Rare ‘shining’ clouds linked to climate change

By Alexandra Back 2 February 2011
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Rare noctilucent, or ‘shining at night’ clouds have been captured by a NASA researcher.

AS IF THE POLAR skies weren’t already brilliant enough with the winter spectacle that are the northern lights, scientists are now observing the growing brightness of the beautiful noctilucent or ‘shining at night’ clouds more often.

These clouds, considered extraordinarily rare, develop within the extreme conditions found near the ‘mesopause’ layer of atmosphere. Roughly 80 to 90 kilometres above sea level, the mesopause is the coldest part of the entire Earth’s environment.

When the temperature in this layer drops below -130ºC the water vapour freezes into ice crystals, materialising into clouds, and reflects sunlight, even after the sun itself has set. From our perspective on Earth, the contours and ridges of the clouds appear boldly illuminated from below the horizon.

To witness these mesospheric clouds yourself, you’lll need to head towards a latitude of between 55 and 65 degrees from the equator – in Norway or Russia for example – or if you’re based in the south, Antarctica or Argentina are the best viewing locations. Ideally you would visit in the summer months at which time, contrary to what you might expect, the mesosphere is at its coldest, and cast your eyes skyward about two hours after sunset when the sky turns dark enough to see them.

Matthew DeLand, an atmospheric scientist with NASA’s Goddard Space Flight Center, has been styding the increase in both formation and brightness of these clouds over the previous decades. He has found that changes in temperature and humidity in the mesosphere make the clouds brighter and more frequent. That is, colder temperatures allow more water to freeze, while an increase in water vapour gives rise to more ice cloud formation.

On NASA’s Earth Observatory website, Matthew suggests the upward trend in brightness and occurrence is related to the mesosphere becoming colder and more humid over time – a change he hints could be linked to the presence of greenhouse gases in the atmosphere.

The clouds have been observed thousands of times in the Northern Hemisphere, but fewer than 100 observations have been reported from the Southern Hemisphere. Whether this is due to hemispheric differences of temperature and water vapour or simply a lack of observers, is yet to be determined and is currently the subject of an Australian Antarctic Division study.