Noctilucent clouds seen over Scotland
The fine-structured blue clouds floating above the dark, ordinary storm clouds are the NLCs. "It was hard to tell the full extent of the display due to a storm system passing through," says Ferrier. "But the noctilucent clouds were definitely there." Jimmy Fraser of Alness, Scotland, also photographed the display. "It was a great start for the NLC season here in northern Scotland," says Fraser.
NLCs are Earth's highest clouds. Seeded by meteoroids, they float at the edge of space more than 80 km above the planet's surface. The clouds are very cold and filled with tiny ice crystals. When sunbeams hit those crystals, they glow electric-blue.
Noctilucent clouds first appeared in the 19th century after the eruption of super-volcano Krakatoa. At the time, people thought NLCs were caused by the eruption, but long after Krakatoa's ash settled, the clouds remained. In recent years, NLCs have intensified and spread with sightings as far south as Utah and Colorado. This could be a sign of increasing greenhouse gases in Earth's atmosphere.
Observing tips:
Look west 30 to 60 minutes after sunset when the Sun has dipped 6o to 16o below the horizon. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud.
Radar Echoes From The Noctilucent Zone:Every summer since the late 1970s, radars probing Earth's upper atmosphere have detected strong echoes from altitudes between 80 km and 90 km. These altitudes comprise the "noctilucent zone," where water vapor crystallizes around meteor smoke to form icy noctilucent clouds (NLCs). The first NLCs of the 2015 northern summer season were spotted by NASA's AIM spacecraft on May 19th. The radar echoes have followed close behind.
Les Dean of the MST Radar Facility in Aberystwyth, Wales, reports: "We detected our first echoes of the summer season on May 26th."
Researchers call them "Polar Mesospheric Summer Echoes" or "PMSEs." They occur over the Arctic during the months of May through August, and over the Antarctic during the months of November through February. These are the same months that NLCs appear.
But do the radar echoes actually come from noctilucent clouds? "The association is controversial," notes Dean. A leading theory holds that the ice particles in noctilucent clouds are electrically charged, and this makes them good reflectors of HF radio waves. However, NLCs are not always visible when the radar echoes are observed and vice versa. So the connection is not clear-cut.
One thing is sure: the northern season for both NLCs PMSEs has begun. Stay tuned for more echoes from the noctilucent zone.
"What is happening 90 km above Earth's surface?" wonders researcher Rob Stammes at the Polarlightcenter in Lofoten, Norway. For the past two nights, he has detected intense radio reflections using a forward scatter meteor radar. The phenomenon is almost surely linked to the PMSEs and noctilucent clouds reported above.
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