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in the meteorological conditions which reach or pass
specified threshold values and last for a period of less than
one hour in each instance and, in the aggregate, cover less
than one-half of the forecast period during which the fluctuations
are expected to occu,: If the temporary fluctuation is
expected to last one hour or longer, the change group
"BECMC;" should be used in accordance with 1.3.3 or the
validity period should be subdivided in accordance with 1.3.5 .
1.3.5 Recommendation.- Where one set of prevailing
weather conditions is expected to change significantly and
more or less completely to a dgffrent set of conditions, the
period of validity should be subdivided into self-contained
periorLF using the abbreviation "FM" followed immediat+y by
a four-figure time group in whole hours and minutes UTC
indicating the time the change is expected to occur The subdivided
period following the abbreviation "FM" should be
self-contained and all forecast conditions given before the
abbreviation should be superseded by those following the
abbreviation.
1.4 Use of probability groups
Recommendation.- The probability of occurrence of an
alternative value of a forecast element or elements should be
indicated, as necessary, by use of the abbreviation "PROB"
followed by the probability in tens of per cent and the time
period during which the alternative valuets) is (are) expected
to apply. The probability information should be placed after
the element or elements forecast and be followed by the
alternative value of the element or elements. The probability of
a forecast of temporary fluctuations in meteorological
conditions should be indicated, as necessary, by use of the
abbreviation "PROB" followed by theprobability in tens of
per cent, placed before the change indicator "TEMPO" and
associated time group. A probability of an alternative value or
change of less than 30 per cent should not be considered
suficiently significant to be indicated. A probability of an
alternative value or change of 50per cent or more, for aviation
purposes, should not be considered a probability but
instead should be indicated, as necessary, by use of the change
indicators "BECMG" or "TEMPO" or by subdivision of the
validity period using the abbreviation "FM". The probability
group should not be used to qualifjt the change indicator
"BECMG" nor the time indicator "FM".
1.5 Numbers of change and probability groups
Recommendation.- The number of change and probability
groups should be kept to a minimum and should not
normally exceed five groups.
1.6 Dissemination of TAF
TAF and amendments thereto shall be disseminated to
international OPMET data banks and the centres designated by
regional air navigation agreement for the operation of
aeronautical fixed service satellite distribution systems, in
accordance with regional air navigation agreement.
2. CRITERIA RELATED TO
TREND FORECASTS
2.1 Format of trend forecasts
Trend forecasts shall be issued in accordance with the
templates shown in Appendix 3, Tables A3-1 and A3-2. The
units and scales used in the trend forecast shall be the same as
those used in the.report to which it is appended.
Note.- Examples of trend forecasts are given in
Appendix 3.
2.2 Inclusion of meteorological elements
in trend forecasts
2.2.1 General provisions
The trend forecast shall indicate significant changes in respect
of one or more of the elements: surface wind, visibility,
weather and clouds. Only those elements shall be included for
which a significant change is expected. However, in the case
of significant changes in respect of cloud, all cloud groups,
including layers or masses not expected to change, shall be
indicated. In the case of a significant change in visibility, the
phenomenon causing the reduction of visibility shall also be
indicated. When no change is expected to occur, this shall be
indicated by the term "NOSIG".
APP 5-3 2511 1104
Annex 3 - Meteorological Service for International Air Navigation Appendix 5
2.2.2 Surface wind
The trend forecast shall indicate changes in the surface wind
which involve:
a) a change in the mean wind direction of 60" or more, the
mean speed before andlor after the change being 20 kmih
(1 0 kt) or more;
b) a change in mean wind speed of 20 kmih (10 kt) or
more; and
2.2.4.2 The trend forecast shall indicate the expected
onset or cessation of one or more, up to a maximum of three,
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