Every summer, above the North Pole, ice crystals begin to cling to dust and particles high in the atmosphere, forming electric-blue, rippled clouds – called noctilucent or “night-shining” clouds – that stretch across the sky at sunset. Their season is eagerly anticipated by skywatchers in the high latitudes.
This year, noctilucent clouds got an early start. NASA’s Aeronomy of Ice in the Mesosphere (AIM) spacecraft first saw them on May 13. The season started a week earlier than any other season that AIM has observed, and quite possibly earlier than ever before, said Cora Randall of the Laboratory for Atmospheric and Space Physics at the University of Colorado.
The four images above show Earth’s upper atmosphere, centered on the North Pole, as observed by the AIM satellite. The image on the upper right shows noctilucent clouds on May 23, 2013; the upper left image compares the same week from 2012. The two bottom images show the extent of noctilucent clouds in mid-June of each year. The brighter the clouds in each image, the denser the ice particles. Areas with no data appear in black, and coastal outlines are traced in white. You can view a daily composite projection of noctilucent clouds by clicking here during the northern summer months.
Noctilucent clouds were first described in the mid-19th century after the eruption of Krakatau. Volcanic ash spread through the atmosphere, painting vivid sunsets around the world and provoking the first written observations of night-shining clouds. At first people thought they were a side-effect of the volcano, but long after Krakatau’s ash settled, the wispy, glowing clouds remained.
When AIM was launched in 2007, the cause of noctilucent clouds was still unknown. Researchers knew that they formed about 80 kilometers (50 miles) above Earth’s surface—where the atmosphere meets the vacuum of space—but that’s about all they knew. AIM has quickly filled in the gaps.
James Russell is the principal investigator of AIM and a professor at Hampton University. He said:
It turns out that meteoroids play an important role in the formation of noctilucent clouds. Specks of debris from disintegrating meteors act as nucleating points where water molecules can gather and crystallize.
Ash and dust from volcanoes—and even rocket exhaust—also can serve their nuclei.
Night-shining clouds most often appear during spring and summer because more water molecules are wafted up from the lower atmosphere to mix with the meteor debris and ash. The warmest months in the troposphere (lower atmosphere) are also the coldest in the mesosphere (where noctilucent clouds form).
According to Randall and other scientists, noctilucent clouds are becoming more frequent and widespread. In the 19th century, reports of NLCs were mostly confined to high latitudes. In recent years, however, they have been sighted as far south as Utah, Colorado, and Nebraska. Some researchers assert that this is a sign of greenhouse warming, as methane has become more abundant in Earth’s atmosphere. Russell said:
When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor. This extra water vapor is then available to grow ice crystals for noctilucent clouds.
Randall suggested that the earlier start in 2013 may be the result of a change in atmospheric “teleconnections,” or the way changes in one part of the atmosphere affect another. Randall said:
Half-a-world away from where the noctilucent clouds are forming, strong winds in the southern stratosphere are altering global circulation patterns. This year, more water vapor is being pushed into the high atmosphere, and the air there is getting colder.
Bottom line: In 2013, the noctilucent – or night-shining – cloud season got an early start. NASA’s AIM spacecraft first saw them on May 13. The season started a week earlier than any other season that AIM has observed, and quite possibly earlier than ever before, said Cora Randall of the Laboratory for Atmospheric and Space Physics at the University of Colorado. An AIM satellite image shows noctilucent clouds in Earth’s upper atmosphere.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.