Noctilucent clouds
© YouTube/Gerd Baumgarten (screen capture)
They're back. Noctilucent clouds (NLCs), recently missing, are once again circling the South Pole. And, in an unexpected twist, they've just appeared over Argentina as well.

"This is a very rare event," reports Gerd Baumgarten of Germany's Leibniz-Institute of Atmospheric Physics, whose automated cameras caught the clouds rippling over Rio Grande, Argentina (53.8S) on Jan. 3rd:


What's so strange about that? At this time of year, noctilucent clouds are supposed to be confined to Antarctic latitudes--not Argentina. In the whole history of atmospheric research, NLCs have been sighted at mid-southern latitudes only a handful of times.

"Personally, I am thrilled to see NLCs in Argentina, as I had not expected them to occur so far north," says Natalie Kaifler of the German Aerospace Center (DLR), who operates a lidar (laser radar) alongside one of Baumgarten's cameras.

Lidar echoes captured during the display confirm that these are genuine NLCs floating more than 80 km above Earth's surface:

Lidar echoes

Above: The ~hour-long oscillations in these lidar echoes may be caused by gravity waves propagating upward from the Andes 82 km below.
NLCs are Earth's highest clouds. They form when summertime wisps of water vapor rise up from the poles to the edge of space. Water crystallizing around specks of meteor dust ~83 km above Earth's surface create beautiful electric-blue structures, typically visible from November to February in the south, and May to August in the north.

This season has been unusual. The normal onset of NLCs in the south has been delayed for more than a month as strange weather patterns played out over Antarctica. Now, suddenly, they're back, and showing up in unexpected places.

Baumgarten has set up two cameras in southern Argentina to catch unexpected NLCs. "If it happens again," he says, "we'll let you know." Stay tuned!