Storms

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Storms Essay, Research Paper

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.

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