I don’t know why I get so excited about noctilucent clouds. Normally I’m cursing out the clouds because they come at the most inopportune times, blocking some rare celestial treat from view. Not last night. On the way to dark country skies, I noticed a wisp of white in the northern sky in mid-twilight around 10:15 p.m. By 10:30, the twilight had deepened enough to reveal a low bank of noodly blue clouds. Ah-hah! The noctilucent clouds were back.
Night-shining or noctilucent clouds (NLCs) are visible on and off at higher latitudes during the early summer months. They’re not common by any stretch. In the past 30 years of sky watching I’ve seen only a half dozen displays.
Cirrus clouds, the ones that look like feathers wafting across the daytime sky, are typically about 10 miles high. Composed of ice crystals, they float near the top of the lowest, thickest layer of air called the troposphere. Noctilucent clouds share the realm of the Greek gods, basking in sunlight well into the night at an altitude of some 50 miles. That’s nearly as high as the aurora borealis, which can shimmy down to a tickly 60 miles.
NLCs are creatures of the Earth’s mesosphere, a rarefied blanket of air extending from 30 to 53 miles high. Most meteors burn up in this layer. It’s also extremely cold up there – temperatures at the top drop to 130 below F. Like lower clouds, the night-shining variety are composed of ice crystals, either condensed directly from the thin air or seeded by tiny dust particles. Likely sources for the dust include meteor debris, volcanic dust and even water exhaust from the era of the space shuttle.
Because of their great height, noctilucent clouds reflect sunlight long after sunset when other clouds have gone grey and colorless. That’s why they’re called “night-shining” in the first place. Their color is imparted by the ozone layer located 12-19 miles overhead. The reds and oranges of reflected sunlight are absorbed by ozone on their way down to our eyes, tinting the clouds blue. Pretty cool.
Between 10:30 and 11 o’clock last night the NLCs grew brighter and more detailed. Stripes, undulations, curls and streaks were all plainly visible to the naked eye. Last night’s was the best show I’ve ever seen. Still, I wished I would have carried binoculars along to better appreciate the subtle folds and ripples the photos picked up so clearly. Next time, next time.
Between 10:45 and 11 ( 1.5-2 hours after sunset) the clouds slowly began to fade from top to bottom, but this was also when they were most striking. In a dark sky, they glowed an eerie plasma blue as if powered by electricity. By 11:15 the sun was too far below the horizon to provide the light to sustain them. What was once an amazing display disappeared with the coming of night.
The clouds are best viewed from the northern tier of the U.S., Canada and the northern half of Europe, though they’re occasionally sighted at lower latitudes. Displays usually start in June and wrap up by early August.
To find them, go out during twilight at hour or more after sunset and look low in the northern sky. Regular clouds appear dark grey at that time, but NLCs will grow brighter against the darkening sky. They also don’t move as fast or change shape as quickly as lower clouds due to their high altitude. Think of airplanes. Close by, they move quickly, but higher up, they take more time to cross the sky.
I’ll be out the next clear night on cloud watch and hope you will too. To learn more about NLCs and how you can contribute your observations to help improve our understanding of them, check out the Noctilucent Cloud Observers’ Homepage.