Deadly lightning in Africa
Uganda is looking for answers as about 20 students and a teacher were killed June 28 by lightning that struck their school in this highland nation in Eastern Africa. With dozens of children also injured by electricity, Ugandans wonder if the serious string of lightning strikes is related to climate changes, or are just the consequence of an unusually heavy stream of moist air coming from the Atlantic.
We can’t answer, but the tragedy did get us Why Filers to thinking about lightning. Although lightning bolts killed “only” an average of 39 Americans over a recent 10-year stretch, the injuries, which concentrate on the vulnerable nervous system, can be severe and lifelong.
Satellites tell us that 1.2 billion lightning flashes occur in the atmosphere each year — although not all reach Earth.
What is lightning? How does it injure and kill? And what has been learned in the past few years from the millions spent studying nature’s electricity?
Thunder — the cracking or rumbling you often hear — is caused by thermal expansion and contraction. Lightning bolts can get far hotter than the sun’s surface — up to 20,000° Celsius. That heats the air, causing it to expand, and starting a shock wave that moves as sound waves — thunder.
If you’re close to the lightning bolt, you’ll hear a cracking; further away, you’ll hear rumbling because that sound has come from several parts of the bolt, and been reflected from buildings and hills.
And yes, if you start counting “one Mississippi,” when you see the flash, you can estimate the distance to the bolt: Light essentially reaches you instantly, but sound takes about five seconds to travel one mile. Divide the number of seconds by five to find miles, or by three for kilometers.
Silence is — mysterious
One of the many lightning mysteries is this: Sometimes you hear the thunder, and sometimes you don’t. For example, “heat lightning” is an eerie, silent flash that often lights clouds in thunderstorms.
The sound has been gobbled by an audio version of the visual mirages that cause trekkers to see water in stone-dry desert. These visual mirages are caused by heat that bends light waves. You look straight ahead, but you actually see the sky, shimmering like a tempting lake.
Similarly, in a thunderstorm, the sharp boundaries between warm and cool air can channel sound waves away from the observer, as you can see from the nifty applet, below.
Much the same phenomenon was noticed during the Civil War, when artillery was visible in the distance but audible only in some parts of the battlefield.
Nature’s lighting foundry
We think of clouds as billowy places, couches for angels in Renaissance paintings. In thunderclouds, however, air and water – liquid, frozen and in between — may be whizzing up and down at a furious clip — up to 100 miles an hour.
That’s a place where angels fear to tread.
The motion in these cumulonimbus clouds is powered by convection, a force that separates fluids based on density. The dense, cold air falls while the warmer air rises. Smaller water droplets hitchhike up on the updrafts, which can’t support the larger droplets.
Because smaller particles tend to carry positive charges, the movement caused by temperature, humidity and density (which can include snow, ice, and water vapor) segregates electrical charges: The top of a cloud becomes positive and the bottom negative.
Regions of different charge can only exist if surrounded by an insulator — namely air. Insulators, however, eventually fail when they are overwhelmed by electric “pressure.” In a thunderstorm, that “failure” results in lightning.
Having trouble envisioning this? Imagine a chain holding a greasy V-8 motor above a ’63 Ford Fairlane in a shade-tree auto mechanic’s backyard. If the engine is too heavy, or the chain too weak, the chain will snap as it is overwhelmed by the gravitational attraction between Earth and engine.
Substitute air’s insulation for the chain, and electrical attraction between positive and negative charges for gravity, and you have a greasy-fingered picture of how air can separate electrical charges during a thunderstorm.
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. Electrical potential is the “pressure” that’s “trying” to start an electric current between areas of opposite charge.
(Opposite electrical charges are like 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.
In a 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 ground. The streamer makes an ionized (conducting) pathway of plasma, allowing current to flow.
The key to lightning
Lightning researchers follow the famous footsteps of Benny Franklin, the Philadelphia printer and rabble-rouser who studied lightning in the mid-18th century. Thinking that lightning was an electric current, Franklin hung an iron key from a kite string and flew the kite in a thunderstorm in 1752.
Why was the future rebel not fried when he held his hand near the key?
The current must have passed through or around Ben’s bod and into the ground. Although we’d hate to run this little gag past a human-subjects review board, Benny proved that lightning was an electric charge in the cloud.
Where am I safe?
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.
For a 2001 Why File on lightning, David Rust, who was then director of forecast research and development at the National Severe Storms Laboratory, told us that the safety of a building is determined by the degree of grounding. A steel building that’s securely grounded, he said, will be safer than a wooden one that’s not, even if the steel building is taller. Steel and other conductive 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. (More on lightning safety.)
We’ve heard that a big cloud-to-ground bolt carries one trillion watts of electricity. If that estimate is right, during its fraction-of-a-millisecond life, the flash carries about the same current as the total U.S. generating capability. (Watts measure the flow of electric current at any instant. The more familiar watt-hours measures an hour of flow of a given current; 1 kilowatt hour equals 1,000 watt hours.)
But nobody has figured out how to put this energy to work. Though we have heard one proposal, the currents are insanely high and the strikes are too brief and too unpredictable.
Keeping a close watch on lightning
Our understanding of lightning grows with improvements in technology, and a new instrument on trusty weather balloons has pointed to a surprising source for the electric charge. The process involves a small, spongy relative of hail called graupel, says Don MacGorman a physicist at NOAA’s National Severe Storms Laboratory.
“As graupel accumulates tiny, pristine ice particles, and then falls through liquid water, there can be some charge exchange in collisions where the tiny ice particles rebound,” MacGorman says. In the lab, this interaction seems powerful enough to be main source of electricity – and therefore lightning — in large areas of the storm.
Within a few years, a better understanding of lightning formation could improve predictions, MacGorman says. “We will not be able to say lightning will a hit particular location. Lightning is too random for that, but we are getting to the place where it may be possible to say that a storm will produce a little or lot of lightning, and that would be helpful for storm safety.”
The graupel explanation, however, raises a question: If the interaction of water and ice creates the electric charge, why is lightning found in dry sectors of the storm, including the large “anvil” structure that exhausts cold, dry air above the storm? “We have seen lightning initiated almost 100 kilometers from the heavy precipitation area, so something else must be going on in the anvil,” says MacGorman. “This does not accord with how we’d viewed anvils.”
Scientists are also probing cloud flashes, caused by the flow of current between regions of clouds with opposite charges and does not hit the ground. Formerly dissed because they don’t kill people, cloud flashes are getting some respect.
For one thing, they are the most common type of lightning, accounting for perhaps one-quarter of all lightning flashes. Adding cloud-to-ground and cloud-to-cloud lightning gives a better indicator of total storm intensity than ground flashes alone, “which have very little relationship to storm severity,” says MacGorman. “You can have huge ground flashes in a relatively innocuous storm, but total lightning is well related to things that affect severity and strength: the size of the updraft, the amount of ice in the clouds, and so it gives us clues as to how intense the storm is.”
The biggest recent discovery on lightning, says MacGorman, concerns storms that produce a large amount of positively charged cloud-to-ground lightning rather than the usual negative currents. During a field research program called STEPS, in a lightning-rich region of the high plains, some storms contained negative charges in places that normally would be positive, and vice versa. In these conditions, instead of dropping the normal negative charge to the ground, the lightning bolts were positive.
The unusual phenomenon could arise in clouds containing a high concentration of liquid water, MacGorman says, and that would also raise the odds of large hail. “Hail typically forms because graupel or another seed particle starts collecting liquid water faster than it can freeze, and the water spreads over the surface, then freezes into a solid layer of ice.”
These dense particles are more likely to happen in an area with a lot of liquid water, and therefore, these positive lightning strikes could be a harbinger of large, destructive, hail.
The view from on high
For the next stage in lightning observations, scientists will go to GOES-R, a series of geostationary satellites scheduled for launch in 2015. These high-orbital spyglasses will carry an optical gadget that should “see” upwards of 90 percent of total lightning activity. “The viewing area will cover pretty much all of the continental United States, and parts of Africa and South America, and eventually, half of the Pacific Ocean,” says MacGorman. “This will allow us to detect thunderstorms over the oceans, which we have not had good way to see in the past.”
That should help airplanes dodge storms, but also aid weather prediction, MacGorman says, since thunderstorms can trigger other thunderstorms. They also add water vapor to the lower atmosphere, which also feeds storms.
Nothing light about lightning
Lightning gathers myths. Whether it’s Zeus throwing thunderbolts from the ancient Greek sky, or the moronic misconception that victims become untouchables because they retain an electric charge, these bolts spark the imagination.
But lightning can change your life, as Steven Marshburn, Sr., of Jacksonville, N.C., told us in 2001. Marshburn was struck in 1969 while working in a bank. Although the sky was blue and no storm was in sight, a bolt entered through a wire from the drive-up window.
Afterwards, Marshburn “suffered from severe headaches, chronic daily pain, grand mal [epileptic] seizures, dizziness, problems with my eyes going blurry. Many health problems persist. I have had 20 lightning-related surgeries…”
In 1989, in response to his brush with death, he formed Lightning Strike & Electric Shock Survivors International to investigate the medical aspects of lightning and to support victims and families. In 2001, he told us that members had talked 13 fellow survivors out of suicide.
A shock to the nervous system
Lightning usually kills by attacking the heart, which runs on electrical impulses. While high-voltage electrical injuries often cause severe burns, they are rare with lightning, likely because the bolts — lasting only 0.1 to 1 millisecond –- are too brief to cause severe burns.
Although burns may result if clothing ignites or sweat boils and steam is trapped under clothing, wet, sweaty clothing may actually conduct a heavy current outside the body and reduce the damage.
Raphael Lee, a professor of surgery and medicine at the University of Chicago, and an expert on the effects of lightning strike, told us that most of the initial current in a lightning strike does not pass through the body. However, two electromagnetic phenomena can produce a strong voltage drop across the body:
Strong electric fields are a problem for nerves and muscles, Lee says, because they “have been structured through evolution to be very sensitive to tiny electric fields.” That, combined with their physical length, which spans a large electrical gradient, “makes them very sensitive to lightning.”
Nerve cells can be a meter long, and by extending into different parts of an electric field, they are exposed to high voltages, Lee says. One focus of concern is the cell membrane which can die if strong voltages poke holes in it. Voltage can also wreak havoc in the pores in the membrane, which regulate the cell’s physiology by controlling how ions enter and leave the cell. Normally, for example, the potassium concentration is 1,000 times higher inside a cell, and damage to the pores can result in malfunction or cell death.
Lightning = thunder in the brain?
Although electricity is the natural focus of lightning damage, Lee suspects that an acoustic pulse, or shock wave, plays a major role, and perhaps a dominant one. A lightning bolt is surrounded by hot, ionized gas that arises in nanoseconds or microseconds and whose temperature may exceed 10,000 ° C. “When you heat something in a small area in such a short period, there are going to be shock waves,” he says.
The power of this acoustic wave is obvious when lightning hits and splits a tree, Lee adds. But inside the brain, the shock can trigger traumatic injuries similar to those caused by a roadside bomb or artillery shell.
Neurological injury: no passing matter
Lightning injury can be severe, long-lasting, and hard to treat, and it “may affect any or all parts of the nervous system,” according to Mary Ann Cooper, an emerita professor of emergency medicine at the University of Illinois-Chicago.
In a 2009 study of survivors of lightning and other electric shocks, 78 percent of the survivors had at least one psychiatric diagnosis; many of the troubles related to learning, memory and executive function.
In 2001, Cooper told The Why Files that confusion, caused by slowed information processing, is a hallmark of lightning injury. Symptoms include “difficulty in short-term memory, coding new information and accessing old information, multitasking, distractibility, irritability and personality change.”
Damage to the frontal lobe, the site of much higher thinking, is common, according to Cooper. “Many suffer personality changes because of frontal lobe damage and become quite irritable and easy to anger. The person who ‘wakes up’ after the injury often does not have the ability to express what is wrong with them…and cannot carry on a conversation, work at their previous job, or do the same activities that they used to handle. As a result, many self-isolate, withdrawing from church, friends, family and other activities.”
Cooper said some cell types continue suffering for weeks after the injury, and that nerve cells seem to “spend a long period trying to heal themselves, until finally the cell body is exhausted” and the cell dies. That process accounts for a delayed disability syndrome among survivors.
Help at hand?
Long-term neurological consequences are a major research area, Lee says, because they also occur in traumatic brain injury. “People are trying to sort out what is the best treatment, and understand why some people are more susceptible to delayed neurological problems. The body is very complicated and … the weight of evidence suggests there are genetic predispositions to complications after a blast causes traumatic injury to the brain, and lightning injury may be no different. Many people recover, but some don’t. What is different about the people who don’t?”
– David J. Tenenbaum
- Are Uganda lightning strikes becoming more common? ↩
- Latest lightning strikes. ↩
- Lightning injuries in sports. ↩
- Lightning basics. ↩
- Lightning science and safety. ↩
- National Lightning Safety Institute. ↩
- Bolts from the blue. ↩
- U.S. weather fatality statistics. ↩
- National Lightning Detection Network. ↩
- Lightning Strike and Electric Shock Survivors International, Inc. ↩
- Medical effects of lightning. ↩
- When people and lightning converge. ↩
- Behavioral consequences of lightning injury (PDF). ↩
- Understanding a strike survivor’s brain. ↩
- Keraunomedicine: the study of lightning casualties. ↩
- Human lightning rod. ↩
- Photojournalism of the Uganda lightning strike tragedy. ↩