Muddy Rain and Dirty Snow

Forrest M. Mims III

Amateur scientists are sometimes better equipped to respond to unusual natural events than their professional counterparts, as demonstrated during a recent weather event that I studied and reported on in my science column for the San Antonio Express-News ("The Country Scientist," 31 March 2008).

Muddy rain fell across South-Central Texas the afternoon of March 18, 2008. It soiled cars, windows and lawn furniture. It also coated the array of delicate solar instruments that I manage for Colorado State University on a rooftop at Texas Lutheran University in Seguin.

The dirty rain triggered a surge of news stories on radio, TV and in newspapers. Callers to radio talk shows complained about their mud covered cars and wanted to know what had happened.

An early report by a spokesman for the National Weather Service said the rain contained mainly ash from a fire in Mexico with perhaps some dust from wind storms that swept across Monterrey, Mexico.

Because the residue left by the rain was tan in color, the ash theory didn't seem to hold water.

San Antonio TV meteorologist Steve Browne (KSAT 12) came to the same conclusion. He showed satellite images that depicted streams of dust from Mexico passing across South Texas. He also showed a close-up of the stuff on a windshield that looked identical to what fell at my country office. Other TV meteorologists in San Antonio agreed that the stuff was dust, not ash.

In an effort to solve this mystery, I inspected some of the residue through the microscope my daughter Sarah used to find microbes in smoke from Yucatan several years ago.

The TV weather people were correct. At a magnification of 400 (Fig. 1) , it was immediately obvious that the stuff was mainly mineral grains that form dust. There were only a few particles of soot, and they resembled those from brush and forest fires. The sample also included some live oak pollen grains, several fungal spores and some microscopic fragments of vegetation.

Figure 1. Microphotograph of muddy rain residue from Mims' office window. Nearly all the particles are dust, an exception being the soot particle indicated by the pointer. Photograph by Forrest M. Mims III.

So the muddy rain was indeed mud. The National Weather Service agreed after they took another look at the satellite images.

Dirty rain has fallen many times across Texas. However, the event of March 18th was quite unusual because of the very large volume of dust entrained within the rain that quite literally resulted in muddy rain.

There is still snow in the mountains of New Mexico, and before the muddy rain fell in South Texas I visited New Mexico to inspect some of its snow through a microscope.

The “pure” white snow atop Sandia Mountain looked good enough to eat. But the microscope revealed that it contained very small dust particles and even a few spores and carbon particles. The snow further south was much, much dirtier.

All this is a reminder that the air we breathe is really a soup of gases and particles, most of which are good and some of which are not. This is also a reminder that some very simple tools of science can allow the amateur to make a contribution to solving unusual natural events.

See my web site for more about stuff in the air.

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