|
Optical
Weather Phenomena
|
The
clear, almost pollution-free skies of Antarctica's interior combined
with extremely low humidities make it an ideal location for viewing
spectacular optical displays. Wide
open vistas unobstructed by trees, buildings or other clutter
give panoramas of the sky that stretch for dizzying distances.
The frigid temperatures mean that little or no water vapor is
held in the air, instead it freezes and falls out as ice crystals,
builds up on surfaces as frost, or is suspended in the air. Depending
on the particular atmospheric conditions, these ice crystals can
reflect sunlight in a variety of ways resulting in many fantastic
and fascinating atmospheric phenomena.
|
|
|
A fun thing to do in -60°F cold weather is to throw
a pot of boiling water into the air.
Take a pot and fill it with boiling
water.
Throw it all up into the air. Voila,
instant ice crystals!
As the +212ºF water meets the
-60°F, it instantly vaporizes.
Most of it is turned into a cloud
of steam that drifts gently away.
But some of the droplets are turned
into tiny crystals which make a tinkling sound as they
land on the hard surface of the ground.
Often, a solar halo is formed around
the sun by the ice crystals in the air.
|
Solar
Haloes occur
around the sun when light is refracted through ice crystals. They
are usually encountered in the winter when lower temperatures
make such occurrences more likely.
Sun
dogs are luminous spots on both sides
of the sun that occasionally occur with a halo.
Air
of different temperatures refracts light in different ways.
Mirages
are commonly seen on the horizon in the winter or at the end
of winter when the sea-ice has just broken up. They are a result
of temperature differences in the bottom few yards just above
the ice or sea surface.
This is the same phenomena responsible for "heat
haze" as seen above a road on a very hot day.
The
fata morgana is a complex mirage in which distant
flat objects appear to have tall cliffs, columns, and pedestals.
A
superior mirage occurs when
an image of an object appears above the actual object, due to
the downward refraction of light in cold, dense air.
Ice
blink
refers to a white glare seen on the underside of low clouds
indicating the presence of ice which may be beyond the range
of vision.
Water
sky refers to the dark appearance
of the underside of a cloud layer when it is over a surface
of open water.
A
corona is a ring of deflected
light that surrounds the sun or moon, sometimes forming a colorful
disk with the sun or moon at its center.
A
fog bow is similar to a
rainbow, but without any colors due to the very small size of
the water droplets.
A Solar
Pillar occurs when the sun is reflected so strongly
that the reflection is almost as bright as the sun itself. The
pillar appears to move when the observer moves, but always remains
directly below the sun. Like a rainbow, this sight is dependent
on where the light is coming from and where the observer is
standing.
|
|
|
Low cloud cover over snow usually
leads to a condition known as a "white-out"
during which no visual points of reference exist,
causing a loss of depth perception -- for both humans
and wildlife.
Navigation under these conditions
is exceedingly difficult and a compass is essential.
If surface vehicles are operated
regularly in places prone to white-outs and blizzards,
the regular routes will be marked by fuel drums or metal
tagged canes that can be picked up by radar.
|
Ice
fog occurs when seawater exposed to much colder air
above pack ice condenses into fog. It is often seen at "tide-cracks"--
openings that form around offshore rocks and small islands when
the tide rises and falls with continuous sea-ice present. As
the ice is not flexible it cracks and as it does, exposes an
amount of open water to the air.
Antarctic
sea water temperatures vary between about 34ºF and 28ºF
over the course of a year, so the exposed sea water is often
more than 70ºF warmer than the surrounding air. Sunshine
makes the fog more visible and different temperature layers
in the air cause it form in bands above the clearer air close
to the ice surface.
A
Föhn bank is formed
by a warm contour-hugging wind (a Föhn
wind) that is blowing across the land. As the warmer wind blows
across the much cooler land, it causes snow and ice to sublime
(change from solid directly to gas). The overall effect as seen
from a distance is that the land is covered by a very large
blanket of condensed vapor. The gross contours can be seen through
the cloud layer, but all of the finer detail is obscured.
Noctilucent
clouds are clouds that form in the
cold, summer mesopause at latitudes of 60 to 70 degrees north
and south and are believed to be made of ice crystals. They
are seen at extremely high altitudes (about 50 miles) and are
thus, illuminated by the sun long after it has set over the
horizon. The terms 'nocti' and 'lucent' are derived from the
Latin where they roughly translate to 'night' and 'luminous'.
They appear pearly white in color and often appear very delicate
in texture.
|