METARs are routine aerodrome reports. Routine weather observations are
hour. They give two temperatures: e.g. 09 /07, where 09 is the actual temperature and 07 is the dewpoint temperature - the difference between them acting as a guide to the possibility of mist / fog that will occur if the temperature and the dewpoint become the same.
Trends (or Landing Forecasts)
It is common to attach a forecast TREND to an Aerodrome Report (i.e. an observation of actual weather) in order to improve the meteoro-logical information. The Trend Forecast indicates what the weather tendency for the next two hours is expected to be. It is valid only until two hours after the time of observation - a much shorter period than the duration of a normal Aerodrome Forecast (9 hours); so it must be more accurate. A TREND is commonly referred to as a landing fore-cast. If no significant change is expected, the ob-servation will be followed by the trend statement: NOSIG (no significant change).
Weather reports and trends for selected aerodromes are broadcast continuously on VHF frequencies. This service is called VOLMET.
The VOLMET broadcast for each aero-drome is updated each hour and half-hour and indicates:
„the actual weather report; „landing forecast;
„SIGMET (significant weather, if any); „the forecast trend for the two hours fol-
lowing the time of report.
Information is also obtained from the Au-tomatic Terminal Information Service (ATIS) - a tape-recorded message of the current aero-drome information and is broadcast on appropri-ate VOR or VHF frequencies to off-load the ATC communication frequencies.
Some aerodromes have both an arrival and departure ATIS.
CLOUDS
The amount of cloud coverage in Area Forecasts (and in TAFs and METARs) is ex-pressed in eights of the sky covered, or oktas.
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4 oktas means that half of the sky is covered by the cloud mentioned, and 8 oktas means complete sky coverage.
In Aerodrome Forecasts and Reports, the amount of cloud coverage is indicated by the following abbreviations:
KC - Sky clear (0 okta) FEW - Few (1- 2 oktas) SCT - Scattered (3 -4 oktas) BKN - Broken (5 - 7 oktas) OVC - Overcast (8 oktas)
Thunderstorms (TS), which are best avoided by aircraft, are associated with cumu-lonimbus (CB) clouds meaning hail, moderate or severe icing and/or turbulence. The amount of CB cloud in an area is indicated by the fol-lowing abbreviations:
ISOL Isolated for individual CB clouds OCNL Occasional - for well-separated CB clouds FRQ Frequent - for CB clouds with little
or no separation
EMBD Embedded - CB clouds contained in layers of other clouds
CAVOK
The term CAVOK is often used. It means that the following conditions occur simultaneous-ly:
„visibility 10 km or more;
„no cloud below 5,000 ft above aero-drome level (aal) or below the highest minimum sector altitude,
„no cumulonimbus; no significant weather phenomena at or in the vicinity of the aerodrome.
CHANGING WEATHER IN FORECASTS
Temporary change (TEMPO)
While the cumulonimbus cloud asso-ciated with a thunderstorm may exist for hours, its passage through the immediate vicin-ity of an aerodrome may take only a short pe-riod - less than 60 minutes, or an even shorter time. During those t emporary periods the weather in the vicinity of the aerodrome may be quite different when compared with the prevailing conditions.
In such a situation, the Aerodrome and Landing Forecasts will state the general condi-tions existing at the aerodrome (i.e. the pre-vailing conditions), and any temporary changes to the conditions will be indicated by the term TEMPO.
TEMPO. Temporary variation lasting less than 60 minutes, or lasting in total less than half the TREND (or TAF) period; that is, changes take place quite infrequently for the prevailing conditions.
Note: TEMPO can relate to improvements as well as deteriorations
in wind, visibility, weather or cloud.
Lasting changes
While TEMPO is used to indicate a tempo-rary variation from the prevailing weather, when lasting changes in the prevailing weather are fore-cast, the term BECMG (becoming) is used pre-ceding an expected permanent change in the weather conditions.
In an Aerodrome Forecast (TAF), BECMG will be followed by a four-figure time group; in a TREND Forecast that is attached to an aero-drome report (METAR), BECMG may be fol-lowed by a four-figure time group hours and mi-nutes preceded by one of the abbreviations: FM (from), TL (until) or AT (at).
Other descriptive terms that appear on Trend statements are:
NOSIG: no significant change, NSC: no signifi-cant cloud, and
NSW: no significant weather.
Probability. Sometimes the forecaster is uncer-tain of whether conditions will occur and, if he or she assesses the probability of them occurring as 50 % or less, the message may be prefaced with a PROB (probability) percentage.
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