At what altitude does the horizon cease to be visible

At altitudes greater than a few kilometers the horizon is indistinct because of contrast reduction, even on extraordinarily clear days. An airplane passenger flying over an ocean cannot point to the apparent boundary between earth and sky and confidently proclaim its distance. To determine the distance to the horizon to within 10% requires knowing its angular position to within a few minutes of arc. This is unattainable in a realistic atmosphere, even one that is very clean. Scattering by the air molecules themselves is not quite sufficient to blur the horizon, but it does not take many particles before the horizon becomes indistinct.