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1995-03-31
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Haloes Forecast Storms NEWSCIENCE
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If you know where and when to look, you can treat yourself
to a colourful display of atmospheric haloes, spots and
pillars. These images can tell you something about the
clouds overhead and possible changes in the weather.
All of these images are created by light shining through
cirrostratus clouds. These clouds occur at an altitude of
6,000-12,000 metres. They appear as a thin sheet or layer
(strata) that is pure white. The layer of cloud is so
thin (only 100-450 metres) that is doesn't obscure the sun
or moon, so you should be able to see your shadow.
Cirrostratus is made of many types of ice crystals.
However, four crystal shapes are responsible for producing
most of the commonly see haloes-plate crystals, columns,
capped columns and bullets.
The most obvious halo is found around the sun. If the
layer of cirrostratus is extensive, you'll see an entire
ring. Within the layer of cloud, sunlight is striking and
passing through the sides of randomly-oriented ice
crystals. As the sunlight passes through each crystal,
the light changes direction, or refracts. The radius of
the hale depends on the amount of change in the direction
of the sun's light. Usually this is 22 degrees. Since
the sun is 1/2 of a degree across, the radius of the halo
is 44 sun-widths. Occasionally you may see a second halo
at 46 degrees from the sun (that is, with a radius of 92
sun-widths). This is produced by sunlight passing through
both the side and bottom of each crystal. Moonlight will
also produce a halo, around the moon, with the proper
layer of cirrostatus.
Another common optical effect is known as "mock suns" or
"sun dogs" or "parhelia" (Greek for "with the sun").
These bright spots on either side of the sun, outside of
the halo, occur when sunlight passes through the sides of
capped columns, bullets and plate crystals, when these
crystals are arranged with their sides vertical. The
crystals wobble, diffusing and smearing the colours of the
mock sun.
You can see haloes and mock suns more clearly if you block
out your view of the real sun by holding your hand in
front of it at arm's length. Another spectactular optical
effect is the solar pillar. This is a vertical shaft of
light the same colour as the sun stretching upwards from
the sun and is most often seen at sunset or sunrise. It's
produced by sunlight reflecting of the base of plate and
capped column crystals in the clouds. You can also see
pillars in an ice fog, when it's illuminated by
streelights, or airport runway lights, for instance.
The appearance of all these optical images is a good
indication that the weather will change. Strong vertical
air currents associated with low pressure storms carry
moist air skyward, where the water freezes. High speed
winds above the storm system push the ice crystals on
ahead. When you see haloes around the sun or moon, you
can be sure of two things-there are cirrostratus clounds
above and, in a day or two, the skies will darken with an
approaching storm.