Winter Weather Shows

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Throughout the winter months, nature provides a show of its own. From the mystical magic of the aurora borealis to the chilling appearance of halos, sundogs, and pillars, conditions in the winter months provide favorable environments for these phenomena.

Halos are visible throughout the year, but generally occur in the upper levels of the atmosphere because they are caused by refraction of light through ice crystals. Ice crystals, which are in the shape of pillars, refract the light twice, once when it enters and once when it leaves the crystal. The light is refracted 22 degrees, producing a ring of light around the original light source. If the ice crystals are oriented in such a manner that the light enters or exits through the ends of the crystal, a ring of light at 46 degrees is created around the light source.


Sun pillars can be viewed most commonly at sunrise or sunset as light is reflected off of small ice crystals falling towards the surface. Generally these ice crystals are high up in the atmosphere, but in frigid conditions with temperatures below 0, these ice crystals can form near the surface. Often referred to as diamond dust, these thin ice crystals can reflect light off of sources near the surface such as street lamps creating pillars of light.

Aurora Borealis occur because of interaction between the Earth’s magnetic field and charged particles originating from the Sun. The Earth’s magnetic field is strongest at the poles and weakest near the equator. Charged particles, which are in constant motion around the Earth, are guided by the magnetic field. At times, these particles become energetically charged by a fast moving stream of particles from the Sun. When these particles become charged and then collide with gas atoms in a layer of the atmosphere called the Ionosphere, they give off light. The stronger the magnetic field, the less intense the stream of particles originating from the Sun needs to be to be seen. Although auroras occur throughout the year, they are most commonly viewed in the northern hemisphere in the winter months during the long nights. The light can appear in various colors, including green, blue, red and purple.
To help determine where there is an abnormally large number of charged particles originating from the Sun headed for the earth, an index has been devised based on the amount of geomagnetic activity measured in Gottingen, Germany. The index, which updates every three hours, varies from 0 to 9, with 9 being extreme activity. The latest values of this index, called the KP Index, can be found at http://www.sec.noaa....lots/kp_3d.html . In addition, information on the amount of upcoming solar activity can be found at http://www.spaceweather.com

Auroras can be viewed on a semi regular basis across the Upper Midwest. When looking for an aurora display, be sure to wait for a clear night and avoid light pollution from near by towns. Auroras are best viewed on cold nights when there is little moisture in the atmosphere. To view pictures of recent aurora events in the Central Iowa area, please go to: http://www.nightskyevents.com/.

Unfortunately, my pictures of sun pillars and halos are packed right now, but there are numerous great examples on the web. A few good ones I have found include http://www.extremein...om/03-02-16.htm , http://antwrp.gsfc.n...d/ap011107.html and http://apollo.lsc.vs...pter4/halo.html . In upcoming articles, we will explore tools used in the forecast process and brush up on severe weather terminology.