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past 10 minutes shall be reported when the maximum
wind speed exceeds the mean speed by 20 km/h (10 kt)
or more;
d) when a wind speed of less than 2 lan/h (1 kt) is
reported, it shall be indicated as calm;
e) when a wind speed of 200 lan/h (100 kt) or more is
reported, it shall be indicated to be more than 199 lan/h
(99 kt); and
f) when the 10-minute period includes a marked discontinuity
in the wind direction andlor speed, only variations
from the mean wind direction and mean wind speed
occurring since the discontinuity shall be reported.
Note.- See note under 4.1.3.1.
4.1.4.3 In local routine and special reports:
a) if the surface wind is observed from more than one
location along the runway, the locations for which these
values are representative shall be indicated;
b) when there is more than one runway in use and the
surface wind related to these runways is observed, the
available wind values for each runway shall be given,
and the runways to which the values refer shall be
reported;
c) when variations from the mean wind direction are
reported in accordance with 4.1.4.2 b) 2), the two
extreme directions between which the surface wind has
varied shall be reported; and
d) when variations from the mean wind speed (gusts) are
reported in accordance with 4.1.4.2 c), they shall be
reported as the maximum and minimum values of the
wind speed attained.
4.1.4.4 In METAR and SPECI, when variations from the
mean wind speed (gusts) are reported in accordance with
4.1.4.2 c), the maximum value of the wind speed attained shall
be reported.
4.2 Visibility
4.2.1 Siting
Recommendation.- When instrumented systems are used
for the measurement of visibility, representative visibility
observations should be obtained by the use of sensors appropriately
sited. Sensors for visibility observations for local
routine and special reports should be sited to give the best
practicable indications of visibility along the runway and
touchdown zone. The visibility should be measured at a height
of approximately 2.5 m (7.5 ft) above the runway.
4.2.2 Displays
Recommendation.- When instrumented systems are used
for the measurement of visibility, visibility displays relating to
each sensor should be located in the meteorological station
with corresponding displays in the appropriate air traffic services
units. The displays in the meteorological station and in
the air traffic services units should relate to the same sensors,
and where separate sensors are required as specified in 4.2.1,
25/11/04 APP 3-4
Appendix 3 Annex 3 - Meteorological Service for International Air Navigation
the displays should be clearly marked to identrfj, the area, e.g. b) when there is more than one runway in use and the
runway and section of runway, monitored by each sensor visibility is observed related to these runways, the available
visibility values for each runway should be
4.2.3 Averaging
reported, and the runways to which the values refer
should be indicated.
4.2.4.4 Recommendation.- In METAR and SPECI, Recommendation.- When instrumented systems are used visibility should be reported as prevailing visibility, as defined
for the measurement of visibility, their output should be in Chapter When the visibility is not the same in dllfwnt
updated at least every 60 secondr to permit provision of directions and
current representative values. The averagingperiod should be:
a) when the lowest visibility is drferentfhm the prevailing
a) I minute for local routine and special reports and for visibility, and less than 1 500 m or less than 50 per cent
visibility displays in air trafic services units; and of the prevailing visibility, the lowest visibility observed
should also be reported and its general direction in reb)
I0 minutes for METAR and SPECI, except that when the lation to the aerodrome indicated by reference to one of
I0-minute period immediately preceding the observation the eight points of the compass. If the lowest visibility is
includes a marked discontinuity in the visibility, only observed in more than one direction, then the most
those values occurring afier the discontinuity should be operationally significant direction should be reported;
used for obtaining mean values. and
Note.- A marked discontinuity occurs when there is an b) when the visibility is fluctuating rapidly, and the
abrupt and sustained change in visibility, lasting at least prevailing visibility cannot be determined, only the low-
2 minutes, which reaches or passes through criteria for the est visibility should be reported, with no indication of
issuance of SPECI reports given in 2.3. direction.
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