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AIC
REPUBLIC OF SEYCHELLES CIVIL AVIATION AUTHORITY AERONAUTICAL INFORMATION SERVICE P.O.BOX 181, VICTORIA SEYCHELLES
02/2010 01 NOV 2010
IMPLEMENTION OF PREVAILING VISIBILITY IN METAR, TAF, SPECIAL REPORTS, TREND REPORTS 1.
Amendment 73 to ICAO Annex 3, Standards and Recommended Practices – Meteorological Service for International Navigation details changed to the way in which visibility is reported and forecast. This will affect the visibility reporting used in METAR, TAF, Special Reports and TREND Reports. This change is being implemented in Seychelles on 01 December 2010.
2.
Definitions
2.1
ICAO defines prevailing visibility in the following way: ‘Prevailing visibility is the visibility value that 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
2.3
The ICAO Annex 3 Amendment 73 proposal for the observing and reporting of prevailing visibility is as follows:
the prevailing visibility shall be reported;
if the visibility in one direction, which is not the prevailing visibility, is less than 1500 m or less than 50% of the prevailing visibility, the lowest visibility observed shall also be reported and its general direction in relation to the aerodrome indicated by reference to one of the eight points of the compass. If the lowest visibility is observed in more than one direction, then the most operationally significant direction shall be reported (this should take into consideration the runway in use);
when the visibility is fluctuating rapidly and the prevailing visibility cannot be determined, only the lowest visibility shall be reported, with no indication of direction.
Examples of how prevailing visibility observations and reporting will be made are given at the Annex to this document.
3.
Prevailing Visibility Observations and Reporting
3.1
Forecasters will forecast the prevailing visibility in TREND forecasts and TAFs. However, when the visibility is forecast to vary in different directions and the prevailing visibility cannot be forecast, only the lowest visibility will be included.
3.2
MET Observers and Forecasters will use the new visibility reporting with effect from 0001 on 01 December 2010.
4.
Application of Prevailing Visibility to operational decisions
4.1
Additional guidance is being provided to operators of aircraft (that operate under an (AOC) and to organisations that provide Air Traffic Services.
Mrs C. Maria (AISO) For: Chief Executive Officer Seychelles Civil Aviation Authority
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ANNEX A.
ASSESSING PREVAILING VISIBILITY 1
Mentally divide the horizon circle into as many sectors of equal visibility as needed to measure the differing visibilities, as in the following example: Good visibility in this sector
Visibility affected by rain in thedistance in this sector 12 Km
4 Km
8 Km
6 Km Visibility affected by smoke in this sector
Prevailing Visibility = 8 kilometers 2
Prevailing visibility is the value which is reached or exceeded within at least half the horizon circle or at least half of the aerodrome; in the above example, the visibility value which is reached within at least half the horizon circle is 8km (the area covered by the 8 km and 12 km visibility sectors).
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The lowest visibility is also reported in the METAR when it is less than 1500 m (providing it is not the prevailing visibility) or less than 50% of the prevailing visibility, along with its general direction. In the above case, the lowest visibility (4 km) is not reported, as it is not less than 50% of the prevailing visibility (meaning not less than half of 8 km).
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In the example below the visibility value which is reached within half of the horizon circle is 10 km. This is reported as the Prevailing Visibility. The lowest visibility of 3000 m is also reported as the minimum visibility as it is less than half (50%) of the prevailing visibility. Note that the minimum visibility is always reported with its general direction in relation to the aerodrome using the 8 compass points.
Good visibility in these sectors
Visibility affected by rain in this sector 10 Km
3000 m
10 Km
10 Km
Prevailing Visibility = 10 kilometers Minimum Visibility = 3000 m NE
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B.
Situation of Prevailing Visibility at Seychelles International Airport 1
Due to the prevalence of convective precipitation in Seychelles, situations in which aerodrome visibility in any direction could be significantly different are quite common. At present, only the lowest (minimum) visibility is reported which may not have any operational significance. In the example below visibility has been reduced to 2000 m due to convective showers over part of the northwestern sector whilst over 75% of the horizon circle, good visibility prevails. Under present reporting procedures the visibility to be reported in METARs would be 2000 m. In the context of Prevailing Visibility using the same example, a Prevailing Visibility of 10 km would be reported together with a minimum visibility of 2000 m with an indication of the general direction (NW). The significance of this for operational decisions is that whilst an approach to RWY 13 could be critical for a pilot, a RWY 31 approach poses no problem given other Met’ conditions such as wind are favourable. Low visibility (2000m) due to convective showers in the North West sector
Good visibility in these sectors 2000 m
10 Km
10 Km
10 Km
Prevailing Visibility = 10 kilometers Minimum Visibility = 2000 m NW
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