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23 January 2004

The paradox of juniper pollen

by Forrest M. Mims III

South Texas experienced two exceedingly rare atmospheric phenomena recently. One was rain.

The second was even rarer. It was the exotic pollen corona caused by thick clouds of juniper pollen drifting overhead.

These tiny cones generate the pollen that is emitted by male cedar trees. Photograph by Forrest M. Mims. Click image to enlarge.

Pollen coronas are rainbow-like halos that form closely spaced rings around the Sun. The one Saturday was unusual because it formed two concentric halos. It lasted most of the afternoon.

Coronas and haloes around the Sun caused by ice crystals are common. Coronas caused by pollen are among the rarest phenomenon in the sky.

I can find only two sets of papers in the formal scientific literature that describe pollen coronas. Both are in the journal Applied Optics. A team of scientists in Finland wrote the first, and I wrote the second.

The pollen corona brings to mind two paradoxes about the juniper tree. The first paradox is that Junipers have bad timing. Instead of releasing their pollen during spring or fall like normal trees, they pollinate during winter.

The second paradox is that the same pollen that causes such a spectacular apparition around the Sun causes severe allergic reactions in from 10 to 30 percent of the adult population.

The symptoms of a mountain cedar attack are not pleasant, especially for asthmatics. They range from the standard runny nose and sneezing to much worse. Sensitive people who inhale a strong enough dose become much too sick to enjoy a pollen corona.

Some people are so sensitive to the pollen that small amounts brought indoors on their clothing can cause problems.

Allergists recommend that people who are affected by juniper pollen stay indoors during days when the pollen is dense. They also recommend wearing a dust mask having pores small enough to trap the pollen. How small? Typical juniper pollen grains are spheres around 22 micrometers in diameter. This is around a fourth the diameter of a hair on your head.

Junipers have separate male and female trees. Thousands of tiny cones on the male trees produce their pollen.

Some years the cones are so thick with pollen that the normally green trees appear orange. Bump into a tree at this time, and pollen will be released like clouds of yellow smoke.

The females receive some of this pollen on their round, berrylike cones. Most ends up high overhead, where it can drift hundreds of miles with the wind.

Juniper trees drink lots of Texas water. Aside from fence posts, their wood has only nominal value, for it burns much too fast and furiously for regular fireplace use.

Next time cedar pollen makes you ill, you can get revenge by buying some of those long-lasting cedar fence posts.

Meanwhile, keep an eye out for the beautiful pollen corona. Be safe and wear dark sunglasses. Never look directly at the Sun. Instead, be sure the Sun is behind a roof line before looking for those spectacular rainbows. Be especially careful when children are viewing a pollen corona.

Forrest M. Mims III and his science are featured online at www.forrestmims.org.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.