Noctilucent clouds are very high clouds that look like cirrus clouds,
but are much higher (75-90 km above the Earth's surface) than clouds
that we observe every day. They are optically thin and can only be
observed during twilight hours, when the sun is just below the horizon
and only shines on the uppermost atmosphere.

In this image, the limb of the Earth at the bottom transitions into the
orange-colored troposphere, the lowest and most dense portion of the
Earth's atmosphere. The troposphere ends abruptly at the tropopause,
which appears in the image as the sharp boundary between the orange-
and blue- colored atmosphere. The silvery-blue noctilucent clouds are
far above this boundary.

http://eol.jsc.nasa.gov/debrief/Iss0...07-E-10974.htm