Answer blows in wind, swirls in soap.

Besides using up energy to move sand dunes, create waves, and otherwise rearrange the scenery, winds exhaust their force on inner processes, too. Collisions between gas molecules, for instance, convert kinetic energy to heat in so-called viscous processes.

Using a soap film that mimics the atmosphere of Earth or other planets (SN: 8/22/98, p.118), physicists now have measured how much energy a thin sheet of turbulent fluid gives up to those types of losses. The scientists say they can use flat fluid systems to model large-scale motions in atmospheres because, compared with the diameter of a planet, an atmosphere is hardly more than a vaporous, two-dimensional skin.

To conduct the experiment, Michael Rivera and Xiao-Lun Wu, both of the University of Pittsburgh, devised an electromagnetic means of stirring fluid films. As they report in the July 31 PHYSICAL REVIEW LETTERS, salt ions driven by electric and magnetic fields make the fluid swirl.

The team finds that at least as much energy drains off into the surroundings of their model system as goes into the fluid's viscous processes. Wu says the study confirms behavior of two-dimensional fluids that has been suspected by scientist but never before measured. The new results also may improve the accuracy of simulations of atmospheric circulation and climate, he adds.