2:50 PM (Tuesday) | *A spectacular outbreak of polar stratospheric clouds (PSC) continues around the Arctic Circle*

Polar stratospheric clouds (PSC) as seen today over Sweden.

Overview

Normally, the stratosphere has no clouds at all as it is an extremely dry layer in Earth’s atmosphere.  Every once in awhile, however, it gets so cold up there that the sparse water molecules can actually assemble themselves into icy clouds. Conditions for the past couple of days have been conducive to the formation of such clouds as it is extremely cold in the stratosphere over the Arctic Circle with temperatures as low as -85 degrees (C).  As a result, there has been an outbreak of these polar stratospheric clouds and some reports suggest they are more prevalent than ever filling up as much as 25% of the sky.

Stratospheric temperatures as low as 85 degrees (C) below zero currently exist over the Arctic Circle; map courtesy NOAA

Discussion

The stratosphere is the second major layer of Earth’s atmosphere, just above the troposphere and below the mesosphere.  This layer is stratified (i.e., layered) in temperature with warmer layers higher and cooler layers closer to the Earth.  The stratosphere is known as being home to the ozone layer and is not known for its clouds as there usually are none.  When temperatures drop to extremely low levels, however, icy clouds can form, but it is far rarer for this to happen than, for example, the northern lights.  These particular clouds are sometimes referred to as “mother-of-pearl” clouds and the more technical term is “nacreous” clouds.

This picture of polar stratospheric clouds was taken earlier today by Per-Anders Gustavsson in Jukkasjärvi, Sweden and it shows why some onlookers mistake them for daytime auroras; photo courtesy spaceweather.com

The current outbreak of polar stratospheric clouds began on Monday, December 30th and the Scandinavian countries such as Sweden have reported seeing the most activity.  The latest reading of temperatures at stratospheric levels (10 millibars or about 15 miles high) indicate there is indeed some extreme cold of between 80 and 85 degrees (C) below zero that is centered near and over Scandinavia. Polar stratospheric clouds are very colorful because they are made up of a special type of ice.  High-altitude sunlight shining through the microscopic crystals only about 10 microns across produce a bright iridescent glow that is often mistaken as the northern lights. These clouds are best observed during twilight when the sun is between 1 and 6 degrees below the horizon.

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com