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.