Admiring the noctilucent clouds...
They are the highest clouds in the Earth's atmosphere, located in the
mesosphere at altitudes of around 75 to 85 kilometers (47 to 53 mi).
They are normally too faint to be seen, and are visible only when
illuminated by sunlight from below the horizon while the lower layers
of the atmosphere are in the Earth's shadow. Noctilucent clouds are
not fully understood as a meteorological phenomenon. Clouds generally are
not able to reach such high altitudes with such thin air pressures.
This was the first time the Captain or I had ever seen them.