16 July 2004

The wet season

Forrest M. Mims III

Because the axis of Earth is tilted, much of our planet experiences seasons as it orbits around the Sun. During winter in the northern hemisphere, the north side of the Earth is tilted away from the Sun. During summer, it is tilted toward the Sun.

The opposite occurs in the Southern Hemisphere, which is why Christmas in Australia is much warmer than it is in the northern hemisphere.

If you have lived in South Texas for more than a year, you know that our seasons are much less distinct than those up North. We can have hot winter days. And we can go directly from winter to summer with scarcely any spring in between. All this comes as a major surprise to visitors from areas where snow covers the ground each winter.

Folks to our South in the tropics experience even less seasonal influences than we do. In many regions, people don't even think about such strange terms, at least to them, as "winter" and "summer." Instead, they think about the dry season and the wet season.

The Southwestern United States experiences a summer wet season in the form of a monsoon that brings in thick layers of moisture from far to the south.

When this moist, warm air bumps into the mountains of New Mexico, Arizona and Colorado, powerful thunderstorms are formed. These storms produce prodigious lightning, which often ignites brush and forest fires. Sometimes these storms can dump thick layers of marble-sized hail in adjacent foothills.

The summer monsoon often affects West Texas weather. Here in South Central Texas, most of our rain falls during spring and fall.

June's near record rains have raised water tables, filled stock ponds and confused wild flowers. A pair of Indian paintbrushes in our field testifies to the latter. They should have stopped blooming weeks ago.

The abundant rains have also stimulated the growth of various kinds of molds, including toadstools and mushrooms have popped up in parks and lawns.

Many mold spores can cause allergic reactions when inhaled, including those shed by the millions by many mushrooms and toadstools.

But there's a good side to these fungi, for they are the reproductive portions of vast underground networks that help provide the roots of trees and other plants with essential nutrients from the soil.

Some of these fruiting bodies can even be eaten. But since some are extremely poisonous, use great caution should you miss lunch and chance upon a delicious looking mushroom under a convenient shade tree.


Forrest M. Mims III and his science are featured online at http://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.