Earth’s skies are filled with electrical charge, even in fair weather. Physicists have traditionally viewed the atmosphere as a sort of spherical capacitor, with a negative end at Earth’s surface and a positive end high in the ionosphere. In the 1920s, C.T.R. Wilson suggested that charge moves through the air in a continuous current that’s now known as the global electrical circuit (GEC). Wilson proposed that charge is generated in disturbed-weather regions, such as in thunderstorms, before dispersing via the GEC and flowing through fair-weather regions in the rest of the atmosphere.
But those studying the GEC have ignored the weather in between: neither disturbed nor fair. Now research led by Giles Harrison and Keri Nicoll at the University of Reading is challenging the traditional picture of the GEC. Their experiments show that charge gathers in layer clouds—also known as stratiform clouds—in regions of what Harrison and Nicoll call semi-fair weather. Harrison, Nicoll, and colleagues reviewed their progress in June in Proceedings of the Royal Society A.
Semi-fair weather can cover as much as 30% of Earth’s surface at any moment. “We’re particularly interested in layer clouds because they are very passive,” Harrison says. “They’re about the simplest clouds that you can think of, which are just layers of droplets.” That makes layer clouds different from the storm clouds that generate electricity and the fair-weather atmosphere through which charge disperses. If Earth’s atmosphere is a capacitor, large regions of semi-fair weather are akin to conductive layers between the capacitor plates. Charge passing through layer clouds gathers on their constituent droplets.
In 2013 the Reading group began launching balloons carrying charge-measuring instruments to investigate layer clouds’ role in the GEC. As the balloons ascended, the instruments detected a noticeable gradient: negative charge at the cloud base and positive charge at the top. Separately, the group recently measured changes in electric fields on the ground due to layer clouds passing overhead, which allowed the researchers to remotely infer cloud base charge. The remote observations agreed with the balloon observations that negative charge gathers at the cloud base.
The idea that layer clouds might carry charge is a new development, says Karen Aplin, a researcher at Bristol University in the UK who has worked with the Reading group. “Layer clouds are a key part of the climate system,” she says, “so it is important that their role in the GEC is properly considered.”
Exactly how charge affects those clouds, and in turn the climate, remains unknown. According to Harrison, it’s possible that droplet charging can affect how raindrops form. It’s also possible that more charge is being fed back into the atmosphere as the warming climate exacerbates storms and creates more lightning strikes, he says.
For Harrison, layer-cloud research is especially valuable because of its interdisciplinary nature. “Traditionally, the electrical processes of the atmosphere and the meteorological processes of the atmosphere have tended to be quite separate in the way people have studied them,” he says. Even though the GEC is deeply intertwined with weather events, he says, only now are researchers beginning to appreciate their connectedness.
