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NOAA - 12/24/2023


Now we know why polar stratospheric clouds (PSCs) have suddenly exploded. According to NASA's MERRA-2 climate model, temperatures in the Arctic stratosphere just hit a 40-year record low for the month of December:


Cold air in the stratosphere is exactly what PSCs require. Normally, the stratosphere has no clouds at all. But when the temperature drops to a staggeringly-low -85 C, widely-spaced water molecules coalesce into ice crystals and PSCs begin to form. Their aurora-like colors make them the most beautiful clouds on Earth.

PSCs are normally confined to the Arctic where the stratosphere is coldest. During this week's extreme cold wave, the clouds descended all the way to mid-latitudes. Here they are over Locarno,Switzerland (+46N):


"I saw these clouds for the first time on Dec. 22nd," says photographer Branca Cristina. "The colors were amazing!"

At the same time, the clouds were sighted in Torun, Italy (+45N); the next morning they appeared again in Lausanne, Switzerland (+47N). These are extraordinary excursions from normal PSC habitat.

The season for PSCs usually starts in January. The current cold wave has given the season an early start, and could herald many more clouds in the weeks to come. To help sky watchers catch these rare clouds, we will henceforth publish daily predictions of temperatures in the Arctic stratosphere. When the air is cold, it's time to look! Look here for the forecast.

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