More than an hour after sunset a veil of glowing clouds appears low in the northern sky. Whirls and billows are visible as the sky shines. What is going on? Few Americans have witnessed this phenomenon, which is known as noctilucent, or night-shining, clouds (NLCs). That’s because NLCs usually appear north of 50 degrees latitude, which in North America lies just above the Canadian border. "Usually," but this may be changing.
NLCs form in a narrow region two miles thick, 50 miles above the Earth’s surface, more than ten times higher than typical clouds, and over seven times higher than commercial airplanes fly. They probably are made of water ice crystals, and they are still in sunlight when night has fallen on the ground below.
Typically, they are seen above the polar regions in early summer, when a weather pattern of rising and cooling air drives temperatures below -220 degrees Fahrenheit, lower than anywhere else in the Earth’s atmosphere.
The first unambiguous, published description of NLCs was written by Robert C. Leslie, who observed the phenomenon from Southampton, England, in July 1885. In a letter to the journal Nature, he wrote:
"Ever since the sunsets of 1883 and last year there has been at times an abnormal glare both before and after sundown. But I have seen nothing in the way of twilight effect so strange as that of Monday evening, the 6th, when about 10 P.M. a sea of luminous silvery white cloud lay above a belt of ordinary clear twilight sky, which was rather low in tone and colour. These clouds were wave-like in form, and evidently at a great elevation, and though they must have received their light from the sun, it was not easy to think so, as upon the dark sky they looked brighter and paler than clouds under a full moon. A friend who was with me aptly compared the light on these clouds to that which shines from white phosphor paint."
Coordinated ground-based observations show that the number of NLCs is growing by the decade. As sightings increase, scientists want to know if NLCs are harbingers of more widespread change in the Earth’s atmosphere.
My first encounter with NLCs came as a complete surprise, on a muggy night in Florida in August 1997. I was on a small science team at Cape Kennedy working the midnight-to-noon shift during the second mission of the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI), launched aboard the space shuttle a week earlier. The instrument, about the size of a coffee table, rode on a satellite deployed by the space shuttle. I had been to Cape Kennedy for a couple of simulations, but this was the first time I had participated in an actual mission.
We were measuring the ultraviolet emission of hydroxyl (OH). Near NLCs, OH is created when water vapor is destroyed by solar radiation. OH is also very reactive, and lower in the atmosphere it can contribute to the destruction of ozone. We wanted to find out how the observed OH compared with model predictions.
Working around the clock at the Payload Operations Center, a small building in a quiet section of Cape Kennedy, we monitored data sent from the satellite. One night I was among a handful of sleepy scientists huddled around a few flickering monitors as the data trickled down from space.
MAHRSI simultaneously observed sunlight scattered from the Earth’s atmosphere as well as OH. This background atmosphere normally becomes steadily brighter at lower altitudes. That night we were unwittingly collecting data around NLCs.
Staring at the monitor at 2 A.M., I noticed that as the satellite reached the Arctic, the data behaved strangely at the top of the scans: bright at one altitude, dim right below that, then bright again at the next lowest altitude.
Now, experience as a research scientist has taught me that data frequently look peculiar. In fact, for every hundred times I say to myself, "Wow, that’s interesting," there is maybe one time where something new is actually revealed. This is especially true when looking at uncalibrated data at 2 A.M. There remained a few orbits, and opportunities to see new Arctic data, before sunrise in Florida. At 7 A.M., Robert Conway, the principal investigator of MAHRSI, made his customary telephone call to the overnight team from his hotel room a few miles away.
"What’s hot?" he asked, regrettably not wasting any time.
"Well, not much really," I began, unwilling to play the role of overenthusiastic young scientist imagining things in data.
But he sensed my excitement. "What? What is it?" he prodded. I took a breath and continued.
"It’s the high latitude data. There seems to be a lot of structure at the top of the emission profile," I said, meaning that the Arctic scans had bumps and wiggles near the top.
"I’ll be right there," Conway said. He arrived soon after and we scrapped our carefully planned observations, to scan the higher altitudes more rapidly.
As the astronauts prepared to retrieve the satellite, Conway went directly to the operators in the next room to negotiate for the earliest possible opportunity to communicate with MAHRSI. He didn’t have time to look at the data carefully. Meanwhile, I squinted at the monitor, uncertain of exactly what I was seeing as MAHRSI relentlessly approached the Arctic again. I will never forget the feelings of tense excitement that crawled through me that morning.
In the end, we were able to send up the commands expeditiously, and MAHRSI performed flawlessly. A few weeks later I looked more closely at the data and found unambiguous evidence of late-season NLCs. The bumps and wiggles near the top of the Arctic scans were due to the scattering of sunlight by tiny ice crystals in the atmosphere.
Then, in 1999, a spectacular display of NLCs was observed in June over Colorado and Utah—much farther south (about 40 degrees N) than they are usually observed. This piqued the curiosity of the public and left scientists scratching their heads.
Even more provocative is evidence from Europe showing that NLC sightings overall increased in the late 20th century. Nobody really knows why.
Last year, NASA selected for study a space mission exclusively dedicated to understanding why NLCs form and why they vary. If chosen to fly, this mission will measure NLCs, temperature, and all the constituents thought critical to ice cloud formation. Measuring OH, for example, a proxy for water vapor, helps calculate what is available for NLC growth. By monitoring both NLCs and their environment in this way, scientists seek to identify their catalyst for change.
Whether they are an "abnormal glare" observed from the ground or a lofty layer of scattered sunlight observed from space, these clouds continue to spark interest. Given the increase in sightings, many who study NLCs would like to know how humanity influences them. Perhaps another accidental discovery will bring us closer to the answer.