Page 126 - UK ATM ANS Regulations (Consolidated) 201121

Part MET - ANNEX V - Specific Requirements for the Providers of Meteorological
Services

cumulonimbus only.
MET.TR.205(e)(1) AMC2 Reporting of meteorological elements
CLOUD AUTOMATIC OBSERVING SYSTEM
When an automatic observing system is used to report local routine reports and local special reports
and METAR:
(a) when the cloud type cannot be observed, the cloud type in each cloud group should be
replaced by '///'';
(b) when no clouds are detected, it should be indicated by using the abbreviation 'NCD';
(c) when cumulonimbus clouds or towering cumulus clouds are detected and the cloud
amount and/or the height of cloud base cannot be observed, the cloud amount and/or the
height of cloud base should be replaced by '///'; and
(d) when the sky is obscured and the value of the vertical visibility cannot be determined due
to a temporary failure of the system/sensor, the vertical visibility should be replaced by '///'.
MET.TR.205(e)(3) AMC1 Reporting of meteorological elements
CLOUD HEIGHT OF CLOUD BASE
At aerodromes where low-visibility procedures are established for approach and landing, as agreed
between the meteorological station and the appropriate ATS unit, in local routine reports and local
special reports, the height of cloud base should be reported in steps of 50 ft up to and including 300 ft
(90 m) and in steps of 100 ft (30 m) between 300 ft (90 m) and 10 000 ft (3 000 m), and the vertical
visibility in steps of 50 ft (15 m) up to and including 300 ft (90 m) and in steps of 100 ft (30 m)
between 300 ft (90 m) and 2 000 ft (600 m).
MET.TR.210 Observing meteorological elements
The following meteorological elements shall be observed and/or measured with specified accuracy
and disseminated by automatic or semi-automatic meteorological observing system.
Surface wind direction and speed
(a)
(a) Surface wind direction and speed
The mean direction and the mean speed of the surface wind shall be measured, as well
as significant variations of the wind direction and speed (gusts), and reported in degrees
true and knots, respectively.
(1) Siting
The meteorological instrument used to measure surface wind direction and speed
shall be situated in such a way as to provide data which is representative of the
area for which the measurements are required.
(2) Display
Surface wind displays relating to each sensor shall be located in the meteorological
station. 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, the
displays shall be clearly marked to identify the runway and section of runway
monitored by each sensor.
(3) Averaging
The averaging period for surface wind observations shall be:
(i) 2 minutes for local routine report and local special report and for wind
displays in ATS units;
(ii) 10 minutes for METAR, except that when the 10-minute period includes a
marked discontinuity in the wind direction and/or speed; only data occurring
after the discontinuity shall be used for obtaining mean values; hence, the
time interval in these circumstances shall be correspondingly reduced.
(b) Visibility
(1) The visibility shall be measured or observed, and reported in metres or kilometres.
(2) Siting
The meteorological instrument used to measure visibility shall be situated in such a
way as to supply data which is representative of the area for which the
measurements are required.
(3) Displays
When instrumented systems are used for the measurement of visibility, visibility
displays relating to each sensor shall be located in the meteorological station. 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, the displays shall
be clearly marked to identify the area monitored by each sensor.
(4) Averaging
The averaging period shall be 10 minutes for METAR, except that when the
10minute period immediately preceding the observation includes a marked
discontinuity in the visibility, only those values occurring after the discontinuity shall
be used for obtaining mean values.
(c) Runway visual range (RVR)
(1) Siting
The meteorological instrument used to assess the RVR shall be situated in such a
way as to provide data which is representative of the area for which the
observations are required.
(2) Instrumented systems
Instrumented systems based on transmissometers or forwardscatter meters shall
be used to assess RVR on runways intended for Categories II and III instrument
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