Turbulence leads to early rain of ash. (Earth Science: from San Francisco, at the 2001 fall meeting of the American Geophysical Union).

A new aerodynamic analysis suggests that small particles of ash in a turbulent volcanic plume can fall nearer their source than researchers had thought.

On Aug. 18, 1992, Mount Spurr--a 3,374-meter-tall volcano about 125 kilometers west of Anchorage, Alaska--sent a plume of ash more than 10 km into the sky. The cloud passed directly over Anchorage and dropped a two-nickel-thick layer of sand-sized ash particles there. Another 125 km downwind, the particles that blanketed the ground were smaller and came primarily in two disparate sizes--the larger was about 90 micrometers across, and the smaller was about 20 [micro]m.

That's surprising because volcanologists had expected that the lighter particles would remain airborne until they had traveled much farther from the volcano, says William I. Rose of Michigan Technological University in Houghton. However, an aerodynamic parameter that describes the behavior of particles in turbulent flows, known as the Stokes number, may help explain the early fall of the smaller ash grains, Rose notes.

When suspended flecks of ash are very light, their Stokes number is much less than 1, and the particles stay suspended within a cloud. For heavy particles, the Stokes number is much greater than 1, and the ash drops out of the cloud. But when an ash particle's Stokes number is approximately equal to 1, the grain is flung to the outside edges of the cloud's swirling eddies, where it's much more likely to bump into another ash particle.

What caused the 20-[micro]m particles in the ash cloud from Mount Spurr to clump together and fall prematurely isn't yet clear, says Rose. Whatever the reason, he notes, measurements from weather instruments lofted into the cloud as it passed over Anchorage suggest that ash particles 22 [micro]m in diameter had a Stokes number of 1. That size closely matches the anomalously small size of the grains that fell 250 km from the volcano.

The new finding could help scientists better predict where fine-grained ash will fall, says Rose. That's important because small, light particles such as those that dropped early from the Mount Spurr ash easily reach areas deep in people's lungs and so pose health risks. --S.P.