Weather quiz

[1739T]

Can anyone tell me why visibility on METARs are given in SM instead of NM?

[Dave Chael]

Visibilty as defined in the AIM:

"Visibility is reported as statute miles, hundreds of feet or meters."

As to why, my best guess is that ground distances (for example, distance from a known landmark to the control tower at an airport) are estimated in statute miles, probably due to the fact that most distances scales on maps are in statute miles. The actual difference is not all that significant though. 1 SM = 5280', 1 NM = 6076.115', a difference of around 796' per mile. If the vis is "three miles" (remember it's a subjective WAG [wild ass guess] anyway), you'd be talking about a difference of less than 1/2 a mile. Any other thoughts?

Dave

[leardvr]

because metar's are the survace opervations used around the world, not all countries have high tech stuff to measure and therefor a human does it. Humans see in SM and not NM.

Go Figure!!

[wxwatcher]

Actually, it goes back to the very first weather observations, also known as Surface Analysis (SA) observations. And, you're right. That system was instituted in the U.S. and we used statute miles when measuring distances across land masses, not nautical miles or kilometers.

When the SA's were replaced by the METAR system a couple years back, the US kept statute miles for visibility because almost all visibility marker charts in the US were and still are calculated in SM.

BTW.. Those visibilities made by actual observers are not WAG's. A survey is done at each observation location and a select set of prominent objects/landmarks for daytime and unfocused lights for night time are chosen. The distance from the observation point to each of those landmarks, etc is measured and then plotted on a visibility marker chart specific to that observation location.

The observer then looks at all visibility markers through the full 360 degrees around the observation point. The "prevailing visibility" is the greatest distance that the markers can be clearly seen through at least 50% of the circle. If an observer can see all the markers listed for one mile, but only one or two at three miles in one area, the prevailing (official) visibility will be one mile. In the remarks section you may see something like VSB NW 3, to indicate a significant difference from prevailing in a specific direction from the observing location. You could also see a remark VSB NW 3, SE 1/4. That tells you something else, visibilities to the northwest are better than average, but to the southeast, they really stink.

Any other questions, let me know..

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Comments and opinions are my own and do not reflect those of my employer.