turtlehurricane Posted January 7, 2011 Share Posted January 7, 2011 Since the visibility at night in the middle of nowhere is basically zero to the human eye, how is visibility defined by airports and ASOS? Do they use a laser or something? I bring this up since quarter mile visibility is known as extremely low, yet it feels like when quarter mile visibility is being reported it's basically a whiteout and you can't see even that far. It makes me wonder how useful our current metrics are. Link to comment Share on other sites More sharing options...
Hokie7 Posted January 8, 2011 Share Posted January 8, 2011 If there are human observers, they use various landmarks around the aerodrome to determine vis...light towers, bldgs, terrain, signs etc to judge the vis. This is generally very accurate and reliable. The automated systems do use lasers as you suggested. They are extremely accurate, but can be slow to react to quickly changing conditions as they generally use an average of the previous ten minutes. Link to comment Share on other sites More sharing options...
patrick7032 Posted January 8, 2011 Share Posted January 8, 2011 Since the visibility at night in the middle of nowhere is basically zero to the human eye, how is visibility defined by airports and ASOS? Do they use a laser or something? I bring this up since quarter mile visibility is known as extremely low, yet it feels like when quarter mile visibility is being reported it's basically a whiteout and you can't see even that far. It makes me wonder how useful our current metrics are. It is best to try to look at obs that are not automated (AUTO) to get a good idea. Last night I had obs with a vis down to 3SM and had -SN BR where as ASOS was "trying" to report 9SM and no snow or obscurations....also most airfields have only one sensor unless they are a large airport such as I am....where I have two sensors for ASOS....even then the equipment can miss stuff....being slow to respond as Hokie7 stated, but also reading low....thus you may be at 1/16 actual vis when the local automated ASOS may be as high as 3/4SM. Link to comment Share on other sites More sharing options...
patrick7032 Posted January 8, 2011 Share Posted January 8, 2011 If there are human observers, they use various landmarks around the aerodrome to determine vis...light towers, bldgs, terrain, signs etc to judge the vis. This is generally very accurate and reliable. The automated systems do use lasers as you suggested. They are extremely accurate, but can be slow to react to quickly changing conditions as they generally use an average of the previous ten minutes. Agreed....you hit it exactly....earlier it took ASOS about 14 minutes (if not longer) to drop down close to the vis I was reporting and even then it showed 1/2SM and I my obs shown below but to be honest I almost went down to 1/8SM....even my RVR was low and it is on the runway that has a terminal on each side of it near where the sensor for the RVR is located (shielded some). KDTW 072153Z 31004KT 1/4SM R04R/2200V3500FT +SN FZFG BKN014 BKN026 OVC033 M07/M08 A2935 RMK AO2 TWR VIS 1/2 SNB24 SLP949 P0002 T10671083 Link to comment Share on other sites More sharing options...
turtlehurricane Posted January 8, 2011 Author Share Posted January 8, 2011 Thanks guys, that clears it up. Link to comment Share on other sites More sharing options...
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