Noctilucent Clouds


Some fascinating sights in the night sky lie close to home. Meteors, aurorae, and skyglow, for example, are all captivating phenomena visible just at the edge of space. During the recent Slooh broadcast of the 2015 Lyrid meteor shower, Slooh astronomers discussed another phenomenon of the upper atmosphere: the ethereal electric-blue glow of noctilucent clouds high above the horizon after the sun sets.

Found in the mesosphere at an altitude of 50-80 km, noctilucent (“night shining”) clouds are Earth’s highest cloud formations. During much of the year, the air is bone dry this high up in the atmosphere. But during summer, upwells from the lower atmosphere carry moisture up to the frigid mesosphere where it’s seeded into microscopic ice crystals by tiny micrometeorites and other space dust. The ice scatters sunlight and becomes visible to Earthbound observers as wispy blue clouds well above the horizon about an hour after sunset.

Noctilucent clouds appear to be a newly observed phenomenon. They were first seen in 1885, two years after the eruption of the Krakatoa volcano in Indonesia, an event which sent huge amounts of volcanic ash high into the upper atmosphere. But the clouds remained visible even as the ash dissipated, so it’s not clear if they were caused by volcanic ash or if they were simply noticed because so many people were watching the colorful sunsets and sky colors in the months after the eruption.

Whatever their cause, noctilucent clouds seem to be occurring more frequently. A century ago these clouds were visible only at far northern or southern latitudes. They are now visible occasionally as far south as Utah and Colorado. Some researchers suggest the increasing frequency of noctilucent clouds may be caused by man-made pollutants in the atmosphere or by climate change. So it may not be a coincidence that they first appeared after the start of the industrial revolution in the late 19th century. No one knows for sure.

Nor can scientists explain the apparent connection of these clouds to phenomena lower in the atmosphere and to the Earth’s climate in general. A surprise discovery in 2014 using data from NASA’s AIM spacecraft (Aeronomy of Ice in the Mesosphere) suggests the frequency of these clouds over Antarctica is directly correlated to the winter air temperature over North America, for example. Again, no one knows why.


Their increasing frequency makes this a good time for skywatchers to look for these clouds at the edge of space. In the summer, look for luminous blue-white tendrils spreading across the sky about 30-60 minutes after sunset. You have a better chance observing from sites north or south of 40 degrees latitude in the northern and southern hemispheres, respectively.

Slooh is investigating the possibility of a live member broadcast of noctilucent clouds from our observatories. Look for more details here in the near future.

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