© Valtteri ImmonenDecember 3, 2017 @ Muonio, Lapland, Finland


Rainbows usually require liquid water. Droplets falling out of the sky intercept beams of sunlight, reflecting them back in a colorful spray of red, green, and blue. Yesterday, Valtteri Immonen saw such a rainbow over Muonio, Finland, but something was missing: the raindrops. "It was -6 degrees C, no rain and no snowfall," he says. "Yet there was a huge rainbow across the sky."

"I have never seen a rainbow during the winter and I didn't even know that it is possible for them to form when the temperature is below 0 degrees Celsius," he marvels.

© Valtteri ImmonenDecember 3, 2017 @ Muonio, Lapland, Finland
Atmospheric optics expert Les Cowley says that raindrops must have been present: "The rainbow's appearance, narrow with a supernumerary, says that it was produced with reasonable size water drops, perhaps about 0.5 mm in diameter. I surmise that the there was high level precipitation that did not necessarily reach the ground. The drops could also have been supercooled below the normal freezing point of water," he speculates.

Supercooled raindrops can form when droplets of liquid water fall through layers of subfreezing air. Droplets containing specks of dust or even microbes readily freeze as ice crystals form around the impurities. But when rain droplets are especially pure, they can remain in a liquid state even when the temperature drops below freezing.

Last winter, supercooled rainbows were sighted in the Arctic as strange weather gripped the poles. Immonen's photo suggests that more of these rare rainbows may be in the offing. Stay tuned.


Red sprites tend to be a summertime phenomenon. They shoot up from the tops of intense thunderstorms, reaching toward the edge of space, while their cousins, normal lightning bolts, lance down to the ground below. Summer is the season for sprites. So it came as a surprise, on the night of Dec. 2nd, when Martin Popek photographed a phalanx of winter sprites leaping above the snowy landscape of the Czech republic:
© Martin PopekTaken December 1st 2017
"This is a very rare thing," says Popek. "I've been monitoring sprites from my private observatory since 2011. This is only the second storm with red sprites I have ever seen in December."

Sprites are a true space weather phenomenon. Some researchers believe they are linked to cosmic rays: subatomic particles from deep space striking the top of Earth's atmosphere produce secondary electrons that, in turn, provide the spark that triggers sprites. If this is true, then sprites could multiply in the years ahead as cosmic rays intensify due to the decline of the solar cycle. Indeed, increasing levels of cosmic rays might explain why the strange red forms are spilling over into the "off-season."

Although sprites have been seen for at least a century, most scientists did not believe they existed until after 1989 when sprites were photographed by cameras onboard the space shuttle. Now "sprite chasers" routinely photograph sprites from their own homes. "I used up a Watec 910HX security camera with UFOCapture software to catch my sprites," says Popek. Give it a try! Diagram: How to Look for Sprites