A Rare Pollen Corona Causes Colorful Rings Around the Sun

Forrest M. Mims III

On 5 January 2007, the sky was clear and blue at solar noon while I was preparing to make sun and sky measurements. Overhead a brilliant set of colorful rings encircled the sun.

Ice crystals and water droplets in clouds can cause colorful rings around the sun known as coronas. But the display I saw was caused by pollen.

If there was ever a silver lining to the juniper pollen that aggravates so many Texans each winter, it is the colorful coronas they sometimes cause. These rare pollen coronas occur only when the pollen is thick and the sky is deep blue. Air pollution washes them out.

Figure 1. This exceptionally rare pollen corona recently appeared over the field in South Texas known as the Geronimo Creek Observatory. Photograph copyright by Forrest M. Mims III.

The first time I saw a pollen corona was in the winter of 1997. I had no idea what caused such a spectacular display around the sun on a perfectly clear day. Eventually a scientist contacted over the Internet suggested that pollen might be the cause. Measurements of the diameter of the rings pointed to juniper pollen from the Texas Hill Country. (The trees are commonly known as mountain cedar. They are actually Juniperus asheii.)

Pollen was confirmed during experiments in which branches of male juniper trees were agitated until they released clouds of orange pollen. The pollen caused coronas around the sun identical to those observed near Seguin.

Two scientists suggested that I write a scientific paper about the Texas pollen coronas. After all, one noted, Finland was then the only place with confirmed pollen coronas. "Solar corona caused by juniper pollen in Texas," was published in Applied Optics in March 1998.

Pollen coronas occur on very clear days during juniper pollen season. Even when they occur, they are rarely seen due to our natural aversion against looking toward the sun. Dark sunglasses are absolutely essential, but even they will not fully protect your eyes from the direct rays of the sun. So great caution must be used when attempting to see one.

The sun must be completely hidden behind a roof or other large object, and the sky away from the sun should be viewed with caution. Never attempt to photograph a pollen corona while looking through a camera viewfinder.

The safest way to view a pollen corona is to see Fig. 1. For much more on safely viewing and photographing a pollen corona, see my web site www.sunandsky.org and click "Sky Photos."

Usually we have no more than one good pollen corona each winter, but so far this winter we have had two. I've made a 16-second video of the second corona that you can see on YouTube. This begins with bright sunlight shining around the top of a fence post. The camera then pans across the top of the post. When the sun is blocked by the post, a beautiful corona suddenly pops into view.

If you observe a pollen corona, please send a report and, if possible, a photograph to "Backscatter." Coronas caused by ice crystals in high cirrus clouds are much more common than rare pollen coronas. So be sure to include information about the sky and clouds and the corona's diameter with respect to that of the sun.

"World of Science" columns are selected and sometimes revised from columns published in the San Antonio Express-News or the Seguin Gazette-Enterprise. The columns are intended for a general audience. Forrest M. Mims III and his science are featured online at www.forrestmims.org and www.sunandsky.org .