11 June 2004

Rain rocks and tropical smoke

Forrest M. Mims III

Saturday, April 24, 2004, was supposed to be rain day in the San Antonio area. The National Weather Service forecast predicted a 70 percent chance of rain. Up to 10 cm (4 inches) were expected.

Saturday came and went with only a few sprinkles. What happened?

San Antonio has many festivals and fiestas. The folks who run Night in Old San Antonio (NIOSA) might want to take credit for ruining the rain forecast. They don't want rain to dampen their crowds, so they employ a rain rock.

This rock, when suitably named and installed, supposedly prevents rain from falling during NIOSA. But I'm rather skeptical that the rain rock did what it was supposed to do.

Several other factors, either on their own or conspiring together, could have ruined the weather forecast. The most likely answer for the missing rain was simply a bad forecast.

Forecasts are educated guesses by living meteorologists and inanimate computers. So why was the forecast wrong?

Meteorologists can probably come up with some answers by analyzing the weather conditions for last Saturday. But I've come up with a reason they might not consider.

The rain forecast might have been ruined by thick smoke from agricultural fires in Yucatan that blanketed Central Texas on Saturday. Much more rain fell Sunday, after the smoke had been pushed south by a cool front.

Several years ago, scientists studying thick smoke over Brazil learned that smoke can sometimes switch off rain clouds. This finding has been published in scientific journals.

Rain can help clear smoke, so this is not good. Worse, however, is that the regional suppression of rain by giant clouds of smoke can affect crops, trees, the water table and even regional climate.

Texas will be better off without smoke invasions that can cause ozone violations, asthma attacks and dried up rain clouds. Unfortunately, I've learned from serving on various government air pollution committees that little is being done to address this problem.

So I expect we'll continue to see smoke arriving here from Central America every spring and Louisiana every fall.

Folks who don't live in Texas are not immune to smoke invasions. During summer significant portions of the United States and Canada are often covered by smoke from forest fires, prescribed burns and agricultural fires. Smoke from Mexico and Central America can penetrate to the Great Lakes and beyond. In May 1998, it reached the Eastern U.S. and crossed the Atlantic to England.

Forrest M. Mims III has measured smoke, haze and the ozone layer since 1989. His science is 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.