OK, they’re not much to look at … at first. All you see are these thin, pale streaks low in the northern sky just about the time the stars come out. When you realize you’re looking at clouds as nearly as high as the aurora borealis, you quickly find yourself paying closer attention.
Noctilucent or night-shining clouds were out in force last night, appearing at first glance like the pale tendrils of cirrus clouds. They spread like a thin veil very low across the northern sky. What gave them away was not only their pale blue-white coloration, but they first came into view in a darkening sky when the stars began to show. I spotted the first faint wisps an hour after sunset at 10:10 p.m. Their visibility and contrast improved through about 10:40 p.m. when the Big Dipper was easy to see. By 11 p.m. the clouds had faded away into the darkness.
These oddballs of the cloud world are unbelievably high, floating 31-55 miles overhead in a layer of Earth’s atmosphere called the mesosphere. More familiar high clouds, like the feathery cirrus we see on a summer’s day, form 4 to 8 miles high, about the same altitude as a transcontinental jet flight. High clouds like cirrus are composed of ice crystals.
In order for a cloud to form and grow, water needs to stick to something.Â In regular clouds, dust from wind storms – especially from the world’s deserts – supplies the necessary “nuclei” for the formation of water droplets and ice crystals. Since it’s next to impossible to get dust up high enough to provide nuclei for noctilucent cloud (NLC) formation, scientists suspect “outer space” dust from meteroids and comets may provide the necessary material on which ice can condense. As Earth travels around the sun, it sweeps up some 40,000 tons of interplanetary dust a year, plenty to get the job done. Summer storm winds carry the water vapor into the mesosphere from the lower atmosphere, which explains why NLCs appear in summer.
“Extreme cold is required to form ice in a dry environment like the mesosphere,” says Gary Thomas, a professor at the University of Colorado who studies NLCs. “Ironically, global warming helps. While greenhouse gases warm Earth’s surface, they actually lower temperatures in the high atmosphere.” Thomas notes that noctilucent clouds were first spotted during the Industrial Revolution–a time of rising greenhouse gas production.
Although most NLCs are visible from latitudes of 50 northÂ – give or take – they’ve recently been spotted in Colorado and other more southerly locations. Will they be out tonight? Early summer is the peak time to look. Find a place with a view as close down to the northern horizon as you can get and start watching about an hour after sunset. If the entire sky is clear except for wispy streaks low in the north, you’re probably seeing an NLC display.
Because of their great height, NLCs are the only clouds still shining in sunlight when all the others have gone dark. Don’t forget binoculars. A little magnification and extra light gathering power will help you appreciate the subtle ripple and wave textures in these fascinating clouds.