What's in a number? When it reflects visitation to national parks, it's not always as obvious as it might seem.

The National Park Service website has a colorful and comprehensive page that details each national park unit in the country. But by digging down deep into the site map, you can find NPS statistics pages of gray columns of numbers that tell a great story. The statistics concentrate on the number of visitors, though there’s also information on size of the park and its monetary impact on the community.

Fiscal Year Report 10/1/2008 to 9/30/2009 is the most fascinating. The Blue Ridge Parkway attracted the most recreational visitors (16,070,849) while Aniakchak National Monument and Preserve had only 14 visitors.

If we look at "national parks" only, as opposed to any unit of the National Park System, Great Smoky Mountains National Park had the most visitors (9,430,794) during that year, with Grand Canyon National Park being a distant second (4,343,620). This makes sense since the Blue Ridge Parkway and the Smokies are within a day’s drive of half of the U.S. population. But Shenandoah National Park in northern Virginia is in the same urban belt and yet it hosted only 1,153,133 visitors. On the low side, just 1,626 visitors went to Kobuk Valley National Park in Alaska.

The chart also includes "visitor days," defined as number of visitor hours divided by 12. If we divide the number of visitor days by recreational visitors, we get the average stay per visitor. Only 18 national park units out of a total of 392 had visitors stay more than a day. How is that possible?

On average, people only spent 15 minutes in Thaddeus Kosciuszko National Monument in Philadelphia and 1.45 hour in Saguaro National Park in Arizona, the lowest average stay in a "national park."

Most people would consider Denali National Park in Alaska to be a trip of a lifetime. Yet visitors only stayed 15.3 hours. I would not have believed this number until I talked to a ranger who worked in Denali a few years ago. She recalled that visitors came to the visitor desk and said, “I have two hours. What can I do?” Many visitors stop in Denali as part of larger tour of Alaska.

So you can conclude that visitors spend very little time in any one park. But are these numbers realistic? I tested these low averages on park sites I know.

* The Smokies might be the most-visited park in the nation, but the average stay is less than 7.5 hours. That’s the length of an all-day hike. Of course, most visitors don't hike all day; they drive through the park on Newfound Gap Road, stop at the two main visitor centers and several overlooks, and feel they’ve “done” the park.

* Carl Sandburg Home National Historic Site in Flat Rock, North Carolina, was the last home of the famous writer and poet and his equally famous wife, who was known internationally as a goat breeder. You can take a 30-minute tour of the home and visit the goat farm. Locals may also walk some of the five miles of beautifully manicured trails. So in less than two hours (1.87 hours), you see the house, pet the goats, walk to the lake and you’re done.

* Congaree National Park is a small park outside of Columbia, South Carolina, where the average visitor stays 3.7 hours. The park attracts lots of school groups; the children may listen to a talk by a ranger, walk the boardwalk trail, buy souvenirs at the visitor center, and eat their picnic lunch. That probably takes less than four hours.

In only six parks, did visitors only stay more than two days. The parks with the longest stay are not the most popular ones but are remote. Yukon-Charley Rivers National Preserve in Alaska along the Canadian border can only be accessed via water or air. The average stay is five days.

Similarly, you can only visit Isle Royale National Park in Michigan by sea plane or boat. The park website explains that "This island gem houses the highest backcountry overnight use per acre of any national park and at the same time it is one of the least visited."

In other words, once you get there, you’re going to stay for a while.

Rio Grande Wild and Scenic River has the longest average stay, 8.7 days. That would be about the length of a float trip.

I've included the numbers and a chart (attached)that details the number of parks broken out by average stay. I welcome other analysis and interpretations of these numbers.

Several caveats: There are 392 units of the National Park Service, but only 360 are in this spreadsheet. Most reports can be seen on the web or downloaded as a spreadsheet. Some reports are for the fiscal year, others for the calendar year. Some reports can only be seen as PDF files. This means it is very difficult to extract the numbers and manipulate them. But even with all these inconsistencies, the numbers are fascinating.