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Recent satellite data suggests that there are more than 3 million lightning flashes worldwide per day, or more than 30 flashes per second on average. This includes flashes within or between clouds as well as flashes extending from cloud to ground. The amount of lightning found by satellites is considerably less than scientists once thought existed across the planet.The most accepted global measure of lightning frequency is the thunderstorm day--a day on which thunder is heard at a reporting site. By this standard, the tropics are the earth's lightning capital. From 100 to 200 thunderstorm days are reported each year across the equatorial belt from South America to Africa, southeast Asia, and northern Australia. However, thunderstorm days are not the ideal index of lightning, since this measure does not distinguish between a single clap of thunder and a prolonged severe storm.
In the middle latitudes, North America receives the most lightning due to its unique geography conducive to thunderstorms. Lightning detectors show an average of about 20 million cloud-to-ground flashes per year across the United States.Two U.S. regions are especially prone to strikes. Florida is the overall leader: its peninsular shape causes ocean-land heat contrast and air circulations that trigger storms year-round. The High Plains and foothills of the Rocky Mountains receive intense summer lightning due to elevated heating, moisture from the Gulf of Mexico, and their high altitude. (Even small clouds over the Rockies are cold enough to carry the ice crystals crucial to lightning formation).
Automatic devices to detect cloud-to-ground (CG) strikes were developed in the 1970s and have since become common in America, Europe, and Australia. These detection networks sense the radio-frequency pulses that travel outward from a lightning bolt. Each system has several antennae, separated by hundreds of kilometers, that give the direction of a strike; the strike's actual location is where the vectors intersect. The United States has been monitored since 1994 by a single combined network operated by Global Atmospherics, Inc., whose displays often show up on television weathercasts. This ground-based network does not provide information on in-cloud or cloud-to-cloud (CC) lightning.Satellites can also observe lightning. Two NASA satellites are now keeping tabs on lightning around the globe. The Optical Transient Detector was launched in 1995 and provides daytime as well as nighttime reports of lightning activity. A similar, recently upgraded instrument, the Lightning Imaging Sensor, was deployed on the Tropical Rainfall Measuring Mission in 1997; it correlates total lightning with rainfall amounts and locations. Data from these instruments is now being used in conjunction with the ground-based CG detection networks to deduce the amount of CC lightning in storms.
On the scale of a single thunderstorm, lightning is a discharge--a means of releasing the tremendous electrical energy built up by the storm. But on a global scale, thunderstorms actually separate charge. Lightning and other storm-related electrical features act to maintain a permanent potential of some 300 kilovolts between the earth's crust, which is negatively charged, and the ionosphere (well above 30 miles, or 50 kilometers), which is positively charged. In between, the slightly conductive lower atmosphere allows current to flow between the two regions. Were it not for the constant recharging from thunderstorms, the earth-atmosphere potential would disappear in a mere five minutes.
The answer lies in the structure of thunderstorms. For reasons unclear--but probably involving millions of collisions among ice crystals and small hailstones or graupel--storms evolve with positive charge near the top and negative charge from middle to cloud base. In a typical cloud-to-ground strike, negative charge descends from cloud base to ground. In response, trees, poles, and other objects release positive charge upward--thus keeping the earth's overall charge negative.
Clouds vary greatly in their ability to become electrified and produce lightning, and the process of charge separation still puzzles scientists. This research topic has been investigated at NCAR though use of an instrumented Schweizer 2-32, an all-metal sailplane flown into developing storms. Among other things, NCAR's sailplane studies have found that:
A 1996 experiment in northeast Colorado, the Stratosphere-Troposphere Experiment: Radiation, Aerosols, and Ozone (STERAO), explored the electrical and chemical aspects of thunderstorms. Another field experiment, the Severe Thunderstorm Electrification and Precipitation Study (STEPS-2000), is scheduled for the summer of 2000 near the intersection of Colorado, Kansas, and Nebraska to examine the electrification and microphysics of High Plains thunderstorms, especially those producing little rain but considerable lightning. This work will help further clarify the processes that help to electrify a storm.
- Clouds become electrified only after significant amounts of ice particles and supercooled water form at heights above the freezing level.
- Negative cloud charge tends to develop in cells or blobs, rather than the uniform layers previously theorized.
- The amounts of charge on single particles generally agree with laboratory findings on particle collisions and charge transfer--but they are more variable.
Once enough charge has been separated in a growing storm, a lightning flash can occur. These normally travel within or between clouds (abbreviated CC) or from cloud to ground (CG). Most storms produce more CC than CG flashes--about six times as many in tropical storms and two times as many in midlatitudes. Sometimes a flash will travel from cloud to air or simply occur within "clear" air.Exactly what triggers flashes is still uncertain and an area of continued research. It seems that very concentrated electric fields (perhaps at the ends of pointed surfaces or single particles) are needed to accelerate charged particles, or ions. Once moving with sufficient energy, the ions appear to blaze a path toward opposite charge in cascading fashion.
Despite its confident appearance, a lightning flash develops in fits and starts. The path of a typical cloud-to-ground (CG) flash lowering negative charge to earth is carved by a series of stepped leaders, each moving a bundle of charge a distance on the order of a city block. Each step takes only 1 microsecond or so, but the pauses between steps are much longer--on the order of 50 microseconds. At each step, the bolt may shift direction toward a stronger electric field, thus creating its crooked appearance. As a CG flash approaches several regions of opposite charge on the ground, it often branches into several parts.Just before it reaches ground, the step leader induces a huge electric potential (some 10 million volts), enough to bring up surges of positive charge from sharp objects or irregularities near the ground. Once the impulses meet--a few tens of meters above earth--the connection is established and the return stroke zips upward at a rate much faster than the stepped leader's descent. It is this return stroke that produces the visible flash as it heats surrounding air to 30,000 degrees C (54,000 degrees F), which in turn creates the shock wave we hear as thunder.
Some flashes end after a single return stroke, but more often than not, there are sequels. Negative charge close to the top of the channel takes advantage of the already-created path, descending as a dart leader. This is a continuous and usually unbranching pulse traveling about ten times faster than the stepped leader. Each dart leader is discharged by a subsequent return stroke that carries perhaps half as much current as the initial stroke (or even less). A typical flash has four strokes; occasionally, more than ten are observed. The time between strokes is on the order of a twentieth of a second. Since this is just within the range of human perception, a set of multiple strokes appears to flicker. A multistroke flash may continue for as long as a second.
The renegade of the lightning family is the positive flash--one that lowers positive charge to earth. Comprising 10-20% of all cloud-to-ground flashes, these powerful bolts carry as much as ten times the current of negative CGs and often last longer. They frequently emerge from the cirrus anvils that sweep downwind of thunderstorms, rather than from a storm's core.Some storms feature many more positive flashes than usual. The presence of smoke, dust, or pollution (such as downwind from urban areas) seems to encourage the development of positive flashes. This is probably because of the particles' effect on the number and sizes of ice crystals within storms. A study in the journal Science (10/2/98) examined thunderstorms in the Southern Plains during the spring of 1998, when smoke from Mexican forest fires was flowing northward over the region. Up to three times the usual number of positive flashes were observed in these smoke-altered storms.
Just as a thunderstorm can bring charge to earth through lightning, it also can send charge into the upper atmosphere above the storm. This happens through several recently discovered forms of storm electricity called sprites, elves, and blue jets. Much fainter than lightning, these phenomena are usually too dim to be seen by the naked eye, although some sprites have been observed from as far away as 400 miles (640 km). Sprites were discovered in 1989; they and their cousins have been studied in the 1990s through ground-based television cameras specially adjusted to pick up the subtle light they give off. Aircraft, satellites, and the space shuttle also have detected these features.A sprite is a large-scale but low-intensity pulse that can extend upward from the top of a thunderstorm to heights approaching 60 miles (100 kilometers). Sometimes a sprite is preceded by a short-lived, pancake-shaped area of charge called an elf that forms several dozen miles above the top of a thunderstorm. Every sprite or sprite cluster is connected to an intracloud or cloud-to-ground lightning flash; however, only about one in every 100 to 200 flashes produces a sprite. Just as lightning helps to reduce the electric field between a storm and the earth, sprites are believed to dispel charge differences between a thunderstorm and the ionosphere, an electrically charged region of the upper atmosphere.
Blue jets are narrow cones of energy shooting upward from thunderstorm tops at roughly 60 mi (100 km) per second to heights of 25-30 mi (40-50 km). Discovered by aircraft, blue jets are even more rare than sprites. They appear to be disconnected to the magnetic field in the storms beneath them, and the role they play in atmospheric electricity is unknown.
Lightning is known to produce nitrogen oxides within thunderstorms. These chemicals can react with others in the presence of sunlight to produce ozone. Since most lightning occurs inside a storm, the added ozone tends to show up several miles high rather than near the earth's surface, so it doesn't add significantly to ozone pollution at ground level.In 1996, NCAR and several other institutions studied the chemical environment of thunderstorms across the northeast plains of Colorado in the STERAO experiment noted above. This study confirmed that nitrogen oxides are more prevalent in the storm anvils rather than at the cloud bases. This lends support to the idea that thunderstorms have only a minor influence on ozone levels close to the ground.
Until recently, most studies of ozone and lightning have focused on measuring the production of nitrogen oxides in the immediate vicinity of storms. However, the resulting ozone has a long lifetime in the upper troposphere (a few miles above the ground), so it could be carried over long distances. According to an NCAR analysis, ozone from storms across southern Africa is being transported by the subtropical jet stream eastward to Australia, where it causes significant rises in ozone levels in the upper troposphere.
On average, lightning strikes kill about 100 Americans each year, more than hurricanes, tornadoes, or any other single kind of bad weather except floods. Some studies have shown that U.S. lightning deaths may be underreported by 20 to 30% and lightning injuries by more than 40%. The lightning death toll has dropped from its 1940 level of 400 a year as people moved from rural to urban settings. Recently it has held constant, due to an increase in outdoor recreation.According to the National Lightning Safety Institute, people under the age of 35 represent some 85% of lightning victims. One out of five strike victims die, and 70% of those who survive suffer serious long-term after effects.
The true impact of lightning on nature and culture is hidden by the widely dispersed nature of lightning itself. Forest fires are perhaps the most dramatic events caused by lightning. Positive C-Gs are a prime culprit in forest fires, since they tend to be strong and separated from rain-bearing parts of a storm. The need for quick detection of lightning-caused fires gave a boost to automatic lightning detection systems in the mountain West and Alaska before they spread elsewhere.
Benjamin Franklin's invention of the lightning rod made buildings much less prone to lightning-induced fires. The mechanics of protecting buildings from lightning are now well understood, though certain locations--nuclear power plants, munitions depots, blasting operations, and the like--must take special precautions. A research project involving NCAR and other institutions studied the frequent lightning strikes at Kennedy Space Center, which often delay shuttle missions.
Also vulnerable to lightning are power and telecommunication grids. A lightning strike in upstate New York led, through a chain of events, to the blackout that paralyzed New York City in 1977. Research continues on how such outages can be avoided by predicting highly electrified storms and understanding power surges--how much current and how large a voltage occur and the time they take to peak and fall.
Going indoors during a thunderstorm is by far the best way to avoid lightning. New guidelines recommend taking shelter as soon as you notice thunder arriving less than 30 seconds after a lightning flash. Since it takes five seconds for thunder to travel one mile, the 30-second interval means a flash is less than six miles away. This, in turn, means that the next flash might strike your area soon. Outdoor activities such as baseball or football games should be interrupted for shelter as soon as the 30-second rule is met. (An entire football team of 11 players was killed by a lightning strike in Africa in the fall of 1998.)Shelter is not failsafe. Lightning can strike though telephones, except for the cellular variety. You should avoid taking showers or standing by windows, screen doors, or patios. To protect household appliances, unplug them before (but not during!) electrical storms.
Outdoors, the idea is to avoid being near--or being--the highest object around. Get away from isolated trees, metal fences, wire clotheslines, and the like, and avoid standing in an exposed area or near water. If you are the tallest thing around, or in a boat on open water, crouch down to reduce your height (but don't lie flat). Lay down metal sports equipment and dismount bicycles. Take especially swift action if your hair stands on end, as that means charged particles are starting to use your body as a pathway. The safest form of vehicle is one with a fully enclosed, all-metal body, which helps to channel electricity around the interior. Make sure the car's windows and doors are completely closed.
Finally, remember that lightning can, and often does, strike the same spot more than once--even the same person. U.S. park ranger Roy Sullivan reportedly was struck seven times between 1942 and 1977.
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