Fog is a form of a cloud and it is said clouds are visible moisture. We all know that mist (baby rain) is also moisture so I am curious when will a METAR say BR (mist) or FG (fog)?
Quoting from the METAR decoder:
You may ask "Why 5/8th of a statute mile?" That's because 5/8ths of a mile is 1,006 meters, or about 1 kilometer.
The main difference between the fog and mist is their density and as a consequence, the visibility. From UK Met office:
This is the same one given in FAA Aeronautical Information Manual (Page 7-1-62 in Dec 2015 Version). For visibility below 5/8 statute miles (~1000m), the term fog is used, while for values above that, mist is used.
Image from FAA AIM, page 7-1-62