I think I remember seeing somewhere that since it's an observation at the surface, statute miles are used for the visibility. Would be nice if someone had a reference.
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1$\begingroup$ In Europe, it's in meters. $\endgroup$ – assylias Jul 30 '18 at 10:50
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1$\begingroup$ ...not just in Europe, practically everywhere outside America. Thus, lacking the country-specific tag, metres should be the correct default answer. $\endgroup$ – Zeus Jul 31 '18 at 6:03
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The FAA has this handy chart to decode METARs (which are the printed version of the ATIS) and TAFs.
Prevailing visibility; in U.S., Statute Miles & fractions; above 6 miles in TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as required, lowest value with direction)
