Noctilucent clouds are rare bluish or white clouds visible against the
night sky after the sun has set. They are most common at high latitudes
during summer. The noctilucent clouds are very high they occur at an altitude
of 70-90 km and can only be seen when the sun is between six and sixteen
degrees below the horizon. The clouds are composed of very small ice crystals.
How ice crystals form at that high altitude is a mystery. One theory is
that water condense on meteoric dust particles.