For years, scientists have noticed rapidly varying electric fields inside dust storms and dust devils, the dirty whirlwinds that skitter across many desert areas. Some even wondered how those fields might alter the size of the storms, but no one had made any measurements. Now, first-of-their-kind field tests in the western Sahara reveal that the fields—generated when windblown sand grains rub together—loft desert dust much more effectively than previously recognized, creating larger and longer lasting storms than wind alone.
“I’m pleased [the new] results show that what we’d previously theorized,” says Nilton Renno, an atmospheric scientist at the University of Michigan, Ann Arbor, who was not involved in the new work. “But I didn’t expect to see the effect so clearly in the data.”
When wind begins to blow across a sandy, dusty surface, the lightest particles aren’t the first to move. That’s because much of the dust is either stuck to larger particles or tucked between them. But when sand grains start to bounce across the surface, they strike other grains and shake loose the dust, which then rises into the air just above the ground. All that bouncing and jostling also generates static electricity—the geological version of shuffling your feet across the carpet.
When this happens, the larger sand grains typically lose electrons to the lighter dust particles, giving the dust a negative charge. The dust particles are blown higher into the air more readily, whereas the now positively charged sand grains usually remain closer to ground level. That separation of charges creates an electric field that may help electrify some of the dust still bound to sand grains, thus boosting even more of it into the air.
Previous studies have suggested that electric fields generated during the early stages of a sandstorm would have that effect, but nobody had made field measurements to support the idea, says Francesca Esposito, a planetary scientist at the National Institute for Astrophysics in Naples, Italy. So she and her colleagues set out to do just that. At a broad, flat site in southeastern Morocco, they set up a weather station that would constantly measure wind speed, temperature, humidity, barometric pressure, and sunlight intensity. Extra sensors measured the electric field 2 meters above the ground. The team collected data during the height of the Saharan dust storm seasons in 2013 and 2014.
The instruments chronicled several dust storms and dust devils. And in each of those events, the electric field grew stronger than normal, often in just a matter of seconds—bolstering the idea that the shuffling of sand grains generates static electricity. But the data showed another trend, too: When the wind blew above certain speeds, as much as 10 times the expected amount of dust rose from the ground, the researchers report online in Geophysical Research Letters. On each occasion, the rise was very rapid, suggesting that the dust emissions and the electric field were reinforcing each other, Esposito says.
Renno, who co-authored a paper suggesting such a dust-boosting feedback loop in 2008, says he is seeing similar phenomena in field studies at California’s Owens Lake. In some cases, the electric fields differ in direction from those Esposito and her colleagues measured—possibly, Renno says, because different minerals make up the sand and dust at the two sites.
The findings might be a boon for climate scientists. Atmospheric dust may have a powerful effect on climate, absorbing sunlight and warming the atmosphere at some altitudes while shading and cooling underlying layers of air. Some of the largest uncertainties in current climate models stem from their wide-ranging estimates of the size and number of dust particles in the atmosphere. Many of those models estimate the sizes and numbers of dust particles in the atmosphere based on weather conditions, but they don’t include the effects of electric fields. Reducing uncertainties in the models could lead to better long-term assessments of climate, Esposito says.