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Chasing Rainbows at the South Pole

Robert Greenler letured at the APS Centennial Meeting this past March.
Robert Greenler lectured at the APS Centennial Meeting this past March.

Along the Antarctic coast one finds a rich source of wildlife: penguins, sea lions, whales and sea birds, for example. But further inland the raw materials are simply ice and air, making the Pole a desolate area where the nearest rock is two miles below the icy surface. The isolation makes it an ideal site for scientists studying neutrinos from exploding stars and meteorological air quality. And of course, the spectacular ice crystal halo effects that occasionally appear in the atmosphere. The latter are the passion of Robert Greenler, a professor (now emeritus) of the University of Wisconsin, Milwaukee and one of the featured public lecturers at the APS Centennial meeting in Atlanta.

A former president of the Optical Society of America, Greenler has made three trips to the South Pole over the last 20 years to directly observe ice crystal halos, most recently last November and December. He traces his interest to a childhood fascination with rainbows that continued long after he'd become a physicist. He helped develop computer simulations to trace rays of light through ice crystal forms to explain the origins of the phenomena, and eventually applied to the National Science Foundation for a grant to study them in situ at the South Pole. In 1980 he published Rainbows, Halos and Glories, to be re-issued this fall in paperback by Peanut Butter Publishing, detailing his investigations of ice crystal halos. Also due out this fall is a new book, Chasing the Rainbow: Recurrences in the Life of a Scientist, which Greenler describes as a recounting of incidents in his life foreshadowed by earlier events, rather than a traditional autobiography.

The halo effects he studies occur when small droplets of water in the atmosphere freeze into hexagonal crystalline form, arising from two varieties in particular: a hexagonal plate crystal, and a cylindrical, pencil-shaped hexagonal crystal. A preponderance of these crystals in the atmosphere, with proper orientation, behave much like prisms when light is shone through them, defracting the ray by specific degrees (22 and 46 degree deviations trigger the most common halo effects), known as the "angular radius." Halo effects are recorded with a device affectionately dubbed "R2D2", which contains a video camera operating in time-lapse mode to track the sun around the sky and keep an extended record of the effects. But the notorious fickleness of Nature in granting such a display can be frustrating, Greenler admits: "You get all your equipment set up and then you wait. And wait. And wait. Either halos come, or they don't."

The importance of his work goes beyond a simple admiration and desire to understand Nature's optical "art." Mapping the characteristics of ice crystals in the atmosphere is of particular interest to meteorologists conducting climate modeling. In fact, Greenler believes he can "teach" a LIDAR system-the radar-like instrument meteorologists use to probe the atmosphere, similar to radar - to recognize the morphology of the crystals based on these optical effects. Still, the halos are a self-described sideline for Greenler. His primary research has been in optics, particularly the phenomenon of iridescence in biological organisms, which he defines as "colors that don't originate from pigments, but from structure." Examples would be colors in an oil film on the surface of a water puddle, or a particular genus of beetle whose color changes depending on the angle from which one views it. He also co-founded UWM's "Science Bag" program 25 years ago, a series of public lectures that has drawn a cumulative attendance of roughly 120,000 since its inception. Many of these have been collected onto videotape for sale to educational institutions.

While his interest in halo effects remains undiminished, Greenler doesn't foresee any more trips to the South Pole in his future. "It's very demanding, both physically and emotionally," he says. Simply getting there requires 9 airline flights over 9 days, and the final leg - an 800-mile flight from the coast of Antarctica to the Pole itself in an LC-140 cargo plane - is dependent on the notoriously unpredictable weather. Once there, individuals often find it difficult to function in an effective air pressure elevation of over 9000 feet. There is also the biting cold, with temperatures averaging -40 degrees F during the research season, and wind chills of more than 100 degrees below zero. Not surprisingly, the average age for those involved in research in Antarctica is 36.

However, Greenler will continue to lecture extensively about ice crystal halos and other optical phenomena, through the "Science Bag" program and other venues. "This particular sideline has given me a lot of pleasure because I've been able to share some of the excitement of science with non-scientists," he says. "And it's taken me places I never dreamed of going when I first started asking questions about what causes all those pretty things in the sky."

For information about Robert Greenler's books or videotapes of "Science Bag" lectures, contact Blue Sky Associates at 414-377-1398.

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Editor: Barrett H. Ripin
Associate Editor: Jennifer Ouellette