Storm clouds from night thunderstorm lit by cloud-to-cloud lightning.
Courtesy NOAA Photo Library

Call it a place where angels fear to tread.

The motion in thunderclouds (technically cumulonimbus clouds) is powered by convection, a force that separates fluids based on density. The relatively dense, cold air falls while the warmer air rises. Smaller water droplets hitchhike upward on the updrafts which can't support the larger droplets.

Because smaller particles tend to carry positive charges, the movement of air of different temperatures and humidities, and of snow, ice, and water vapor -- tends to segregate electrical charges. The top of a cloud becomes positive and the bottom negative. (New evidence reveals that a significant number of lightning flashes are from positively charged cloud bottoms. These "positive-cloud-to-ground" flashes occur more often as a thunderstorm ages).

Regions of different charge can only exist if surrounded by an insulator -- namely air. Insulators, however, eventually fail when the electric "pressure" gets strong enough. In a thunderstorm, that "failure" results in lightning.

The lower part of a thundercloud is usually negatively charged, the upper area is usually positively charged. Lightning from the negative area generally carries a negative charge to Earth and is called a negative flash. Courtesy NASA

Hangin'-motor blues
Think of a chain holding a V-8 engine above a '63 Ford Fairlane in the backyard of a shade-tree auto mechanic. If the engine is too heavy, or the chain too weak, the chain will be overwhelmed by the gravitational attraction between Earth and engine, and snap.

Thud!

Substitute air's insulating ability for the chain, and electrical attraction between positive and negative charges for the gravitational attraction, and you have a greasy-fingered picture of how air can temporarily constrain electrical charges in the atmosphere.

To go further, we need one hunk of physical-science jargon: electrical potential is how fast charge changes with distance, and it's measured in volts per meter. Think of electrical potential as the "pressure" that's "trying" to establish an electric current between areas of opposite charge.

(Opposite electrical charges remind us of young lovers. They will do anything to get together.) Just as an overweight V-8 can snap a skimpy chain, excess electrical potential can "break" air's insulation. When that happens, an electrical current -- in the form of a lightning bolt -- neutralizes the opposing charges.

Flash!
In cloud-to-ground flash, the huge electrical potential -- measured in millions of volts -- eventually overcomes air's electrical resistance, and a "streamer" or "leader" begins reaching, about 50 meters at a time, toward the ground. The streamer makes an ionized (conducting) pathway of plasma, allowing current to flow.

As the current approaches the ground, its electrical potential can cause a surge of oppositely-charged particles to "reach" up toward it. Because this upward current often springs from tall objects, trees and other tall objects make lousy shelter during a storm.

In reality, says David Rust, director of forecast research and development at the National Severe Storms Laboratory, the safety of a building is determined by the degree of grounding. A steel building that's securely grounded, he says, is going to be safer than a wooden one that's not, even if the steel building is taller. Steel and other conducting metals provide an easy pathway to ground for the lightning, and that translates into safety.

Once the ionized pathway is established, electric currents flow back and forth between ground and cloud so quickly that they appear as flickers rather than separate bolts.

We've heard estimates that a big cloud-to-ground bolt carries one trillion watts of electricity. We don't know who made those estimates, but during the fraction of a millisecond or so that the flash is active, it carries about 20 percent more current than total U.S. generating capability. (Watts, remember, measure the flow of electric current at any instant. The more familiar watt-hours measures an hour of flow of a given current.)

Sorry. Nobody has figured out how to put this energy to work. The current are simply too high, and too brief, for that.

What are the medical effects of being struck by lightning?