Atmospheric
Optics
Night Shining Clouds
~ Noctilucent clouds, planet Earth's highest and coldest shine over Vejle Fjord, Denmark. Three images by Anders Falk Jensen on the night of 22/23 June '15.
All images ©Anders Falk Jensen, shown with permission
Look for NLCs in summer. Look to the north (northern hemisphere!) an hour or so after sunset.
Avoid moonlit nights.
Use binoculars where their shifting skein like appearance distinguishes them from lower tropospheric clouds.
NLCs shine because at their high altitude they are still in daylight.
Stratospheric ozone absorbs some of their scattered light to confer a blueish colour
Noctilucent clouds are 80-85 km (50 – 53 mile) high.
Small, 0.1 micron, ice crystals compose them. Very low temperatures below -123 Celsius are needed to form ice in the dry mesosphere.
Nuclei on which the ice crystals can grow are also needed. Meteor dust, lofted oceanic salt, volcanic ash and rocket exhausts are all candidates.
NLCs are possibly becoming more frequent. Paradoxically, as the lower atmosphere warms the mesosphere cools.
