Noctilucent clouds are Earth�s highest. They are composed of sub micron sized ice crystals in a 80-85 km (50-53 mile) high layer that is a few km below the mesopause, the coldest area of the atmosphere.
At these heights the crystals remain in sunlight long after the sun has set at Earth�s surface. They shine purely by reflected light and unlike stratospheric nacreous clouds they do not show iridescent colours � their crystals are too small for that.
Look for them in the Northern Hemisphere from mid May until mid August. That is when they are in sunlight for the longest and when the mesosphere is at its coldest. Temperatures below -123�C are necessary for NLCs to form.
Look when the Moon does not interfere and away from light pollution and the glare of �security� lights. Look to the northern horizon (NW before midnight) for bluish white clouds. NLCs hardly move compared to the motion of lower clouds. Binoculars help to distinguish them from lower clouds � NLCs have a characteristic skein like structure and they appear sharper.
A latitude of 50-65� is best but NLCs are seen much further south. They have been sighted in Austria, Hungary, Italy and southern Germany and in Utah and Colorado in the US.
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