"They're gorgeous, and there are more of them now than before," says James Russell, principal investigator of NASA's Aeronomy of Ice in the Mesosphere (AIM) mission, about noctilucent (literally, "night-shining") clouds. Floating at an altitude of around 50 miles, they are only seen well after sunset on summer nights above 50 degrees latitude in either hemisphere and have been increasing in frequency and brilliance with each passing year. Their origin is a mystery as is the reason for their growing quantity and brightness. But new data from the AIM satellite, launched April 25, may help shed light on these enigmatic bodies as well as answer questions that have arisen about their possible link to global warming.
Noctilucent clouds are made of ice particles 20 to 100 nanometers in size that float in the mesosphere well above normal clouds (the highest of which are seven miles up) as well as the ozone layer (at nine to 22 miles), but below the auroras (60 miles and higher). Although the air there is 100,000 times dryer than the Sahara's desert winds, extremely low temperatures around –225 degrees Fahrenheit (–143 degrees Celsius) crystallize what little moisture there is on tiny dust particles that act as cloud seeds. In an interesting and so far unexplained atmospheric riddle, the air at that altitude is actually colder in summer than in winter, causing these clouds to form only in the summer months in either hemisphere.
Although no one is sure what triggers their formation, one theory is that cosmic dust, or debris from burned-up meteors, seed the ice particles, which is plausible because meteors typically are incinerated in the upper mesosphere at about the same altitude where these clouds form. That would not, however, explain why their frequency has been increasing by 8 percent a decade, because there has been no known systematic long-term increase of meteoric activity over the same period.
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These clouds, technically called polar mesospheric clouds, typically form near the poles and migrate toward the tropics. Traditionally confined to latitudes north of European cities like Frankfurt and Brussels, over the past couple of decades they have been observed in the Northern Hemisphere at latitudes as far south as Budapest, Hungary, (47 degrees) and Logan, Utah, (41 degrees). Their brightness has also been growing, due to either larger or more ice crystals, suggesting a rise in moisture content at those altitudes. Because air temperature significantly alters atmospheric dynamics, which in turn affects moisture transport, scientists speculate that this increase of high altitude moisture may be tied to global warming.
The goal of the AIM mission is to find answers to these questions as well as to their possible connection to climate change over a two-year period. Preliminary data from the satellite will be presented late next month.
