THE POLAR VORTEX WOBBLED IN DECEMBER: Last month, sky watchers in Europe saw something rare and beautiful. A giant bank of polar stratospheric clouds (PSCs) escaped the Arctic Circle, and for more than a week they filled skies with rainbow color as far south as Italy. In the Italian comune of Sanfrè (latitude +45N), Pablo Javier Lucero was able to photograph the clouds at all hours of the day:

Surprise is the correct reaction. Normally, Earth's stratosphere has no clouds at all. Only when the temperature drops to a staggeringly-low -85 C can widely-spaced water molecules assemble into icy polar stratospheric clouds. With colors that rival auroras, PSCs are considered to be the most beautiful clouds on Earth.

During the outbreak, Spaceweather.com received hundreds of photos of PSCs. Curiously, they all came from Europe. Not a single cloud was sighted in North America or Asia. Why not?

Amy Butler of NOAA's Chemical Sciences Laboratory has the answer: "The polar vortex can wobble like a top, and in December it was displaced towards Europe," she explains. "You can see this in the figure below from Zac Lawrence's website stratobserve.com."

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Cold air normally contained over the poles was carried by the wobbling vortex to mid-latitudes. Purple and green contours in the figure show where temperatures were cold enough for Type I and Type II PSCs, respectively.

Since the December outbreak of PSCs, which ended around Christmas, the polar stratosphere has warmed more than 30 degrees Celsius. Butler says this is probably the result of "a strong planetary wave breaking in the stratosphere." Wave energy warmed the air and dispersed the rainbow-colored clouds.

The clouds might return. As January unfolds, the polar stratospheric vortex is still tilted toward mid-latitudes in Europe. If the stratosphere cools again, PSCs could re-appear over many populated areas. You can monitor the situation with daily temperature reports right here on Spaceweather.com.