09 August 2012


One of nature's most ephemeral sights may be a harbinger of climate change.  Noctilucent clouds (NLCs) form high in the mesosphere, 76 to 85 kilometers (47 to 53 miles) above the Earth's surface, at latitudes between 50 and 70 degrees north and south of the equator.  They are so faint and so high that they only visible when the sun is below the horizon.

The term "noctilucent" derives from Latin, literally translating as "night shining".  These wispy, pale blue clouds were first recorded by skywatchers in 1885, two years after the eruption of Krakatoa.  At first it was thought that volcanic ash might be responsible for their formation, but they have persisted long after the ash settled back to the surface.  Now scientists at NASA have announced a new connection ~ the dust around which sparse water vapor condenses at that altitude is space dust.

"The inner solar system is littered with meteoroids of all shapes and sizes ~ from asteroid-sized chunks of rock to microscopic specks of dust.  Every day Earth scoops up tons of the material, mostly the small stuff.  When meteoroids hit our atmosphere and burn up, they leave behind a haze of tiny particles suspended 70 km to 100 km above Earth's surface.  It is no coincidence that NLCs form 83 km high, squarely inside the meteor smoke zone.

" .... In the 19th century, NLCs were confined to high latitudes ~ places like Canada and Scandinavia.  In recent times, however, they have been spotted as far south as Colorado, Utah and Nebraska.  The reason is climate change.  One of the greenhouse gases that has become more abundant in Earth's atmosphere since the 19th century is methane.  It comes from landfills, natural gas and petroleum systems, agricultural activities, and coal mining.  It turns out that methane boosts NLCs.

" .... If this idea is correct, noctilucent clouds are a sort of 'canary in a coal mine' for one of the most important greenhouse gases.  And that is a great reason to study them.  Noctilucent clouds might look alien, but they're telling us something very important about our own planet."

Note that the first sightings of NLCs coincides with the advent of accumulating pollution from the Industrial Revolution, whose chief energy driver was burning coal (hence producing methane, boosting the abundance of water in the upper atmosphere and contributing to the formation of noctilucent clouds.  You can learn more in the original NASA announcement here.  Be sure to watch the embedded video explaining how "meteor smoke" seeds noctilucent clouds.  The image at top was taken from Earth's surface, in Estonia.  The image below was taken from the orbiting International Space Station.  Click on either image to enlarge.

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