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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)?

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    $\begingroup$ I always think of BR as "barely rain" but it actually come from the French word "Brume" meaning mist. cfidarren.com/r-metarmystery.htm $\endgroup$ – Mike Sowsun Mar 25 '16 at 16:47
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    $\begingroup$ Or it could be the French word "brouillard" => fog $\endgroup$ – Antzi Mar 25 '16 at 19:39
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    $\begingroup$ @Antzi Related $\endgroup$ – Pondlife Mar 26 '16 at 15:57
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    $\begingroup$ how would this be off-topic? O_o $\endgroup$ – Federico Apr 11 '16 at 12:23
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    $\begingroup$ I could see the question being off-topic if it stopped at the title question, but the body clearly ties in aviation $\endgroup$ – CGCampbell Apr 11 '16 at 13:29
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Quoting from the METAR decoder:

BR Mist (Foggy conditions with visibilities greater than 5/8 statute mile) 

FG Fog (visibility 5/8 statute mile or less)

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.

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The main difference between the fog and mist is their density and as a consequence, the visibility. From UK Met office:

... they are two distinct terms for a similar phenomenon. Visibility less than 1,000 metres we call 'fog' and obscurity with visibility greater than 1,000 metres we call 'mist'.

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.

Fog and mist

Image from FAA AIM, page 7-1-62

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    $\begingroup$ Visibility less than 1000 metres , classified as fog; visibility more than 1000 metres, classified as mist. These observations may have to be made at sea or ground level. It would be interesting to hear from aviators how visibility is classified at altitude - in particular: is a 'fog ceiling' a useful and recorded statistic. $\endgroup$ – Arif Burhan Mar 26 '16 at 2:13
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    $\begingroup$ What the heck is Snow Grains? $\endgroup$ – Burhan Khalid Mar 26 '16 at 13:06
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    $\begingroup$ @Burhan Khalid. Snow Grains are precipitation of very small, white, and opaque grains of ice. $\endgroup$ – wbeard52 Mar 26 '16 at 16:55

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