"Calculations of the effect of dust in the energy balance of the climate system generally assume that dust is randomly oriented," explained the University of Reading's Giles Harrison. "But if dust is aligned preferentially, those calculations will need to be improved."

Greater organization influences climate by reducing the optical depth of atmospheric dust layers, allowing more solar energy to penetrate. Harrison's co-authors from the University of Hertfordshire found such an optical depth reduction during astronomical observations in the Canary Islands.

"They concluded that the dust particles were lined up vertically, letting energy through selectively, rather like an open Venetian blind," Harrison told environmentalresearchweb. The team thought that electrical charging was the cause and set out to confirm this theory.

In August 2009, Harrison and colleague Keri Nicoll from the University of Reading measured the electrical properties of dust blown from the Sahara into the atmosphere above the Cape Verde Islands using weather balloons. While dust charges have been measured in the atmosphere near the Earth's surface, there is little information from higher altitudes. So the British team exploited two custom-designed instruments mounted aboard a standard meteorological radiosonde.

The first of these instruments, an aerosol particle counter, measures particle size and concentration using light scattered from particles drawn into a sampling chamber by an air pump. The second, a dust charge sensor, consists of a spherical metal electrode in which charge is induced by impacts from electrified particles.

"Because the charges on dust are small the detector has to be very sensitive, but this means it can also easily become swamped if large charges are present," Harrison said. "We had to develop special electronics that would overcome this, otherwise the flight would be wasted." However, while the sensor could measure the dust particles' charge, it was unable to determine their polarity unambiguously.

The team's first balloon sounding saw two layers of dust, one at an altitude of 4 km and one at 2 km; the second sounding saw a single, much denser, layer of dust up to 3.5 km. In each case more than 85% of the particles detected were 0.6–1.4 μm in diameter, with a maximum measured charge density of 25 pCm–3. This is sufficient to influence the dust particles' removal by wet atmospheric processes.

Earle Williams, who studies physical processes in meteorology at Massachussetts Institute of Technology, US, says that the measurements of African dust storms are "new and valuable". "The consideration of electrical forces on particles in situ may be needed to account for their extraordinarily long-range transport," he said. However, Williams feels the lack of polarity information is a major shortcoming, and also hopes that the team can add electrical-field measurements to their detection capabilities. This is exactly what Harrison and his colleagues now intend to do. "We are developing a disposable sensitive electric-field sensor to fly on weather balloons," said Harrison.