AMOSSG/3-SN No. 8

2/1/02

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

THIRD MEETING

Miami, 22 to 25 January 2002

Agenda Item	5:	Review of the requirements for meteorological observations and reports at aerodromes contained in Chapter 4 of Annex 3

PREVAILING VISIBILITY

(Presented by Wil van Dijk )

SUMMARY
This study note discusses the observing and reporting of prevailing visibility.

1.	OBSERVING AND REPORTING VISIBILITY (ANNEX 3, PARAGRAPH 4.6, 2001 EDITION)

1.1	In the 15 years the stated recommendations on observing and reporting visibility are modified significantly and frequently. Typically, very specific details are entered (e.g. inside paragraphs 4.6.3 to 4.6.5). For practical reasons and simplicity, the reporting of visibility should be restrictive in terms of functional requirements to be "operational visibility (for aeronautical purposes)", in line with the first part of paragraph 4.6.3 (Amendment 73: paragraph 4.6.2). Within this context, clear regulations can be stated with regards to the needs and functional requirements of the aeronautical user. Since "operational visibility" is to be interpreted as a variable typically for operational practices, the requirements can be formulated in a simple form.

1.2	In the 2001 Edition of Annex 3, paragraph 4.6, however, the variable "visibility (for aeronauticalpurposes)" is transformed in "prevailing visibility" and with functional requirements in terms of an extensive number of details (e.g. end of paragraph 4.6.3 (Amendment 73, paragraph 4.6.3) and 4.6.5 (Amendment 73, paragraph 4.6.10)). As a consequence the observation techniques (by human beings or by instruments) will give rise to confusion and false interpretation by the user. Moreover, it is not obvious if "prevailing visibility" (especially from paragraph 4.6.5 (Amendment 73, paragraph 4.6.10) can be used straightforward in operational practices.

1.3	A point of concern is the desire to incorporate some "forecast" information in an observation, i.e. to combine the actual visibility on the runway and some lower visibility occurrences in the proximity of the runway (e.g. incoming fog patches). It should be stated that a clear distinction has to be made between the actual situation and the situation in the near future (the expected trend). In principle the actual situation should be given as observation, without any "forecast" details. In terms of operational visibility such a statement implies that this variable will be based on visibility, measured on or along the runway and, if possible, in its touch-down area, i.e. in front of the runway.

2.	RECOMMENDATION

2.1	To improve the clarity and to reduce any confusion with respect to the presentation of observed visibility, the variable visibility should be based only on the measurements along and if possible in front of the runway. The use of existing RVR systems is a cost-effective option, presuming that the observing ranges are sufficient.

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