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b) runway visual range, when all changes of one or more
steps on the reporting scale in use are being reported to
the local air traffic services unit by an observer on the
aerodrome.
Local special reports shall also be made available to the
operators and to other users at the aerodrome.
4. OBSERVING AND REPORTING OF
METEOROLOGICAL ELEMENTS
Introductory Note.- Selected criteria applicable to
meteorological information referred to under 4.1 to 4.8 for
inclusion in aerodrome reports are given in tabular form at
Attachment D.
4.1 Surface wind
4.1.1 Siting
4.1.1.1 Recommendation.- Su$ace wind should be
observed at a height of approximately 10 m (30ft) above the
runway(s).
4.1.1.2 Recommendation.- Representative su$ace wind
observations should be obtained by the use of sensors appropriately
sited. Sensors for su$ace wind observations for local
routine and special reports should be sited to give the best
practicable indication of conditions along the runway and
touchdown zones. At aerodromes whei-e topography or pmvalent
weather conditions cause signzjicant dzflerences in su$ace
wind at various sections of the runway, additional sensors
should be provided.
Note.- Since, in practice, the sugace wind cannot be
measured directly on the runway, su$ace wind observations
for take-ofland landing are expected to be the best practicable
indication of the win& which an aircraft will encounter during
take-ofland landing.
4.1.2 Displays
4.1.2.1 Surface wind displays relating to each sensor shall
be located in the meteorological station with corresponding
displays in the 'appropriate air traffic services units. The
displays in the meteorological station and in the air traffic services
units shall relate to the same sensors, and where separate
sensors are required as specified in 4.1.1.2, the displays shall
be clearly marked to identify the runway and section of runway
monitored by each sensor.
4.1.2.2 Recommendation.- The mean values of; and
significant variations in, the su$ace wind dimction and speed
for each sensor should be derived and displayed by automated
equipment.
4.1.3 Averaging
4.1.3.1 The averaging period for surface wind observations
shall be:
a) 2 minutes for local routine and special reports and for
wind displays in air traffic services units; and
b) 10 minutes for METAR and SPECI, except that when
the 10-minute period includes a marked discontinuity in
the wind direction andlor speed, only data occuning
since the discontinuity shall be used for obtaining mean
values, hence the time interval in these circumstances
should be correspondingly reduced.
APP 3-3
Annex 3 - Meteorological Service for International Air Navigation Appendix 3
Note.- A marked discontinuity occurs when there is an
abrupt and sustained change in wind direction of 30" or more,
with a wind speed of 20 k d h (10 kt) before or ajer the
change, or a change in wind speed of 20 km/h (10 kt) or more,
lasting at least 2 minutes.
4.1.3.2 Recommendation.- The averaging period for
measuring variations fi.om the mean wind speed ( p t s )
reported in accordance with 4.1.4.2 c) should be 3 seconds for
local routine and special reports and for METAR and SPECI
and for wind displays used for depicting variationsfi.om the
mean wind speed (gusts) in air traffic services units.
4.1.4 Reporting
4.1.4.1 In local routine and special reports and in
METAR and SPECI, the surface wind direction and speed
shall be reported in steps of 10 degrees true and 1 kilometre
per hour (or 1 knot), respectively. Any observed value which
does not fit the reporting scale in use shall be rounded to the
nearest step in thescale.
4.1.4.2 In local routine and special reports and in
METAR and SPECI:
a) the units of measurement used for the wind speed shall
be indicated;
b) variations from the mean wind direction during the past
10 minutes shall be reported as follows, if the total
variation is 60" or more:
1) when the total variation is between 60" and 180" and
the wind speed is 6 km/h (3 kt) or more such directional
variations shall be reported as the two extreme
directions between which the surface wind has
varied;
2) when the total variation is between 60" and 180" and
the wind speed is less than 6 km/h (3 kt), the wind
direction shall be reported as variable with no mean
wind direction; or
3) when the total variation is 180" or more, the wind
direction shall be reported as variable with no mean
wind direction;
c) variations from the mean wind speed (gusts) during the
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