Why do we report 10 mile visibility when there's not a cloud in the sky.
Here's an explanation that a view mailed to us at WTAP to help educate us on the subject.
It sounds good to me!
"Visibility is relative to location of the viewer and the type of terrain around him or her.
If you were at Myrtle Beach, standing on the beach looking out over the ocean with no mountains or trees, etc., to restrict vision, visibility is at best 10 miles to the horizon.
Visibility around Parkersburg at ground level will probably be less than that at the beach due to our surrounding terrain. However, if you were in an airplane at 8,000 to10,000 feet over Parkersburg, you could see the lights at Cleveland on a clear night. That is certainly more than 10 miles.
If you were driving west and were approaching Denver from the east, you could see the mountains west of Denver when you were still maybe 80 miles east of Denver on a clear day.
The folks in the tower at the Wood County airport have reference points located around the airport at some known distance from the airport they use in reporting visibility. Such as the cooling towers at Willow Island North East of the airport. If they can see the cooling towers than probably they will report the visibility as 10 miles....
...If the visibility is excellent, a good way of reporting it is: "Clear And Visibility Unlimited", (CAVU), , such as (The visibilty is "CAVU").