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
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?