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A typical METAR or TAF reads something like this:

VRB04KT P6SM SCT025 BKN040 OVC060

P6SM means "greater than 6 statute miles visibility." Why in particular is it 6 miles--why not P7SM or P10SM? Particularly since sometimes METARs and TAFs list actual visibilities greater than 6 statute miles anyway:

28006KT 10SM CLR

Is 6 miles a limitation of commonly used equipment for measuring visibility, or something else?

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The METAR format is defined by the World Meteorological Organization's (WMO) on behalf of ICAO. According to the specification (section 15.6.3), visibility is to be reported in meters. The upper bound for visibility reporting is 9999, which indicates the visibility exceeds 10 kilometers.

In North America, the METARs deviate from the standard format by using non-standard units. Visibility is reported in statue miles. Six statue miles is equivalent to 10 kilometers.

Hence > 6 statue miles is equal to the standard > 10 kilometers.

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    $\begingroup$ Note that automated systems use P10SM, but reports generated by an official weather observer sometimes report more, such as 20SM or 50SM, so I believe that it is a limit of the measuring systems in use. The AWOS wiki also states that it can measure up to 10 SM. $\endgroup$ – Lnafziger Jan 6 '15 at 16:09
  • $\begingroup$ @Lnafziger that's because ICAO uses 4 digits of meters (not km) and USA uses 4 digits of statute miles (not feet), so the US can report up to 9998sm, and ICAO can report upto 9998m. $\endgroup$ – rbp Jan 6 '15 at 16:44
  • $\begingroup$ My comment was meant to address that sometimes you will see METAR reports with something greater than 10 miles, but not from automated systems. He asked if it was an equipment limitation, and based on that I'm guessing that it is. $\endgroup$ – Lnafziger Jan 6 '15 at 18:52
  • $\begingroup$ @Lnafziger There are automatic reporting systems that exceed 6SM: KBLH 061852Z AUTO 00000KT 10SM CLR 16/M04 A3029 RMK AO2 SLP256 T01611044 $\endgroup$ – DeltaLima Jan 6 '15 at 19:26
  • $\begingroup$ You are correct! ASOS & AWOS actually report up to 10SM. It is the TAF which is limited to 6SM. I will updated my original comment and delete the extras. $\endgroup$ – Lnafziger Jan 6 '15 at 20:23
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The reason its reported as "greater than 6 statue miles" is because 6 statute miles is the equivalent of 10 kilometers. ICAO reports METAR visibility in meters, and reserves 4 places in the coded METAR for the number of meters. With only 4 places, 9999 is the largest value, and this corresponds to "10km or more."

Reference: Aerodrome forecast - TAF decode

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P6SM - "P" = Plus 6 Statute Miles...in conversational writing we are more used to seeing the plus sign "after" the number, as in 6+, but that's what P6 means. Usually anything less than 7 miles requires an explanation why the "VSBY" (visibility) is less than 7, so there will be an obstruction of some kind included, such as 3SM BR (BR is French, "Brume", mist).

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