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of the following weather phenomena or combinations thereof:
- ice crystals
- freezing fog
-low drifting dust, sand or snow
- blowing dust, sand or snow
- thunderstorm (with or without precipitation)
- squall
- funnel cloud (tornado or waterspout).
c) changes in the wind through values of operational 2.2.4.3 The expected end of occurrence of the weather
significance. The threshold values should be established phenomena shall be indicated by the abbreviation "NSW'.
by the meteorological authority in consultation with the
appropriate ATS authority and operators concerned,
taking into account changes in the wind which would: 2.2.5 Clouds
1) require a change in runway(s) in use; and
2) indicate that the runway tailwind and crosswind
components will change through values representing
the main operating limits for typical aircraft operating
at the aerodrome.
2.2.3 Visibility
When the visibility is expected to improve and change to or
pass through one or more of the following values, or when the
visibility is expected to deteriorate and pass through one or
more of the following values: 150, 350, 600, 800, 1 500 or
3 000 m, the trend forecast shall indicate the change. When
significant numbers of flights are conducted in accordance
with the visual flight rules, the forecast shall additionally
indicate changes to or passing through 5 000 m.
When the height of the base of a cloud layer of BKN or OVC
extent is expected to lift and change to or pass through one or
more of the following values, or when the height of the base
of a cloud layer of BKN or OVC extent is expected to lower
and pass through one or more of the following values: 30, 60,
150, 300 and 450 m (100, 200, 500, 1 000 and 1 500 ft), the
trend forecast shall indicate the change. When the height of the
base of a cloud layer is below or is expected to fall below or
rise above 450 m (1 500 ft), the trend forecast shall also indicate
changes in cloud amount from SKC, FEW, or SCT
increasing to BKN or OVC, or changes from BKN or OVC
decreasing to SKC, FEW or SCT. When no cumulonimbus and
no cloud below 1 500 m (5 000 ft) or below the highest minimum
sector altitude, whichever is greater, are forecast and
"CAVOK" and "SKC" are not appropriate, the abbreviation
"NSC" shall be used.
2.2.6 Vertical visibility
Note.- In trend forecasts appended to local routine and When the sky is expected to remain or become obscured and
special reports, visibility refers to the forecast visibility along vertical visibility observations are available at the aerodrome,
the runwayts); in trend forecasts appended to METAR and and the vertical visibility is forecast to improve and change to
SPECI, visibility refers to the forecast prevailing visibility. or pass through one or more of the following values, or when
the vertical visibility is forecast to deteriorate and pass through
one or more of thd following values: 30, 60, 150 or 300 k
2.2.4 Weather phenomena
2.2.4.1 The trend forecast shall indicate the expected
onset, cessation or change in intensity of one or more, up to a
maximum of three, of the following weather or
combinations thereof:
- freezing precipitation
- moderate or heavy precipitation (including showers
thereof)
- duststorm
- sandstorm
- other weather phenomena given in Appendix 3, 4.4.2.3,
only if they are expected to cause a significant change in
visibility.
(100,200, 500 or 1 000 ft), the trend forecast shall indicate the
change.
2.2.7 Additional criteria
Criteria for the indication of changes based on local aerodrome
operating minima, additional to those specified in 2.2.2 to
2.2.6, shall be used as agreed between the meteorological
authority and the operator(s) concerned.
2.3 Use of change indicators
2.3.1 When a change is expected to occur, the trend
forecast shall begin with one of the change indicators
"BECMG" or "TEMPO".
25/11/04 APP 5-4
Appendix 5 Annex 3 - Meteorological Service for International Air Navigation
2.3.2 The change indicator "BECMG shall be used to
describe forecast changes where the meteorological conditions
are expected to reach or pass through specified values at a
regular or irregular rate. The period during which, or the time
at which, the change is forecast to occur shall be indicated,
using the abbreviations "FM", "TL", or "AT", as appropriate,
each followed by a time group in hours and minutes. When the
change is forecast to begin and end wholly within the trend
forecast period, the beginning and end of the change shall be
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