Mike McCrea wrote:

One of the things that struck me in the couple of days after the
September 11th attacks was the lack of vapor trails in the sky.

Given the routes that commercial aircraft take it might be hard to
find someplace without vapor trails - antarctica maybe?

California's north coast has very few vapor trails overhead,
and can be paddled year-round on some river or creek somewhere.
There are flights between California cities and Portland or Seattle,
but not ultra frequently, and vapor trails quickly move eastward
away from the coast.

Might be difficult finding a clear day in winter, though! ;-)