Was on a flight over the weekend. At about a crusing height of 30,000ft, and assuming clear weather conditions, how far can you actually see on the ground with the naked eye, I estimated about 65 miles?
From an altitude of approximately 5 miles you should be able to see well over 100 miles under excellent seeing conditions. This is usually seriously limited by an ultraviolet haze. Cameras with UV filters or infrared sensitive CCD detectors can see through this haze. Yellow tinted sunglasses will show a marked improvement also.
Click here and enter 30,000 feet into the slot then click on 'Compute'. It comes up with 233 miles!
Of course, the site is devoted to land- and sea-based sightings, so it's possibly not too accurate at over five miles high!