by Eero Karvinen (more
over Finland July, 2006. ©Eero Karvinen, shown with permission.
Shining eerily blue-white long after sunset, their corrugations knots
and streaks sliding and shifting by the minute, noctilucent
clouds are Earth’s
highest. 80-85km (50-53mile) high in the mesosphere the pressure is only 1/500,000 of that at Earth’s
surface. Minute ice crystals form the clouds when the
temperature falls below -123C.
The low temperatures and formation conditions are, paradoxically,
reached during summer and the time to see them in the Northern Hemisphere
is from mid-May to August.
High latitudes are ideal but they can be seen widely over Europe,
UK, Austria, Italy and southern
Germany. In the US, Utah and Colorado have had them.
They will soon be visible -- Choose a dark, moonless night and
scan the northern horizon. Binoculars help to distinguish
them from lower cloud because NLCs have sharp well-defined