This question may have been asked before, but I can't find it on my searches. When the METAR is reporting visibility of 9999M or 10SM, FS responds properly as unlimited. However if the actual visibility on the ground is 35 miles, what would AS report and what would FS then report. Can't quite understand the 10SM for unlimited when the actual visibility could be greater than 10SM but not unlimited.An explanation of the values would be greatly appreciated.Bob JohnsonDenver

Hi,AS would report it as 35SM, FS would report it at greater than 10SM.Pretty sure that 10SM is the standard value in the US to represent unlimited. Some may know more!Hope this helps,JimActiveSky Sales and Supporthttp://www.hifisim.com/images/asv_dev_team.jpg http://www.hifisim.com/images/asv_proud_supporter.jpg

Jim, thanks for the reply, I think you are right, but can you or Damian confirm that 10SM is the METAR standard for unlimited. It would seem very strange that the NWS metar report would have 10SM as unlimited, when it could be 35SM for real. I know that 9999M is unlimited.Bob

>Jim, thanks for the reply, I think you are right, but can you>or Damian confirm that 10SM is the METAR standard for>unlimited. It would seem very strange that the NWS metar>report would have 10SM as unlimited, when it could be 35SM for>real. I know that 9999M is unlimited.>>BobIt's not unlimited any more than SKC means that no clouds exist. What 10SM or 9999M means is that visibility is a minimum of 10SM or 9999M (TAF = P6SM) just as SKC in a METAR means "clear below 12,000 ft" but does not mean that there can't be an overcast layer at 12,500 ft. If you think about it, how often do you get unlimited visibilty except on very cold days in the winter when arctic high pressure systems come through (at least in the northeast US). During the summer, you may get 10SM, but not a whole lot more.

Neil today flying in Asia, I got a visibility of 4500 meter coming out ZSPD (Shaghai), weathr was not so good (smile). I understand all of what you said, just trying to get to the bottom of NWS visibility standard.Bob

Hi Bob,I searched around quite a bit at NWS and really did not find anything I could put my finger on. On their visibility charts they do list values higher than 10SM, but I don't know how much they are used and under what conditions. Even on the clearest of days in Arizona, with dry air and all, the common visibility is 10SM.Hope this helps,JimActiveSky Sales and Supporthttp://www.hifisim.com/images/asv_dev_team.jpg http://www.hifisim.com/images/asv_proud_supporter.jpg

Hi,SKC is not "clear below 12000 ft", this should be reported as CLR. SKC should be present ONLY if the sky is clear at all altitudes (or at least till the highest altitude measurable by the ceilometer) and this is confirmed by a certified observer (not an automated station). If an automated station is reporting CLR, it is true that an OVC layer could existed above 12000ft.Regarding visibility, here again the automated stations are normally limited to 7 or 10SM, while the human observer can state 12 or 15 or 35 or 50 or whatever he sees (well, not exactly "whatever", the Federal Meteorological Handobook states which visibilties should be reported).A good link for the FMH is:http://www.nws.noaa.gov/oso/oso1/oso12/fmh1/fmh1toc.htm

HLM65, you're correct. My point was that a METAR doesn't report any clouds above 12,000, but you're correct that I should have used CLR and not SKC.