum visibility of 8km to the West is observed. Clearly, this is coded as 9999 in the METAR. When coinciding with the other defining CAVOK criteria, CAVOK is reported. However, clearly some portion of the local vicinity is affected by visibility less than 10km (i.e. not meeting CAVOK criteria). The UK has stated in its AIP that, "The applicability of CAVOK shall be based on the minimum visibility, if reported in the METAR, otherwise on the prevailing visibility".

c)	minimum visibility of less than 1500 metres. When the lowest visibility is different from the prevailing visibility and less than 1500 metres, both visibility values (one prevailing and one minimum) are reported, even when the difference between these values is small, for example "1300 1200W". It was found that a number of observers required assurance that such cases were required to be reported in this way.

d)	in the UK, Air Traffic Control Special VFR (Visual Flight Rules) clearances will only be considered when the meteorological visibility is 5000 metres or more (note that the ICAO definition of Special VFR, detailed in Procedures for Air Navigation Services Air Traffic Management — PANS ATM ICAO Doc. 4444, carries no such visibility restriction). Consider the situation where a prevailing visibility of 6km and a minimum visibility of 3500 metres is observed. This is reported in the METAR as "6000". Procedures have been agreed with the UK Met Authority that certain aerodromes will report any minimum visibility below 5000 metres in the local meteorological report.

3.	ACTION BY THE GROUP

3.1	The group is invited to:

a)	note the issues raised in the paper; and

b)	consider whether any changes to Annex 3 are required in the light of the issues raised in paragraph 2(i), paragraph 2(ii) a) and b).

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