Haloes Forecast Storms Essay, Research Paper

Haloes Forecast StormsIf you know where and when to look, you can treat yourselfto a colourful display of atmospheric haloes, spots andpillars. These images can tell you something about theclouds overhead and possible changes in the weather. All of these images are created by light shining throughcirrostratus clouds. These clouds occur at an altitude of6,000-12,000 metres. They appear as a thin sheet or layer(strata) that is pure white. The layer of cloud is sothin (only 100-450 metres) that is doesn’t obscure the sunor 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 producingmost of the commonly see haloes-plate crystals, columns,capped columns and bullets. The most obvious halo is found around the sun. If thelayer of cirrostratus is extensive, you’ll see an entirering. Within the layer of cloud, sunlight is striking andpassing through the sides of randomly-oriented icecrystals. As the sunlight passes through each crystal,the light changes direction, or refracts. The radius ofthe hale depends on the amount of change in the directionof the sun’s light. Usually this is 22 degrees. Sincethe sun is 1/2 of a degree across, the radius of the halois 44 sun-widths. Occasionally you may see a second haloat 46 degrees from the sun (that is, with a radius of 92sun-widths). This is produced by sunlight passing through

both the side and bottom of each crystal. Moonlight willalso produce a halo, around the moon, with the properlayer 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 ofthe halo, occur when sunlight passes through the sides ofcapped columns, bullets and plate crystals, when thesecrystals are arranged with their sides vertical. Thecrystals wobble, diffusing and smearing the colours of themock sun. You can see haloes and mock suns more clearly if you blockout your view of the real sun by holding your hand infront of it at arm’s length. Another spectactular opticaleffect is the solar pillar. This is a vertical shaft oflight the same colour as the sun stretching upwards fromthe sun and is most often seen at sunset or sunrise. It’sproduced by sunlight reflecting of the base of plate andcapped column crystals in the clouds. You can also seepillars in an ice fog, when it’s illuminated bystreelights, or airport runway lights, for instance. The appearance of all these optical images is a goodindication that the weather will change. Strong verticalair currents associated with low pressure storms carrymoist air skyward, where the water freezes. High speedwinds above the storm system push the ice crystals onahead. When you see haloes around the sun or moon, youcan be sure of two things-there are cirrostratus cloundsabove and, in a day or two, the skies will darken with anapproaching storm.