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Training to be an ATC on VATSIM. I live in the Boston area, so I'm training with VATUSA. In the US, METAR visibility data is the only exception to 'miles' = 'nautical miles' rule (that I've learned about/am aware of). All other use of 'miles' including speeds (knots) are in nautical miles.

It seems dangerous to have multiple standards for the same unit of measure, especially when something you want to underestimate is given in the smaller units (1NM = 1.15SM), meaning an error results in someone expecting greater visibility than they actually have (and possibly overestimating their ability to safely fly). I'm hopeful that there's a sensible reason for this, but I live in the US so I recognize it as hope as opposed to reasonable expectation.

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    $\begingroup$ Some other things related to weather are in statue miles as well e.g. § 91.155 Basic VFR weather minimums; Special VFR minimums; 123 Rule for IFR; Alternate visibility for IFR; Civil airport takeoff minimums.; Student Pilot visibility limitations. $\endgroup$
    – JScarry
    Feb 13, 2022 at 21:02
  • $\begingroup$ The reason I've heard in the past for this is "because visibility is referring to something on the ground". That is, as opposed to in the air (or on the water) where there is no ground reference so nautical miles are used. I am afraid I do not have a reference for this. $\endgroup$ Feb 13, 2022 at 21:10
  • $\begingroup$ My personal view is, that the incoherent use and mixing of standards and units (freedom units vs. oppression units etc.) is just another way to make things more complicated than they really need to be... There simply is no justification and logical reason to use all the possible units ever conceived. $\endgroup$
    – Jpe61
    Feb 14, 2022 at 8:47
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    $\begingroup$ Visibility is a fuzzy thing. The 15% difference between statue miles and nautical miles is smaller than the minute-to-minute (or even observer-to-observer) variation in measurement. $\endgroup$
    – Mark
    Feb 15, 2022 at 1:21
  • $\begingroup$ In the Midwest where rural roads are often spaced exactly 1 mile apart, statue miles may be useful. $\endgroup$
    – Jeff B
    Feb 16, 2022 at 4:37

2 Answers 2

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This page might help. Short answer appears to be “because that’s what pilots are used to.”

The U.S. METAR code is described in Federal Meteorological Handbook (FMH) No. 1 "Surface Observations and Reports", while the U.S. TAF code procedures used by the NWS are described in Weather Service Operations Manual Chapter D-31. Both of these standards are tailored to reflect existing longstanding U.S. national practices. For example, in order to lessen the burden on the U.S. aviation community, a number of exceptions to metric reporting units have been filed by the U.S. Winds will continue to be reported in knots (as opposed to meters per second), cloud layer heights, and runway visual range (RVR) will continue to be reported in feet (as opposed to meters), visibility will continue to be reported in statute miles (as opposed to meters), and altimeter settings will continue to be reported in inches of mercury (as opposed to hectoPascals).

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    $\begingroup$ I think you mean "because that’s what US pilots are used to*. Most of the rest of world would be used to kilometres or expect nautical miles. $\endgroup$
    – Bohemian
    Feb 14, 2022 at 14:29
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    $\begingroup$ Something like half of all pilots are US pilots. And, a large number of non-US pilots trained in the US. So, "what pilots are used to" applies on average to all pilots. $\endgroup$
    – Adam
    Feb 14, 2022 at 14:52
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    $\begingroup$ So, they use miles because pilots are used to it, and pilots are used to it because they use miles. Pretty much the same excuse as every case of the US not moving to metric! $\endgroup$
    – StephenS
    Feb 15, 2022 at 16:18
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The visibility is given in the units the meteorologists use in that region (at least externally, e.g. they may measure rainfall in mm in USA but the public gets inches). In USA it is statue miles, elsewhere it is kilometers. It is not the unit that either aviaton or surface traffic uses for speeds and navigation (some European countries use miles for cars but kilometers for visibility).

Note: The numbers are programmed into the users (pilots) head and do not really need to exactly correspond to what is used for navigation. Fog is defined as visibility under 1 km. So one knows how 1 km visibility looks like. Certain limits (like VMC) have their values defined as round numbers in statue miles or kilometers.

Compare the situation with navigating and measiring speed in nautical miles, but measuring altitude and vertical velocity in feet. Some countries used to use kilometers for vertical velocity before 1990. And the transition to feet was very smooth. In some sense it does not matter whether it is measured in apples or in oranges. It is important that everything is consistent (within that very variable) and that one knows what is a low value, what is a high value and that the values one gets to work with are nice and round (like the flight levels from 001 to 660).

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    $\begingroup$ 'Some European countries use miles for cars but kilometres for visibility' -- I believe 'some' means 'the UK' as the RoI has transitioned away from miles for cars to the best of my knowledge. $\endgroup$
    – Jan
    Feb 14, 2022 at 10:08
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    $\begingroup$ Distance in miles, altitude in feet makes sense because altitude is extremely important to get precise at the sub-mile scale. I'm astonished that anyone used km for it ever. Meters I could see, that's equally useful to feet (the difference between a meter and a foot isn't going to save you when it comes to separation from the ground except in action-movie sequences). $\endgroup$ Feb 16, 2022 at 12:52
  • $\begingroup$ @WilliamWalkerIII Of course, I actually meant meters. But meters or kilometers is the same, really, you just move the decimal point. I never claimed anyone used whole kilometers. The precision issue is unimportant here. The point is that flight levels in feet are nice numbers and are not in meters. You could make them be nice numbers in meters, sure, but then the spacing would be impractical. $\endgroup$ Feb 16, 2022 at 15:57

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