Hi,
Might be an obvious one, but i am wondering what, if any, requirements there would be for an alternate if a TAF provided two different visibility observations, one which wouldn't necessitate an alternate and the other that would.
Would the requirement be to use the worst vis or the best vis?
Thanks for the help
Andrew
Hi Andrew,
A TAF will only specify a single visibility figure which will be valid at any time and this visibility will represent the prevailing visibility (i.e. covering more than half the aerodrome). The visibility may change of course and the TAF will then include a FROM, BECMG, TEMPO or INTER as well as the time(s) these changes apply. You will need to apply appropriate time buffers to these changes when determining whether you need an alternate or can simply hold. See the Operational Meteorology section for more details on this.
Multiple visibilities may be given in a METAR (or SPECI) where the visibility in one direction is less than 5000m but you will not use a METAR (or SPECI) when determining the need for an alternate since it is a report and not a forecast.
Cheers,
Rich
