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| Noctilucent clouds at Sweden |
Night clouds or noctilucent clouds are tenuous cloud-like phenomena that
are the "ragged edge" of a much brighter and pervasive polar cloud
layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight.
They are made of crystals of water ice. Noctilucent roughly means night shining
in Latin.
They are most commonly observed in the summer months at latitudes
between 50° and 70° north and south of the equator.
They can be observed only when the Sun is below the horizon.
They are the highest clouds in Earth's atmosphere, located in the mesosphere
at altitudes of around 76 to 85 kilometres (47 to 53 mi). They are normally too
faint to be seen, and are visible only when illuminated by sunlight from below
the horizon while the lower layers of the atmosphere are in the Earth's
shadow. Noctilucent clouds are not fully understood and are a
recently discovered meteorological phenomenon; there is no record
of their observation before 1885.
Noctilucent clouds can form only under very
restricted conditions; their occurrence can be used as a sensitive guide to
changes in the upper atmosphere. They are a relatively recent classification.
The occurrence of noctilucent clouds appears to be increasing in frequency,
brightness and extent. In 2012 Cumberland's doctoral work in physics supported
the possible interpretation of noctilucent clouds as a Miner's Canary for climate change.
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| Noctilucent clouds at Estonia |
Noctilucent clouds are generally colourless or pale
blue, although occasionally other colours including red and green occur. The
characteristic blue colour comes from absorption by ozone in the path of the
sunlight illuminating the noctilucent cloud. They can appear as featureless
bands, but frequently show distinctive patterns such as streaks, wave-like
undulations, and whirls.[41]
They are considered a "beautiful natural phenomenon". Noctilucent
clouds may be confused with cirrus clouds, but appear sharper under
magnification. Those caused by rocket exhausts tend to show colours other than
silver or blue, because of iridescence caused by the uniform size of the water
droplets produced.
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| Noctilucent clouds atNetherlands |
Noctilucent clouds may be seen by observers at a latitude
of 50° to 65°. They seldom occur at lower latitudes (although there have been
sightings as far south as Paris, Utah,
Italy,
Turkey
and Spain),
and closer to the poles it does not get dark enough for the clouds to become
visible. They occur during summer, from mid-May to mid-August in the northern
hemisphere and between mid-November and mid-February in the southern hemisphere.
They are very faint and tenuous, and may be observed only in twilight
around sunrise and sunset when the clouds of the lower atmosphere are in
shadow, but the noctilucent cloud is illuminated by the Sun. They are best seen
when the Sun is between 6° and 16° below the horizon. Although noctilucent
clouds occur in both hemispheres,
they have been observed thousands of times in the northern hemisphere, but
fewer than 100 times in the southern. Southern hemisphere noctilucent clouds
are fainter and occur less frequently; additionally the southern hemisphere has
a lower population and less land area from which to make observations.
The clouds may show a large variety of different
patterns and forms. An identification scheme was developed by Fogle in 1970
that classified five different forms. These classifications have since been
modified and subdivided.
They may be studied from the ground, from space,
and directly by sounding rocket. Also, some noctilucent clouds
are made of smaller crystals, 30 nm or less, which are invisible to
observers on the ground because they do not scatter enough light.
source : Wikipedia.com
source : Wikipedia.com
