What makes the area even more valuable is that the clean air from this oasis rides a northwest wind to provide the only clear days that emerge on the Colorado Plateau, home to several national parks, including the Grand Canyon.
"It's the one place left in the country that has no major sources of pollution," said Christine Shaver, chief of the policy planning and permit review branch of the park service's air quality division.
Today, no park in the continental United States is a refuge from the waste products produced by humans. Pollution affects visibility more than 90 percent of the time at all the National Park Service monitoring stations in 50 parks. On bad days, when winds blowing from the southwest or east bring pollution
from Los Angeles and power plants, visitors to the Grand Canyon National Park can no longer see the Colorado River roaring along the bottom of the canyon or the opposite rim 10 miles away.
And as bad as it is in the West, it's worse in the East.
Purple mountains' majesties have all but disappeared in Eastern parks, where, on average, the visibility is only 10 percent of that of Western parks. When the Shenandoah National Park was established in Virginia in 1935, visitors could make out the Capitol and the Washington Monument 70 miles away. That, park officials can safely say, will never happen again - the median visual range in the summer hovers around 11 miles and has not edged much above 50 miles on the best days for decades.
Visibility is not the only concern of the National Park Service. In its Lakewood, Colo., offices, several members of the air quality division are struggling with an unpleasant task - how to warn people not to exert themselves when the air in the nation's parks is unhealthful.
