Pan American clipper pilots have found that most of their passengers have surprisingly hazy notions of how far they can see in the air. One passenger, 20,000 feet above Brazil, insisted that she could see the coast of Africa, 1,822 miles away. To set such passengers straight (and help military pilots) Pan Am's clipper captains have worked up a handy "vision range" table.
At sea level the curvature of the earth limits the range of vision to 2.9 miles. The formula for determining how many miles an individual can see at higher levels is the square root of his altitude times 1.225. Thus on a clear day at 1,000 feet a person with normal vision can see 39 miles; at 10,000 feet, 123 miles; at 25,000 feet, 194 miles. With good visibility a pilot at 25,000 feet can see Germany from the English Channel; at the same altitude over Tunisia he can see the middle of Sicily.