What is visibility anyway?

I heard on the radio a translation from visibility in miles to healthy vs unhealthy air quality. ( And found a chart here: pdf )

If visibility is 10 miles & up, air quality due to particulates (smoke) is Good & particulate levels are 0-40
6-9 miles Moderate air quality 41-80
3-5 miles Unhealthy for sensitive groups 81-175
1.5-2.5 miles Unhealthy 176-300
1-1.25 miles Very Unhealthy 301-500
.75 miles or less Hazardous over 500

So my question was, just what do they mean by visible? The very farthest haziest hill? One of the intermediate layers of trees? Just where do we draw that line?

I found a reference to interpreting visiblity data, which might be helpful in answering that almost philosophical question in a useful way. USDA Forest Service- Interpreting visibility data
But I haven't found a real answer yet.

This picture was taken in Nevada City on a recent particularly smoky day. We have had lots of days with particulate numbers over 100 (microgams/cubic meter) and several over 300.

And for days now, it is noticeably hazy between 100 and 200 feet - not as bad as it has been, but smoky. And with an orange moon.


Well, I just checked another search result, and found this at Montana DEQ
1) Face away from the sun.
2) Determine the limit of your visible range by looking for targets
at known distances (miles).
3) Visible range is that point at which even high contrast objects totally disappear.