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5 Answers

Understanding Prevailing Visibility

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Student Pilot, Weather

"Prevailing visibility is the horizontal distance over which objects or bright lights can be seen and identified over at least half of the horizon circle."

I'm simply trying to understand how exactly prevailing visibility is measured, but I don't know what "half of the horizon circle" means. Could any explain?

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5 Answers



  1. Kris Kortokrax on Jan 05, 2015

    Stand facing any direction.
    Start turning left or right.
    When you have rotated 180 degrees, that is half the horizon circle.

    A circle is 360 degrees.

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  2. Drew on Jan 05, 2015

    Could you give me an example of how that would be used to get a specific SM value?

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  3. Kris Kortokrax on Jan 05, 2015

    Following definition is from AC 00-6A Aviation Weather.

    “prevailing visibility – In the U.S., the greatest horizontal visibility which is equaled or exceeded throughout half of the horizon circle; it need not be a continuous half.”

    Interesting that it doesn’t need to be continuous.

    Best bet on how to determine how a specific SM value is derived would be to contact NWS. I would guess that the observer has noted identifiable objects at certain distances and uses those to arrive at the visibility. This however is just a guess.

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  4. Drew on Jan 06, 2015

    Thanks, Kris.

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  5. Scott on Jan 06, 2015

    At manual weather observation stations, you break your horizon into about 8 different sectors(and can do sector visibility). And then start using landmarks in each sector to figure out the visibility there. Then you can rank order them, or go to the 5th lowest sector’s value, and that’s your visibility that you report. It’s been a long time since I’ve done it (so a few of the values might be off a bit), but if any sectors are below 3 miles, then you’re supposed to report each individual sector’s visibility(that’s different from the prevailing) in the remarks of the METAR… so you can do VIS E 2 SE-SW 1 1/2 W 5 NW-NE 3 (In this case, prevailing would be 2 SM, and the E 2 would be omitted from the remarks). The directions are always reported in a clockwise direction starting with N.

    Automated stations just use a sensor of some kind that measures the visibility at the site. The sensor has been described to me as using some detector(laser or similar) to shoot a beam over ~1 foot, and is computed off of that. These may or may not be checked by a human to ensure that it’s being reported accurately.

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