Every clear night in late spring and early summer, I make sure to check the northern horizon for noctilucent clouds (NLCs). They’re the only clouds I’m happy to see when the stars come out.
Noctilucent or night-shining cloud season starts every year in late May, but this year it began a week early in mid-May. Scientists are scratching their heads as to why, but it may be connected to an increase in methane gas in the atmosphere related to human activities. Let’s face it, we’re to blame for just about everything.
Night-shining clouds, visible from higher latitudes in Canada and northern Europe, are an uncommon sight if you live in the U.S. From across the northern tier of states, you might see them half a dozen times each summer but only if you deliberately go out and look. Wispy, wavy NLCs shine with a striking silver-blue very low in the northern sky beginning about an hour after sunset and finally fading from view an hour after that. Here in Duluth, Minn. I start looking around 10 p.m. and call it a wrap around 11:30.
Common clouds are too low (5 miles is typical) to catch the sun’s rays and appear in silhouette against the deepening twilight. The night-shiners form far above the world of daily weather in the bitter-cold mesophere 50 miles up (83 km) and catch the sun’s rays long after other clouds have gone dark.
As for that blue color, it’s 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.
Be wary though. Cirrus clouds – those wispy ones called “mares’ tails” – will sometimes appear pale white when higher in the sky and throw you off at first. Look for the blue glow and peculiar wavy-weavy patterns to know you’re seeing the real thing.
NLCs’ alien appearance is no accident. Clouds need “nuclei” to grow. Water in the air condenses on familiar materials like dust, soot from forest fires and car exhaust alike and even sea salt to form the droplets that build clouds. Unlike the common cumulus, night-shining clouds condense around the sooty remains of meteoroids that burn up upon entering our atmosphere at many thousands of miles an hour.
Summer storms loft the needed water to the higher altitudes where noctilucent clouds form. The clouds are connected to the highs and lows of the sun’s approximately 11-year solar cycle. At solar minimum, when activity is lowest, the clouds appear more frequently than at solar max. Scientists think the clouds thrive when the sun blasts out less water-molecule-destroying ultraviolet light compared to the stormy solar max season.
And that’s what’s so odd. Solar activity for the current 11-year cycle peaks this summer, so why did the noctilucent cloud season begin early? If anything, they should be scarce. One reason may be methane, a greenhouse gas that’s been on the increase since the 19th century from human activities including coal mining, oil drilling and agricultural practices.
“When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor,” says Hampton University Professor James Russell, the principal investigator of AIM. “This extra water vapor is then available to grow ice crystals for NLCs.”
Another possibility is a change in wind patterns in the southern hemisphere’s stratosphere. That’s the dilute layer of air sitting above the troposphere, the lowest atmospheric layer and happy home to weather, clouds and transcontinental airplane flights. This year, strong winds in the southern stratosphere are changing global circulation patterns by delivering more water vapor into the mesosphere where NLCs form. Along with the annual drop in temperature of the mesophere during northern hemisphere summer months, conditions are now ripe for more frequent appearances.
While you’re out in twilight batting away the mosquitos watching the space station arc across the northern sky, keep an eye out for these strange, electric-blue visitors from a distant atmospheric realm.
For more on the topic, check out the Noctilucent Cloud Observers’ Homepage.