It comes with the wind.

It happens maybe 10 inches or so above the ground.

And it's one big reason several thousand tons of soil from fallow field might suddenly blow across the land like grit from a giant sandblaster out to level anything in it way.

Donald Fryrear call it the TSS Factor. He discovered it and he named it. The letters stand for transition from saltation to suspension.

"It's the point when windblown particles of soil are either blown back to earth or kept aloft," he say . "It's the point in time and space when dust storms really begin."

Soil particles in saltation, explain Fryrear, are too heavy to be carried by the wind. So they return to earth—at a greater peed than when they first took off and with a pulverizing force that kicks finer lighter particles into the air.

"The whole cycle repeats, building on it self over and over again," Fryrear say . "Eventually, you have a full-fledged dust storm lasting anywhere from minutes to hours—sometimes even for days—and covering maybe hundreds of square miles. But it all starts when the first bits of oil are swept into the air for a few feet and then get blown back into the ground."

Understanding this initial dynamic in dust storm development, Fryrear says, is critical to predicting the storms and their impact on crops, air quality, and even highway safety.

The TSS factor shows, in particular, how some fallow cropland might contribute significantly to dust storms—even when their surface soil conditions would seem to dictate otherwise. Recent rains, for example, may have caused soil particles to congeal into marble-sized clumps too big and heavy to allow much in the way of air travel. One might conclude that such congealed particles were less vulnerable to the wind.

But not so. "The small clumps can be the biggest culprits of all," Fryrear says. ''They become the instigators, the ground-breaking sledgehammer that make the whole thing happen." Fryrear, a supervisory agricultural engineer at the ARS Conservation and Production Systems Research Unit in Big Spring, Texas, developed the TSS factor while studying windblown soil characteristics at various heights above the ground.

"These were field studies," he points out. ''They weren't done in a wind tunnel. I doubt that a wind tunnel approach would have worked. We needed a lot of space, real space and real wind and real soil, to make our data count for something."

Fryrear got what he needed. In addition to outdoor test plots at Big Spring and several fields on nearby farms, his lab became the whole Great Plains and then some. It included farms and test plots in Oklahoma, Colorado, Kansas, Nebraska, Minnesota, Indiana, Missouri, Florida, New York, Delaware, Arizona, and California—with a little bit of Canada thrown in for good measure.

'In all my years with ARS and that now comes to nearly 35," says Fryrear, "I don 't think we've ever had another research project on windblown soils involve so many locations spread over so vast an area."

At each location, Fryrear installed 75 soil particle collectors in large, circular pattern covering six acres. Such a configuration accommodated winds from any direction, while providing ample space for dust storms, to start. The collector were designed by Fryrear to trap windblown soil particles at different heights up to 40 inches above the ground.

'In some cases, we went up to 20 feet," he says; "but the first 3 feet or so were usually enough to go by. The data in that range told virtually the whole story."

Indeed they did and a startling story it was.

As expected, windblown soil particles decreased in size the higher the point of collection. But the rate and extent of change was unexcepted: soil particles sampled at the 40-inch level were a thousand times smaller than those rolling along the surface. Furthermore, nearly all of that decrease occurred within the first 10 inches above ground.

"It's taken 7 years of data to show how often this occurs," says Fryrear, "and now to show how consistent it is with our theory of the TSS factor. Right now, we're pretty confident about placing the line between suspension and saltation at 10 inches for most types of soil. Our data definitely support this."

If so, monitoring the vertical distribution of windblown soil particles might well become a standard and useful operating procedure in agricultural and semi-arid region where dust storm are most likely to arise.

A significant change in proportion above and below the l0-inch mark at a given location could provide advance warning of an impending dust storm according to Fryrear. A sudden increase below the mark for example, would show that larger particles are on the move. And they might be kicking up smaller particles shortly.

"If we had been tracking soil surface conditions in California last year," says Fryrear, "we might have anticipated the blinding dust storm that hit Interstate 5 suddenly. We might have been prepared. Maybe the highway could have been shut down for a time. Maybe we could have prevented the terrible pileup and wreckage and loss of life that the storm caused." —By Stephen Carl Miller, ARS.