One fact often cited by the private - plane set is this: if the entire civilian fleet of 125,000 planes flew over Arizona at the same altitude, each would have almost a square mile to itself. (si.com)
While the altimeter data also showed that the liquid in some of the canyons around Ligeia Mare is at sea level — the same altitude as the liquid in the sea itself — in others it sits tens to hundreds of feet (tens of meters) higher in elevation. (sciencedaily.com)
Although no one is sure what triggers their formation, one theory is that cosmic dust, or debris from burned - up meteors, seed the ice particles, which is plausible because meteors typically are incinerated in the upper mesosphere at about the same altitude where these clouds form. (scientificamerican.com)