AMOSSG/3-SN No. 9

2/1/01

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

ANALYSIS OF ANNEX 3 VISIBILITY REQUIREMENTS,
DEFINITION OF PREVAILING VISIBILITY

(Presented by Michel Leroy)

SUMMARY
This study note discusses Annex 3, Section 4.6, concerning visibility reports in the METAR/SPECI code forms. It particularly deals with the problem of directional variations in visibility. It proposes a definition of prevailing visibility taking in account these problems.

1.	INTRODUCTION

1.1	Annex 3 (Amendment 72 or draft Amendment 73) refers to visibility with directional variations in reports in the METAR/SPECI code forms. These variations are defined in reference to the site of the meteorological station.

1.2	Annex 3, paragraph 4.6.3 recommends that visibility observations should be representative of the aerodrome and its vicinity.

1.3	It will be shown that this recommendation is incompatible with the recommendation to report directional variations.

2.	DISCUSSION

2.1

2.1.1	If the visibility is homogeneous over the aerodrome and its vicinity (which is hopefully mainly the case), any observation point is representative.

2.1.2	If the visibility is not homogeneous, the word "representative" is not defined. For this situation, the current wording of Annex 3 recommends to give special attention on "significant directional variations".

2.2	Significant directional variations

2.2.1	Directional variations are referring to the site of the meteorological station. It is a very pragmatic wording considering what a human observer can do. The Annex 3 wording is such that a human observer reports what he/she sees. But, what a human observer sees is not the whole aerodrome and its vicinity:

—	If the human observer is located at ground level, it is impossible to view the whole aerodrome and its vicinity (8 km away the aerodrome limits?). There always exist obstructions to vision;

—	If the human observer is located well above ground, the observation is biassed: shallow fog may not be considered;

—	In real world, it never exists visible objects or lights at regular distance from the observer, all over the aerodrome and its vicinity. Some directions or areas are better covered than others.

2.2.2	Obstruction to human observer vision is not the main problem of directional variations. The main problem is to refer to the meteorological station:

—	A direction in reference to a single point cannot be representative of an area of at least 10 km around the centre of the airport;

—	Suppose that the observer reports 300. That means that he/she sees nothing beyond 300 m. The conditions beyond 300 m in any direction are unknown. The observer may be in a fog patch. Large area of the airport may be free of fog. The observation cannot be representative of the whole aerodrome;

—	Worst. Suppose that a fog bank is located 2000 m away from the observer (or an advection fog is coming from south). The observer sees an object 2000 m far, no object beyond. In fog, the visibility may be 300 m, but he/she reports 2000 m. And the current wording of Annex 3 requests him/her to report 2000S, even if a visibility sensor, located in the south area of the airport reports the true visibility of this area, 300 m. Such a circumstance can be very dangerous. Hopefully, for landing, it will not be this visibility, which will be reported to the reported to the pilot, but a visibility representative of the landing area (if possible based on instrumental values, if the observer is unable to see the landing area);

—	The observer cannot know the visibility conditions beyond the last object visible. For low visibility values, he/she cannot see beyond a short distance;

—	The meteorological station is never located at the centre of the aerodrome. And the centre of the aerodrome is not defined. For a small airport, it could be the centre of the runway (examples used to illustrate the US concept of prevailing visibility consider this). But for a large airport with several runways?

—	Visibility reports with directional variation use direction relative to the meteorological station, not direction relative to the airport. South of the observer is not necessarily south of the airport, especially for low visibility values. South of the observer may be north of a runway.

2.3	Why to report directional variations in METAR/SPECI ?

2.3.1	METAR/SPECIs are used for flight planning and by meteorological forecasters. The reports represent actual conditions at the time of observation, not the final conditions during approach. Significant directional variations of visibility mainly exist during meteorological transition periods: an arrival of advection fog, a dissipation of low visibility conditions. These periods are not static periods. Visibility conditions usually change quite quickly and the reported situation in a METAR has a strong probability to have changed one hour after. The usefulness of directional variations reporting is probably the indication of changing and non-homogeneous visibility conditions. Consequently, very strong doubts are expressed about the interest of the directions indicated. See below some examples of METAR reports with directional visibility indications. These examples have been selected just because they include directional variations. They show that the indication of direction is valid only at the time of the METAR observation and can hardly be used even half an hour later.

LFPG 220900Z 03005KT 9999 -RA FEW008 BKN036 BKN090 08/08 Q1012 NOSIG=

LFPG 220830Z 04004KT 3000NW -RA VCFG FEW006 SCT038 BKN090 08/08 Q1012 TEMPO 0800 BCFG BKN003=

LFPG 220800Z 03003KT 0900N 9999SW R27/P1500 R09/0700VP1500U R28/P1500 R10/P1500 -RA BCFG FEW006 SCT031 BKN040 08/08 Q1012 TEMPO 0800 BCFG BKN003=

LFPG 220730Z 00000KT 2000 R09/1100VP1500D -RA VCFG FEW032 SCT042 OVC090 08/08 Q1012 NOSIG=

LFPG 222130Z 27005KT 0250 R27/0300D R09/0250N R28/P1500 R10/0400N FG BKN000 04/04 Q1016 BECMG 0200 FG VV///=

LFPG 222100Z 22004KT 3000W BR FEW000 BKN013 06/05 Q1016 NOSIG=

LFPG 222100Z 20003KT 9999 R10/1100VP1500 BKN013 06/06 Q1016 NOSIG=

LFPG 120500Z 32003KT 0250 R27/0300N R09/0300V0500N R28/P1500 R10/P1500 FG BKN001 BKN100 05/05 Q0998 TEMPO 0200 FG=

LFPG 120430Z 30004KT 1400 R27/1400VP1500 R09/P1500 R28/P1500 R10/P1500 MIFG FEW004 BKN100 05/05 Q0997 TEMPO 0800 FG=

LFPG 120400Z 16002KT 1100SW MIFG FEW002 BKN100 06/06 Q0998 TEMPO 0800 FG=

LFPG 120330Z VRB03KT 0900 BCFG FEW002 BKN100 05/05 Q0998 TEMPO 0800 FG=

LFPG 120300Z 00000KT 3500 MIFG FEW002 BKN100 05/05 Q0998 TEMPO 1500=

LFPG 030230Z 01008KT 0600W R27/P1500 R09/0350N R28/0800N R10/P1500 FZFG VV/// M01/M01 Q1016 NOSIG=

LFPG 030200Z 02009KT 0600E R27/0500N R09/0300N R28/0600U R10/1400VP1500 FZFG VV/// M01/M02 Q1016 NOSIG=

LFPG 030130Z 02008KT 0600W R27/0400D R09/0250N R28/0650V1000U R10/P1500 FG VV/// M01/M01 Q1016 NOSIG=

LFPG 030100Z 01007KT 0400 R27/1200VP1500U R09/0250N R28/0500N R10/P1500 FG VV/// M01/M01 Q1016 NOSIG=

LFPG 030030Z 01007KT 0350 R27/0500D R09/0250N R28/0500N R10/0700V1400U FG VV/// M00/M01 Q1016 NOSIG=

2.4	Conclusion on directional variations

2.4.1	All these facts show that directional variations of visibility are very difficult to report and may be strongly misleading, when reported from the observers location. Hopefully, for the final approach of an aeroplane, an instrumented RVR (visibility) system will give the real RVR (visibility) value for the touchdown zone. The control tower will indicate RVR or the visibility representative of the landing area.

2.4.2	The current wording of Annex 3 (paragraph 4.6.5) does not adequately express the needs concerning the observation and report of visibility in METAR/SPECI. The wording is such that an observer may make the observation. With this wording, a point measurement from an instrument or even multi-point measurements from several instruments (which could regularly cover the aerodrome) cannot fulfil paragraph 4.6.5 recommendations. Instruments could indicate low visibility values, but the observer has to report what he/she sees, which may be much larger if the fog bank is not located over the meteorological station. This is quite paradoxical and potentially dangerous, especially with the current tendency, supported by the performance of the sensors, to install more and more visibility sensors and to decrease the human observations.

2.5	Proposal for a definition of prevailing visibility

2.5.1	The introduction of prevailing visibility should be the occasion to get rid of the current wording problems of annex 3 concerning visibility. It should be the occasion to introduce visibility information really useful in the METAR/SPECI:

—	Information of visibility spatially representative of the aerodrome;

—	Information about possible spatial variation of visibility over the aerodrome (and its vicinity): minimum visibility, without directional information, which could not be representative;

2.5.2	Prevailing visibility could be defined as the value, which is reached or exceeded at least within half of the surface of the aerodrome.

2.5.3	A human observer, without the aid of instruments, would estimate prevailing visibility from directional observations.

2.5.4	On an aerodrome with several visibility instruments, prevailing visibility would be calculated from instrumental values (value reached or exceeded by at least one half of the sensors).

2.5.5	On an aerodrome with a human observer and visibility instruments, prevailing visibility would be derived from directional observations and instrumental values to get the best estimation of the aerodrome conditions.

2.6	Proposed definition for prevailing visibility

2.6.1	"Prevailing visibility. The prevailing visibility is the value which is reached or exceeded at least within half the surface of the aerodrome.

Note. -- This value may be assess from human observation and/or instrumented systems. When several instruments are installed, they are used to get the best estimate of the prevailing visibility."

2.7	Proposed new wording for paragraph 4.6.10 of Annex 3

2.7.1	"4.6.10 Recommendation. In reports in the METAR/SPECI code forms, prevailing visibility should be reported. When the visibility is not the same in different aerodrome locations and when the lowest visibility is less than 1500 m and less than 50 per cent of the prevailing visibility, the lowest visibility should be also reported."

3.	ACTION BY THE GROUP

3.1	The group is invited to:

a)	note the content of the paper;

b)	discuss the proposal for a new definition of prevailing visibility; and

c)	discuss the proposal for a new wording for paragraph 4.6.10.

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