The day the sky stood still: "A contrail is not smoke, as I fancied as a child. It's not even an emission, really, at least not in the true sense of the word. Much of what you see when you look at a contrail is water that was present in the atmosphere before the jet came along. A contrail is a cloud, as surely as a thunderhead boiling up over a prairie on a summer afternoon. A cloud of ice, birthed by the happenstance of a passing jet."