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                     Haloes Forecast Storms
	                      NEWSCIENCE

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.