It's summer at the South Pole, which means it's time for the frozen continent's noctilucent clouds to make an appearance. But over the last several decades, these beautiful electric blue clouds have been appearing earlier than usual — and they're getting bigger. It's yet another example of climate change at work.

Noctilucent clouds, or NLCs, are the highest clouds known to science, forming at the edge of space some 50 miles (83 km) above the Earth's surface. They're seeded by disintegrating meteoroids and form when sunlight hits the tiny ice crystals that make up the clouds. At times they appear to glow a mesmerizing electric blue.

NLCS appear in summer on account of favourable wind patterns and the flow of humidity in the atmosphere. During summer, when the amount of airborne water molecules are at their greatest, they're pulled up from the lower atmosphere to mix with the "meteor smoke" at the edge of space. It's also a time when the upper atmosphere is coldest, allowing the ice crystals of NLCs to form.


Image: James Russell of Hampton University/NASA.

Over the last few decades and years, these clouds have become more intense and widespread. During the 19th century, NLCs could only be viewed in polar regions — but now they're appearing as close to the equator as Colorado and Utah. Some scientists say this is climate change at work. As NASA reports:

One of the greenhouse gases that has become more abundant in Earth's atmosphere since the 19th century is methane.

"When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor," explains Hampton University Professor James Russell, the principal investigator of AIM. "This extra water vapor is then available to grow ice crystals for NLCs."

If this idea, one of several, is correct, noctilucent clouds are a sort of "canary in a coal mine" for one of the most important greenhouse gases. And that, says Russell, is a great reason to study them.

[ Source: NASA | Top image: NASA/AIM ]