Page 138 - UK ATM ANS Regulations (Consolidated) 201121

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

disseminated in the TAF code form.
(c) The period of validity of a routine TAF shall be either 9 or 24 or 30 hours, unless otherwise
prescribed by the competent authority taking into account the traffic requirements for
aerodromes with hours of operation of less than 9 hours. TAF shall be filed for
transmission not earlier than 1 hour before the commencement of their period of validity.
(d) TAF, if disseminated in digital form, shall be:
(1) formatted in accordance with a globally interoperable information exchange model
and shall use geography markup language (GML);
(2) accompanied by the appropriate metadata.
(e) The meteorological elements included in TAF shall be:
(1) Surface wind
(i) In forecasting surface wind, the expected prevailing direction shall be given.
(ii) When it is not possible to forecast a prevailing surface wind direction due to
its expected variability, the forecasted wind direction shall be indicated as
variable
(iii) When the wind is forecasted to be less than 1 kt (0,5 m/s), the forecasted
wind speed shall be indicated as calm.
(iv) When the forecasted maximum speed exceeds the forecasted mean wind
speed by 10 kt (5 m/s) or more, the forecasted maximum wind speed shall
be indicated.
(v) When a wind speed of 100 kt (50 m/s) or more is forecasted, it shall be
indicated to be more than 99 kt (49 m/s).
(2) Visibility
(i) When the visibility is forecasted to be less than 800 m, it shall be expressed
in steps of 50 m; when it is forecasted to be 800 m or more, but less than 5
km, in steps of 100 m; when it is forecasted to be 5 km or more, but less than
10 km, in kilometre steps; and when it is forecasted to be 10 km or more, it
shall be expressed as 10 km, except when conditions of CAVOK are
forecasted to apply. The prevailing visibility shall be forecasted.
(ii) When visibility is forecasted to vary in different directions and the prevailing
visibility cannot be forecasted, the lowest forecasted visibility shall be given.
(3) Weather phenomena
(i) One or more, up to a maximum of three, of the following weather phenomena
or combinations thereof, together with their characteristics and, where
appropriate, intensity, shall be forecasted if they are expected to occur at the
aerodrome:
(A) freezing precipitation;
(B) freezing fog;
(C) moderate or heavy precipitation (including showers thereof);
(D) low drifting dust, sand or snow;
(E) blowing dust, sand or snow;
(F) dust storm;
(G) sandstorm;
(H) thunderstorm (with or without precipitation);
(I) squall;
(J) funnel cloud (tornado or waterspout);
(K) other weather phenomena, as agreed by the aerodrome meteorological
office with the ATS units and operators concerned.
(ii) The expected end of occurrence of those phenomena shall be indicated by
the abbreviation 'NSW'.
(4) Cloud
(i) The cloud amount shall be forecast using the abbreviations 'FEW', 'SCT',
'BKN' or 'OVC', as necessa ry. When it is expected that the sky will remain or
become obscured and clouds cannot be forecasted and information on
vertical visibility is available at the aerodrome, the vertical visibility shall be
forecasted in the form 'VV' followed by the forecasted value of the vertical
visibilitiy.
(ii) When several layers or masses of cloud are forecasted, their amount and
height of base shall be included in the following order:
(A) the lowest layer or mass regardless of amount, to be forecasted as
FEW, SCT, BKN or OVC as appropriate;
(B) the next layer or mass covering more than 2/8, to be forecast as SCT,
BKN or OVC as appropriate;
(C) the next higher layer or mass covering more than 4/8, to be forecast as
BKN or OVC as appropriate;
(D) cumulonimbus clouds and/or towering cumulus clouds, whenever
forecasted and not already included under points (A) to (C).
(iii) Cloud information shall be limited to cloud of operational significance; when
no cloud of operational significance is forecasted and 'CAVOK' is not
appropriate, the abbreviation 'NSC' shall be used.
(f) Use of change groups
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