Tuesday, June 21, 2011
Noctilucent or night-shining clouds (NLCs) are wispy blue-white clouds that glow and ripple in the twilight sky at high altitudes.
In the Southern Hemisphere, NLCs appear from mid-November to mid-February, while in the Northern hemisphere, they can be seen from mid-May to mid-August with now being a prime viewing time.
These clouds form in the mesophere, an upper layer of the Earth's atmosphere, around 50 miles (80 kilometers) above ground.
This mysterious phenomenon was first noticed after Krakatoa erupted back in 1883. The Indonesian volcano sent dust and ash up into the mesosphere, triggering spectacular sunsets for several years.
Then, in 1885, German sky watcher T.W. Backhouse spotted TLCs while observing a fading Krakatoa sunset, and is credited with their discovery by some.
The clouds may be caused by particles released from volcanoes and/or dust that originates from outer space when meteorites collide with the Earth's atmosphere, and a few seem to have formed due to freezing water exhaust from space shuttles.
During a process called nucleation, water molecules adhere to a nucleus, such as dust. For water vapor to reach such high altitudes, upwelling summertime winds are needed, which explains why the phenomenon is seasonal.
YouTube screenshot showing NLCs - thin, wispy clouds, glowing electric blue. Photo courtesy: theepochtimes
NLCs can only be seen under certain conditions:
* The sky is free of ordinary tropospheric clouds;
* Light is present atmospheric region where they form, meaning the sun must be a maximum of 16 degrees below the horizon;
* A dark background against which the luminous clouds can contrast, meaning the sun must be a minimum of 6 degrees below the horizon;
* Viewing location should be at a latitude north of 45 degrees, for example Minneapolis, although in the last few years they have been sighted further south.
There is some speculation these clouds could be linked with climate change as they have been sighted more frequently in recent decades, and with increasing brightness and area.
YouTube screenshot showing low level tropospheric clouds moving in the opposite direction to NLCs above. Photo courtesy: theepochtimes