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CHANGE INDICATOR TO A NEW SET OF MEAN CONDITIONS
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Civil Aviation Safety Authority Australia
PRE-FLIGHT PLANNING METEOROLOGY
09/2001
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AVIATION FORECASTS
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ORIGINATION TIME
The origination time of a TAF is expressed in a six figure group, followed by the
abbreviation “Z”
VALIDITY TIME(S)
The time of an aerodrome weather report is expressed in a four figure group followed by
the abbreviation “Z”. The period of validity of an aerodrome forecast is expressed as a
four figure hour group. UTC to hour UTC.
WIND INFORMATION
Wind direction is given in three figures relating to True North.
When the wind is calm, it is encoded as “00000KT”.
Wind speeds from 1 to 9KT, inclusive, are given in two figures; eg. 5KT is given as 05KT.
Variable wind direction is given as “VRB” and is used when the reporting of a mean wind
direction is not possible, such as:
• In light windy conditions (3KT or less), or
• The wind is veering or backing by 180o or more (eg, passage of a thunderstorm, or
localised wind effect)
Maximum wind speed is given only when it is 10KT or more greater than the mean wind
speed and the mean wind speed is greater than or equal to 15KT. The term “MAX” is not
included, the letter “G” followed by the maximum wind speed is used; eg 280° mean
speed 20KT, maximum speed 35KT, is given as 28020G35KT.
USE OF THE TERM "CAVOK"
"CAVOK" is included in the report or forecast when the following conditions are
observed, or forecast to occur simultaneously:
• visibility 10KM or more
• no cloud below 5,000FT or below the highest minimum sector altitude, whichever
is the greater, and no cumulonimbus; and
• no precipitation, thunderstorm, shallow fog, low drifting snow or dust devils.
Whenever a total of BKN (ie more than 4/8) low or middle cloud cover is present at
or above 5000FT, and CAVOK has been used, cloud amount and base are given.
VISIBILITY
In METAR/SPECI or TAF, the minimum visibility observed OR forecast is always given.
In METAR/SPECI, if the minimum visibility covers more than half the aerodrome,
or when visibility is fluctuating rapidly and significant directional variations cannot be
given, the minimum visibility is the only visibility information reported.
METAR/SPECI visibility will have a directional variation indicated when the minimum
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Civil Aviation Safety Authority Australia
PRE-FLIGHT PLANNING METEOROLOGY
09/2001
2
AVIATION FORECASTS
SECTION
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visibility is less than 5,000M and the visibility in another direction, covering more than
half the aerodrome, is at least 50% greater. Under these conditions, the minimum
visibility will be given first, with the direction indicated by one of the eight points of the
compass, followed by the higher visibility, without a compass point. 1000N 9999
WEATHER
Weather is given using the codes listed on page134. One or more of the codes may be
grouped eg TS or TSGR, SH or SHRA.
There is an option to describe the intensity of the weather which is only used with the
precipitation codes DZ, RA, SN, SH , or TS. In these cases, the weather group is
prefixed by (-) for light, and (+) for heavy. Moderate intensity has no prefix.
METAR/SPECI may provide an indication of weather in the vicinity. If this is included,
one or more of the weather groups on page 134 may be used, preceded by the
abbreviation “VC”.
CLOUD
Cloud height is always given as a three figure group in hundreds of feet, with the last
two digits omitted; eg: cloud at 700 feet is shown as 007.
Cloud information is reported from the lowest to the highest layer or mass in
accordance with the following:
• the lowest layer or mass, regardless of amount, as FEW, SCT, BKN or OVC as
appropriate
• the next layer or mass, covering more than 2/8, as SCT, BKN or OVC as appropriate
• the next higher layer or mass, covering more than 4/8, as BKN or OVC as
appropriate; and
• cumulonimbus and/or towering cumulus clouds, whenever observed, and not
reported in the above.
The cloud type will be identified only for cumulonimbus and towering cumulus when
observed at or near the aerodrome. These will be given as “CB” and “TCU”
　
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