7:15 AM | *Temperatures in the polar stratosphere are about to drop to their lowest levels in decades...the result might be an abundance of rarely seen colorful polar stratospheric clouds*

Photograph of “polar stratospheric clouds” taken in Switzerland on January 24th, 2024 (courtesy Andrzej Blonski, spaceweather.com)

Overview

NASA’s satellite-based MERRA-2 climate model is forecasting temperatures to drop to their lowest levels since before 1978 in the polar stratosphere and this might just result in a major outbreak of rarely seen colorful polar stratospheric clouds. Normally, the stratosphere has no clouds at all; however, when the temperature drops to around -85 C (188 K) or below, water molecules can coalesce into ice crystals and form polar stratospheric clouds – even in the very dry part of the upper atmosphere.

NASA’s MERRA-2 climate model is forecasting polar stratospheric temperatures to plunge in coming days to low levels (indicated by arrow) not seen in decades. Plot courtesy NASA/GSFC

Details

The Arctic Circle could soon be filled with polar stratospheric clouds (PSCs) which are widely regarded as some of the most beautiful clouds ever seen in the Earth’s atmosphere. These clouds are quite uncommon and require exceptionally cold air at the highest levels of the atmosphere. Indeed, NASA’s satellite-based Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) climate model is forecasting polar stratospheric temperatures to drop to their lowest levels in decades in the days ahead to as low as -85 C (188 K). This is the type of exceptionally cold air that can allow widely spaced water molecules to coalesce and form PSCs according to spaceweather.com. The MERRA-2 has been providing data to NASA since 1980 with an advanced data assimilation system and is the first long-term global reanalysis to assimilate space-based observations of aerosols and represent their interactions with other physical processes in the climate system.

Photograph of “polar stratospheric clouds” taken in Norway on January 18th, 2025 (courtesy Marianne Bergli, spaceweather.com)

Polar stratospheric clouds are technically classified as “nacreous” clouds either of “Type I” or “Type II” depending on the ambient temperatures in which they form and also other characteristics. “Type I” clouds are slightly warmer with less bright colors and are composed of a mixture of nitric acid and water. “Type II” clouds - often referred to as “mother-of-pearl” clouds - are composed of ice crystals and can be as colorful as the most stunning auroras seen in the northern latitudes (and occasionally seen in the middle latitudes as was the case during 2024). This coming plunge in temperatures is likely to result in the more spectacular-looking “Type II” polar stratospheric clouds in coming days across the Arctic Circle, and potentially, even at lower latitudes.

Meteorologist Paul Dorian
Arcfield
arcfieldweather.com

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