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What does it mean by flight visibility of 5km and horizontal separation from cloud 1500m in VMC? If a cloud is ahead of us at 1500m then how could the flight visibility be 5km?

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    $\begingroup$ Are you asking about a specific authority's definition, and/or in a specific class of airspace? $\endgroup$
    – Jamiec
    Commented Apr 29 at 13:29

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"Visibility" is taken to mean the farthest distance that can be seen in open air. It is affected by particles suspended in the air surrounding you, like smoke or dust or water vapor (mist/fog), or by larger precipitation falling through the air, like rain or snow.

Something like a tall building or a mountain or even a cloud being in your way does not count as a visibility obscuration. If you were inside the cloud then yes, the particles of the cloud would reduce the visibility. But when you're outside of the cloud you can treat it like a singular object.

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  • $\begingroup$ "the farthest distance that can be seen in open air." Yet it is typically never reported greater than 10nm in the US, which actually isn't that great. Also, maybe add the word "prevailing"? Good answer BTW! $\endgroup$ Commented Apr 29 at 14:32
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    $\begingroup$ @MichaelHall Seeing what actual 10 mile visibility looks like for the first time is quite shocking! ;) $\endgroup$
    – Chris
    Commented Apr 29 at 15:14
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    $\begingroup$ @chris, never more apparent than at sea, watching the tacan DME counting down and looking for "mom"... You knew exactly what the visibility was. $\endgroup$ Commented Apr 29 at 16:07
  • $\begingroup$ @MichaelHall the automated equipment maxes out at 10 statute miles and usually there isn't a real reason to edit it. Good enough for government work. At manual stations like KMWN it's common to see visibilities of up to 100SM. $\endgroup$
    – randomhead
    Commented Apr 29 at 16:32
  • $\begingroup$ Isn't automation great?! Anyway, maybe consider adding that to your answer and I will delete these comments to clean things up? $\endgroup$ Commented Apr 29 at 16:36
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The cloud that is 1500m away from you could be to your side. The 5km visibility standard is the farthest distance you can see.

The idea is simple, you need to be far enough away from the cloud so other traffic have a chance of not hitting you if you are hidden by that cloud. I don't know what rules you are referencing but a VFR plane could be 500m below the cloud and be legal and you are 500m above them past the cloud. When they look up, they will not be able to see you as you are obscured by the cloud. If they were to rapidly climb once past the cloud they could potentially hit you. The 1500m distance is designed to allow other pilots to see you with enough time to avoid hitting you.

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