Unstructured Clouds

After the eruption of Krakatoa, some observers noticed curious, luminous clouds, dubbed "noculent clouds", forming high in the sky. The clouds had a blue glow that persisted into the night.

Scientists, quite reasonably, assumed that they were somehow the product of volcanic ash; however, even after the ash had settled the clouds had remained and continue to remain something of mystery to this day. They are normally only found in northern latitudes but have been slowly creeping south over the years, perhaps as a consequence of global warming.