August 26, 2010 | 27
Why do the roiling, black clouds of a thunderstorm produce lightning? Ben Franklin and others helped prove that such lightning was discharged electricity, but what generates that electricity in such prodigious quantities? After all, storms generate millions of lightning bolts around the globe every year—even volcanoes can get in on the act as the recent eruption of Eyjafjallajökull did when photographs captured bolts of blue in the ash cloud.
Perhaps surprisingly, scientists still debate how exactly lightning forms; theories range from colliding slush and ice particles in convective clouds to, more speculatively, a rain of charged solar particles seeding the skies with electrical charge. Or perhaps the uncertainty about lightning formation is not surprising, given all that remains unknown about clouds and the perils of studying a storm—an electrical discharge can deliver millions of joules of energy in milliseconds.
But Brazilian researchers claim that their lab experiments imply that the water droplets that make up such storms can carry charge—an overturning of decades of scientific understanding that such water droplets must be electrically neutral. Specifically, chemists led by Fernando Galembeck of the University of Campinas found that when electrically isolated metals were exposed to high humidity—lots and lots of tiny water droplets known as vapor—the metals gained a small negative charge.
The same holds true for many other metals, according to Galembeck’s presentation at the American Chemical Society meeting in Boston on August 25—a phenomenon they’ve dubbed hygroelectricity, or humid electricity. "My colleagues and I found that common metals—aluminum, stainless steel and others—acquire charge when they are electrically isolated and exposed to humid air," he says. "This is an extension to previously published results showing that insulators acquire charge under humid air. Thus, air is a charge reservoir."
The finding would seem to confirm anecdotes from the 19th century of workers literally shocked—rather than scalded—by steam. And it might explain how enough charge builds up for lightning, Galembeck argues.
The scientists envision devices to harness this charge out of thick (with water vapor) air—a metal piece, like a lightning rod, connected to one pole of a capacitor, a device for separating and storing electric charge. The other pole of the capacitor is grounded. Expose the metal to high humidity (perhaps within a shielded box) and harvest voltage. "If this could be done safely, it would allow us to have better control of thunderstorms," Galembeck says, envisioning a renewable energy source from the humid air of the tropics and mid-latitudes.
Unfortunately, the finding violates the principle of electric neutrality, in which the differently charged molecules of an electrolyte like water cancel out. And although geophysicists and other atmospheric scientists may not know all the details of how lightning forms, they do have a general sense, and hygroelectricity seems to ignore what is largely understood. "It is utter nonsense," says atmospheric physicist William Beasley of the University of Oklahoma, a lightning researcher. "All seriously considered mechanisms for electrification of thunderstorms that can lead to the kind of electric fields that are required for lightning involve convection and rebounding collisions between graupel [a slush ball] and ice particles in convective storms."
Similar efforts to capture the electricity in a lightning bolt have failed, most recently, Alternate Energy Holdings’s would-be lightning capture tower outside Houston. The wired tower never worked. "This concept has been disproven many times over," Beasley notes. What’s more, the amount of energy in a lightning bolt—never mind its crackling electric grandeur—is but a fraction of the amount of energy required to run even one 100-watt lightbulb, which uses 100 joules every second, for a day.
But taming lightning is a prospect that has tempted experimenters since at least the Olympian thunderbolts of Zeus. Of course, the vast majority of the energy is in the storm itself—hurricanes, for example, have the heat energy of 10,000 nuclear bombs. Capturing that energy might prove frazzling.
Image: Eyjafjallajökull eruption and the lightning it caused. © Marco Fulle (Stromboli Online) / NASA