14 DAY TRIAL // In a paper published in last month's online issue of the Journal of Geophysical Research: Atmospheres, scientists at the Hampton University in Hampton, Virginia, report an increase in the incidence of noctilucent clouds across a broad swath of the Earth, primarily between 40 and 50 degrees latitude north. This conclusion is based on data collected by past and present NASA missions.
One such mission is the Aeronomy of Ice in the Mesosphere (AIM) spacecraft, which the American space agency launched back in 2007 to study noctilucent, or night-shining, clouds over Earth's poles. Though AIM cannot see what is going on in the atmosphere at intermediary latitudes, this gap in its dataset was filled by other space- and ground-based missions, both past and present.
Night-shining clouds have fascinated people since 1885, but a scientific explanation has eluded scientists for a long time. Now, atmospheric scientists know that these phenomena are just the ragged, outer edges of much larger, brighter, and more pervasive polar mesospheric clouds (PMC), which exist in the upper atmosphere. The noctilucent effect is visible only in a deep twilight.
The light shining off these wispy clouds comes from the Sun, but they can only be observed after sunset. The reason why no other types of clouds shine at night is that PMC are the highest clouds in the atmosphere. They usually hang out in the mesosphere, at altitudes 76 to 85 kilometers (47 to 53 miles). As such, they are the last clouds to be illuminated by the Sun before a full night sets in.
When the first noctilucent clouds were discovered, they tended to occur more often at higher latitudes but, over the years, they have moved south increasingly often, reaching the area between the 40th and 50th parallels by the 20th century. AIM was partially launched to help explain why this phenomenon was occurring, so the data it collected was combined with a computer model for this study.
“Noctilucent clouds occur at altitudes of 50 miles above the surface – so high that they can reflect light from the Sun back down to Earth. AIM and other research has shown that in order for the clouds to form, three things are needed: very cold temperatures, water vapor and meteoric dust,” explains HU atmospheric and planetary scientist James Russel, who was the first author of the new study.
“The meteoric dust provides sites that the water vapor can cling to until the cold temperatures cause water ice to form,” the expert goes on to say. His team also used data from NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) and Aura missions for this study. The computer model was developed by GATS investigator and study coauthor Mark Hervig.