Haloes Forecast Storms

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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