Night-shining clouds made of ice and meteor smoke are on the increase. We explore their origins and how to see them.
Saturday night June 8th something remarkable happened. Noctilucent clouds (NLCs), normally visible from about 50° to 70° north latitude, streaked the skies over Oregon, Minnesota, Wisconsin, and Michigan. Skywatchers unfamiliar with the night-shining clouds were struck by their odd appearance and leapt into action with cell phones and digital SLRs, recording them over city and countryside alike.
Chris Juhn of Burnsville, Minnesota, spotted the clouds from a hotel room in downtown Minneapolis (latitude 45° N) and had no idea what they were but was moved to record them with his camera. The next night the clouds returned again, this time over the San Francisco Bay area at latitude 38° N, one of the lowest latitude sightings yet. Curiously, they weren't visible from Minnesota that evening, indicating the patchy nature of their distribution.
Noctilucent clouds return annually from May through August to Northern Hemisphere skies. Outside of that, they're notoriously unpredictable. One night they're there, the next they're not. I've seen them from northern Minnesota (47° N) half a dozen times over the past 5 years. In each case they've climbed to a maximum altitude of 10–12° but more often they sit within ~5° of the northern horizon. If you live north of 50° latitude not only are they more frequent, but the clouds occasionally spread out to fill much of the northern sky.
What makes them special? Altitude and origin. They're far above familiar tropospheric clouds with heights ranging from 76 to 85 km — 10 times higher than the feathery cirrus clouds we skim through when jetting cross-country. This begs the question of how water vapor percolates up into the mesosphere, a very dry, very cold layer of the atmosphere that begins at the top of the stratosphere and reaches to ~100 km.
Gravity waves — disturbances in the lower atmosphere from airflow over mountain ranges or powerful thunderstorms — are thought to loft water vapor into the mesosphere. Surprisingly, temperatures there are coldest in the summer. Cold enough to cause the vapor to condense as extremely tiny ice crystals, some just a micron across. Water vapor from rocket exhaust also plays a minor role in depositing water at these altitudes.
For clouds to form, water vapor must condense on something. Salt, dust, soot, and aerosols serve as condensation nuclei to build clouds in the lower atmosphere. Ice in night-shining clouds condenses instead on the dust from disintegrating meteors and interplanetary dust. Earth sweeps up 40,000 tons of dust each year as it sweeps around the Sun, providing a ready source of material.
Dust blasted high into the atmosphere from powerful volcanoes can also serve as condensation nuclei. In fact, night-shining clouds were first reported in 1885, two years after the 1883 eruption of Krakatoa in Indonesia. It's uncertain whether volcanic dust from that particular eruption was responsible or whether more people began paying closer attention to the garish sunsets and twilights in the wake of the explosion.
Because of their great height, NLCs are visible long after lower clouds go dark, similar to how high cirrus clouds still glow pink when cumulus clouds have faded to lumps of gray. Unlike cirrus and what makes NLCs really stand out is their color — electric blue! The hue is thought to arise from the ozone layer (altitude 10-50 kilometers) which filters out the clouds' warmer hues. Alternatively, I've read that the small size of the clouds' ice crystals is just right for scattering blue light, similar to the way air molecules scatter short wavelength light to cloak the sky in blue. Both may be at play.
Night-shining clouds are best viewed from May through August in the Northern Hemisphere and November through February in the Southern Hemisphere with peak visibility around the solstice. But night to night they're unpredictable. None were visible the night after the big June 8th display. To maximize your chances of seeing them, you'll want to find an observing site with a wide-open view of the northern sky to within about 5° of the horizon. If you can see the bright star Capella in Auriga, that's a good sign. Doggedly check that horizon on as many clear nights as you can.
Start looking 45 minutes to an hour after sunset low in the northern sky, sweeping your gaze back and forth from the sunset point all the way to the darkening northeastern sky. You'll be looking for bright, blue-hued wisps and tendrils in contrast to the gray shades of lower clouds. As the sky darkens and the first stars appear, NLCs will brighten.
From my latitude I see the clouds low in the northwestern sky about the same altitude as Capella. NLCs often linger until 11:30 p.m or midnight, the end of dusk, but the "sweet spot" for northern U.S. observers is from 10–11 p.m. If you miss them in the evening, you can also seek the clouds starting at dawn in the northeastern sky starting about two hours before sunrise.
Night-shining clouds used to be primarily a polar phenomenon with occasional sightings at mid-latitudes, but that's been changing, as evidenced by the recent outbreak over California and Iowa. Their extent and brightness has been on the increase in recent years as documented by NASA's Aeronomy of Ice in the Mesosphere Mission (AIM) spacecraft which keeps a regular watch on noctilucent clouds from its orbit 600 kilometers above the Earth's surface. You can check the most recent AIM NLC images here.
Exactly why the clouds have proliferated and brightened in recent years hasn't been nailed down yet but appears to be related to climate change brought on by increased levels of methane and carbon dioxide. Methane from natural gas, intensive agriculture, coal-mining, and the like rises into the upper atmosphere where it reacts with hydroxyl radicals (OH) derived from ozone to create additional water vapor that freezes onto meteor smoke.
We're already familiar with the warming of the atmosphere from CO2, but a lesser known side effect from increasing carbon dioxide is to chill the mesosphere further, making it more conducive to NLC formation. More frequent and powerful storms brought on by global warming may also slingshot additional moisture into the upper atmosphere through gravity waves.
The up side of all this is that skywatchers at lower latitudes now can set their sights on noctilucent clouds, once the province of hardened northerners only. Keep watch and let us know if you spot them. To improve our understanding of NLCs and alert others to their appearance, I encourage you to report your observations to the Noctilucent Cloud Observing Network (NLCNET). NLCNET also has a Twitter-like feed of current observations you can check to possibly anticipate an upcoming display. Or follow @NLCalerts on Twitter. Finally, the Great Lakes Aurora Hunters maintains an excellent public Facebook page to keep tabs on the aurora and other sky phenomena like NLCs. By all means, click and join.