Noctilucent Clouds (NLC) are exquisite clouds that not many people are aware of and they absolutely deserve all the attention. Noctilucent clouds are the “ragged-edge” of a polar cloud layer (polar mesospheric clouds). These ghostly clouds are mysterious and cause controversy specifically of what their presence could mean or indicate. Noctilucent roughly means night shining in Latin. According to NASA, they're like great
“Geophysical Light Bulbs” – turning on every year in late spring and
can be seen many evenings during the summer months. Those are the short summer nights from mid May to mid August (mid-November to mid-february in
the Southern Hemisphere), of latitudes of 50
– 65° which reveal the highest clouds.
They are never visible in daylight.
Wait until an hour after sunset when twilight has deepened.
The sun should be 6 – 16° below
the horizon, enough to darken the sky but not so low that the NLCs
are not still in sunlight. Search first of all low in the sky towards
the direction of the sun beneath the horizon, northwest before midnight,
northeast afterwards. Avoid moonlit nights when lower clouds and
especially cirrus are lit white against a darker sky. Binoculars help in distinguishing them from lower cirrus clouds because
under magnification they appear sharper. NLCs are seen
further south. They have been sighted
in Europe as far south as Austria, Hungary, Italy and
southern Germany. In the US they have been seen in
Utah and Colorado.
They are colourless
or bluish-white, although reds and greens can occur. Those caused by rocket exhausts tend to show colors other than silver or blue because of the iridiscence caused by the uniform size of water droplets produced. They can be skein-like, rich with undulations,
currugations, knots and streaks clawing upwards into the sky, at
other times they lie close to the horizon as a featureless band.They are made of crystals of water ice.
Above: The optimum viewing geometry for noctilucent
clouds. Sunlight scattered by tiny ice crystals in NLCs is what
gives the clouds their characteristic blue color.
They are the highest clouds in the Earth's atmosphere. Although NLCs look like they're in space,they're inside Earth's atmosphere, in a
layer called the mesosphere ranging from 50 to 85 km high , (up to 53 mi), where 99.99% of the atmosphere is below. 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. The mesosphere is not only very cold (-125 C), but also very
dry, it is said one hundred million times dryer than air from the
Sahara desert. Nevertheless, NLCs are made of water.
clouds consist of tiny ice crystals about the size of particles
in cigarette smoke. How ice crystals form in the arid mesosphere is the essential
mystery of noctilucent clouds. Ice crystals in clouds need two things
to grow: water molecules and something for those molecules to
stick to. Dust, for example. Water gathering on dust to form
droplets or ice crystals is a process called nucleation. It happens
all the time in ordinary clouds. Ordinary clouds, which are close to Earth, get their dust
from sources like desert wind storms. It's hard to waft wind-blown
dust all the way up to the mesosphere, however. Despite the dryness at these altitudes,
convective activity could carry water vapor high into the mesosphere.
However, the origin of dust particles remains a mystery.
Noctilucent clouds are not fully understood and are a recently-discovered meteorological phenomenon; there is no record of their observation before 1885. Noctilucent clouds are first known to have been observed in 1885, two years after the 1883 eruption of Krakatoa It remains unclear whether their appearance had anything to do with the volcano eruption, or whether their discovery was due to more people observing the spectacular sunsets caused by the volcanic debris in the atmosphere. Studies have shown that noctilucent clouds are not caused solely by volcanic activity, although dust and water vapour could be injected into the upper atmosphere by eruptions and contribute to their formation.Scientists at the time assumed the clouds were another manifestation of volcanic ash, but after the ash had settled out of the atmosphere, the noctilucent clouds persisted.
Every day Earth sweeps up tons of meteoroids–tiny bits of debris from comets and asteroids. Most are just the right size to seed noctilucent clouds.The source of water vapor is less controversial. Upwelling winds in the summertime carry water vapor from the moist lower atmosphere toward the mesosphere,this is why NLCs appear during summer.One reason for the recent spread of noctilucent clouds might be global warming. Extreme cold is required to form ice in a dry environment like the mesosphere. Ironically, global warming helps. While greenhouse gases warm Earth's surface, they actually lower temperatures in the high atmosphere. Noctilucent clouds were first spotted during the Industrial Revolution–a time of rising greenhouse gas production.
The mystery surrounding these clouds still exists; NASA has even sent a satellite called AIM into the polar atmosphere in 2007, trying to answer the questions still out there. Says Thomas, one of the co-investigators: “It's a puzzle. Noctilucent clouds have not only persisted, but also spread. In the beginning, the clouds were confined to latitudes above 50 degrees; you had to go to places like Scandinavia, Siberia and Scotland to see them. In recent years, however, they have been sighted from mid-latitudes such as Colorado, Utah and Oregon.”
Definition of a cloud: A cloud is a visible aggregate of minute particles of water or ice, or of both, in the free air. This aggregate may include larger particles of water or ice and particles, such as those present in fumes, smoke or dust.
Appearance of clouds:
The appearance of a cloud is determined by the nature, sizes, number and distribution in space of its constituent particles. Appearance is best described in terms of dimensions, shape, structure texture, luminance and colour of the cloud.
Classification of clouds:
A classification pf the charcteristic forms of clouds, in terms of “genera”, “species” and “varieties” has been established
The classification is essentially based on 10 main groups, called genera, which are mutually exclusive, that is to say, a given cloud can belong to one genus only. The genera are;
Obsever peculiarities in the shape of clouds and differences in their internal structure have led to the subdivision of most of the cloud genera into species.
Clouds may exhibit special characteristics which determine their variety. These characteristics are related to the different arrangements of the could elements.