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.

Category: Science