Introduction to Fire from the Sky
Storm clouds begin to build above the high plains of Kansas near the Colorado border. The sky grows dark, the wind begins to blow, and rain starts pouring down in sheets. Suddenly the sky is alive with zigzagging flashes of lightning. These fingers of fire shoot across the darkened skies, the closest flashes followed by tremendous claps of thunder so loud that they seem to shake the ground.
Most people would be terrified of such an electrical storm, but the more than 30 atmospheric scientists gathered on the high plains of the United States in the summer of 2000 put aside any fears they may have had so they could study lightning up close. The researchers, from government laboratories, universities, and private companies, had come as part of a scientific effort called the Severe Thunderstorm Electrification and Precipitation Studies (STEPS), a program dedicated to unlocking the secrets of lightning and severe weather. Funded by the National Science Foundation (NSF), an independent U.S. government agency responsible for advancing science and engineering, and with support from the National Oceanic and Atmospheric Administration (NOAA), STEPS is the largest study of lightning ever conducted.
The scientists involved in STEPS hope to put together a clearer picture of the causes of lightning and the relationship between lightning and other severe-weather events, such as tornadoes. To probe those secrets the scientists in Kansas had come well-equipped. They brought with them a new lightning-mapping system, a storm-penetrating airplane, advanced radar equipment, and a variety of mobile and balloon-borne sensors to measure atmospheric pressure, temperature, humidity, winds, and electrification in storms. The field-research portion of STEPS ended in the summer of 2000, but the scientists involved will spend several years analyzing the data they obtained and using it to test theories about lightning and to find ways to develop more effective storm warnings.
In addition to trying to learn about lightning, precipitation, and tornadoes, scientists are also investigating mysterious storm-related electrical phenomena called “sprites,” “blue jets,” and “elves.” Until the 1990's, very little was known about these ghostly, colorful lights that sometimes appear briefly high above thunderstorms. Though scientists' understanding of lightning remained incomplete in 2001, many researchers thought that by using advanced equipment and techniques, including satellites in orbit around the Earth, they would one day have a deeper understanding of lightning and other severe-weather phenomena.
Scientists began the first serious studies of lightning in the 1700's. The American scientist and statesman Benjamin Franklin conducted some of the first important experiments with lightning in the mid-1700's. He was among the first to demonstrate that lightning is an electrical phenomenon, which he did by conducting an experiment that involved flying a kite during a thunderstorm. Since Franklin's time, scientists have gathered a great deal of information about lightning.
One thing they have learned is that Franklin could easily have been killed. Lightning is one of the most powerful weather phenomena on Earth. A lightning flash is a giant electric spark in the sky. A lightning discharge that strikes the ground can be several kilometers long and can deliver enough electrical energy to light a 100-watt light bulb for up to three years. Each year, lightning that strikes the ground worldwide carries about as much electrical energy as is produced annually by the entire U.S. power-generating industry.
Putting A Charge Into Storm Clouds
Thunderstorms separate electrical charge in a manner similar to that of an electric battery. A battery uses chemical reactions to separate some of the negatively charged parts of an atom, the electrons, from the positively charged remainder of the atom. The negative and positive particles migrate to opposite ends of the battery. Thus, a battery has a potential difference (difference in electrical potential between two points, measured in volts) between its ends, and is said to be charged. The potential difference is maintained until a conducting path is created between the terminals—as, for example, when a piece of copper wire is attached to the ends. Then electrons can flow, forming an electric current.
In a storm cloud, positive and negative charges are also separated as in a battery, except that the process is physical rather than chemical. Not all scientists agree about the details of how that process occurs, but they do tend to agree that it involves collisions between ice particles and graupel (small, soft, wet particles similar to hail) inside the cloud.
According to the most widely accepted theory, graupel fall through fields of small ice particles and collide with them frequently. The small ice particles sometimes are moving upward in updrafts, currents of air moving toward the top of a cloud. During these collisions, negative charge gets transferred to the graupel, which continue to move downward. The small ice particles are left with a net positive charge and the updrafts continue to carry them upward to higher levels in the storm cloud. The result is that negatively charged particles dominate the lower part of a storm cloud and positively charged particles build up in the upper part of the cloud. At this point, the “battery” of the cloud is charged.
There are two main categories of lightning flashes. The first category, which scientists know the most about, is known as cloud-to-ground (CG) flashes. These are lightning discharges between the cloud and the ground, although the name is misleading because some CG flashes actually move from the ground to a cloud. The second category of lightning flash is in-cloud, or, simply, cloud flashes. Cloud flashes occur more often than CG flashes, but they are not as well understood, in part because it is difficult to tell what is actually happening inside a storm cloud.
Countdown to A Discharge
Scientists do not completely understand the details of how lightning flashes are initiated, but they do agree on a general sequence of events. In the build-up to a typical CG flash, as negative charges accumulate in the lower parts of a storm cloud, the ground beneath the storm becomes positively charged. This occurs because like charges repel each other, so electrons (negative charges) on the ground are pushed away by the electrons in the cloud overhead. Since opposite charges attract, the electrons in the cloud are attracted to positively charged atoms and molecules on the ground that have lost their electrons. But the charges cannot move toward each other very far because they are attached to atoms and molecules in the graupel particles in the cloud, and to objects on the ground. Also, the air between the cloud and the ground provides a poor conducting path. In order for the charges in the cloud to reach the ground, a conducting path must somehow be formed. That is what CG lightning flashes do.
When dealing with electrical charges, scientists often use the term “electric field.” You can think of such a field in the same way that you think of a planet's “gravitational field.” Just as a gravitational field determines the weight a mass will have if placed in the field, an electric field determines what the force on a charged particle within it will be. The more electric charge that is concentrated in part of a storm cloud, the stronger the electric field is in that region. If the charge grows large enough, the electric field in that part of the cloud may become great enough to start a small electrical discharge.
Under certain conditions, such a discharge may grow until it becomes a “stepped leader,” which moves downward toward the ground in a series of jumps. Each section of the negatively charged stepped leader is about 50 meters (160 feet) long and takes about 1 microsecond (one millionth of a second) to complete. At the end of each step, there is a very brief pause, and then another step occurs. Each step may take a slightly different direction, creating a zigzagging path, and the leader is likely to form several branches, though scientists do not know why.
Short-circuiting A Cloud
As the tips of the stepped-leader branches near the ground, the electric field at the ends of the objects on the ground can become so strong that they launch upward-moving discharges known as “streamers” toward the downward-moving tips of the stepped-leader. The potential difference between the stepped-leader tips and streamers can be as great as 100 million volts. When a stepped leader and a streamer meet, the result is what you would expect when you short-circuit a 100-million volt battery: There is an intense surge of electrical current as negative charges, deposited along the narrow channel forged through the air by the stepped leader, rush toward the ground. As the electrons move, they bang into air molecules, making them emit light. As a result, the channel emits a bright flash of light from the point of connection to the ground and back up to the cloud.
This phenomenon, known as the “first return stroke,” travels up the channel so quickly that it appears that the entire channel is illuminated simultaneously. As the luminous region moves upward, the channel becomes an even better conductor, and other electrons deposited by the stepped leader along the channel, as well as electrons within the cloud, rush down to the ground.
Typically, the first return stroke is followed by two or three additional return strokes. Subsequent return strokes are initiated by leaders called “dart leaders,” which move continuously rather than in jumps as stepped leaders do. Dart leaders follow the original channel and move about 10 times as fast as the original stepped leader. Each return stroke produces another bright illumination of the channel, followed by an additional flow of electrons to the ground. Sometimes, when there is a pause between return strokes, the light of the channel may appear to flicker.
The very large electric current in the lightning channel heats the air in the channel to about 30,000 degrees C (54,000 degrees F) for a very brief time. The superheated air in the channel expands violently, creating a shock wave that is heard as thunder.
Forms of Cloud-to-ground Lightning
Cloud-to-ground lightning can appear in several forms. Because CG flashes are almost always branched or forked, they are sometimes called “forked lightning.” Sometimes winds blow the channel sideways, spreading it out and making the lightning flash appear to be very wide, an effect known as “ribbon lightning.” And occasionally, for reasons scientists do not completely understand, a lightning channel will appear to break up into a series of bright spots like beads on a string. This phenomenon is called “bead lightning.”
Cloud flashes can also take different forms. Scientists theorize that cloud flashes occur in much the same way as CG flashes, except that they are between negative and positive charges within a cloud. When a cloud flash occurs deep inside a thunderstorm cloud, it can seem to illuminate the entire cloud. This kind of lightning is often called “sheet lightning.” If the discharge channel appears to travel along the underside of a cloud for a long distance, branching out in many directions, it is often referred to as “rocket lightning” or “spider lightning.”
Another form of lightning, called “heat lightning,” occurs on hot summer nights when an observer sees distant lightning but does not hear thunder. Scientists think that there are two main reasons why heat lightning seems to be silent. The first is that the lightning—which is ordinary CG or cloud lightning—may be occurring so far away that the sound waves of the thunder become too weak to hear by the time they reach the observer. Alternatively, the sound waves of the thunder may be refracted (bent) as they pass through air layers of differing temperatures. When a lightning flash is more than 20 to 30 kilometers (12 to 19 miles) away from an observer, the sound waves may be refracted up and over the observer, making them inaudible.
Probably the least understood form of lightning is “ball lightning.” Those who have witnessed this rarity of nature say it appears as a luminous, slowly moving sphere about 30 centimeters (12 inches) in diameter. Ball lightning has often been reported in association with a nearby CG flash, and some witnesses have told of smelling a sulfurous odor. Scientists do not agree on a theory for how ball lightning occurs.
Though it may be convenient to put lightning flashes in descriptive categories, most of the terms used, such as ribbon lightning or spider lightning, are not very useful scientifically. That is because, to one observer, an entire storm cloud may appear to be illuminated by a lightning flash, with no connection to the ground. However, another observer standing several kilometers away might see the flash hit the ground.
Lightning is an extremely complex phenomenon. Despite considerable progress during the last 50 years, scientists still do not completely understand all of its mysteries. One reason for this is that it is very difficult to make quantitative (measurable) observations in storms. The ability of scientists to collect data in electrical storms depends on being at the right place at the right time. For example, even if they have designed a perfect balloon-borne instrument to record certain data, unless the instrument reaches the right part of the storm, the scientists will not get the information they are seeking.
The Mystery of Lightning Initiation
One of the most challenging puzzles that remained unsolved in 2001 was the question of how lightning flashes get started. Scientists know that in dry air at room temperature and sea-level pressure, the breakdown threshold (the electric field required for an electrical discharge to occur) is about 30,000 volts per centimeter. Under some circumstances, the threshold may be as low as 10,000 volts per centimeter. However, researchers have yet to find a field even that strong.
To measure electric fields, scientists use a device called an electric-field meter. Using balloons, they have flown many of these instruments into storms to measure the electric field at different altitudes. The largest fields recorded have typically been about 1,300 to 1,500 volts per centimeter, far too small to initiate a discharge. Occasionally, scientists have measured electric fields as high as 5,000 volts per centimeter, but even that is still far below the threshold for a discharge to take place. So the question remains, how does a discharge get started?
Over the years, several theories have been advanced to explain how lightning can occur when the electric field in clouds appears to be below the threshold for initiation of an electrical discharge. One explanation is that perhaps the region in which the field exceeds the threshold is very small. If so, the chances of getting a balloon-borne sensor into that region at just the right time to take a reading would also be very small.
One theory of how a lightning discharge is initiated in a small region is for individual ice crystals in electrified storm clouds to augment (greatly increase) the field at the ends of the crystals enough to exceed the breakdown threshold. It is well-known that when a long, thin, electrically conducting object is introduced into an otherwise uniform electric field, the field at the ends of the object is greatly increased relative to the rest of the field. In 2001, physicists John Hallett of the Desert Research Institute in Reno, Nevada, and William Beasley of the University of Oklahoma in Norman, were designing a laboratory experiment to investigate whether ice crystals might be capable of initiating lightning.
Perhaps the most intriguing theory of lightning initiation involves cosmic rays, high-energy particles from space that are constantly bombarding Earth's atmosphere. When these particles strike air molecules, they can knock electrons out of their molecules at very high speeds. If that happens in a highly electrified thunderstorm cloud, scientists theorize, the speeding electrons might be accelerated by the electric field, gaining enough energy to initiate a discharge.
Though many experts on lightning and atmospheric physics are skeptical about the idea that cosmic rays might play a role in lightning initiation, evidence that there can be energetic electrons in thunderstorms was found in 1995. William Beasley and a fellow University of Oklahoma physicist, Kenneth Eack, sent instruments attached to balloons into storms to simultaneously observe electric fields and X rays (high-energy electromagnetic radiation), something that had not been done previously. The presence of X rays indicates that electrons are being accelerated to high speeds. Beasley and Eack found that in regions of a storm in which the electric field was strong enough to produce X rays, the radiation was present until a lightning flash occurred. After each flash, the electric field decreased and the X rays ceased. When the electric field regained strength, X rays were again produced. This finding showed that energetic electrons accelerated by electric fields of sufficient strength to generate X rays were probably present. Those electrons may well have been produced by cosmic rays.
Using Technology to Track Storms
Though there has been much progress in the study of lightning, scientists still have many questions about how electrification may be related to precipitation and other aspects of thunderstorms. STEPS is providing scientists an opportunity to find the answers to those questions.
One of the key observational tools used in STEPS is a new lightning-sensing network known as the Lightning Mapping Array (LMA), developed by scientists at the New Mexico Institute of Mining and Technology (New Mexico Tech) in Socorro. The LMA is a system of 13 stations deployed across parts of Kansas and Colorado. Whenever a lightning flash occurs, the stations receive very-high-frequency (VHF) radio signals generated by the discharge and record with great precision the times the signals arrive. By examining these data and comparing when the signal arrived at each station, scientists can determine precisely where and when the discharges that caused the VHF signals occurred. They can then use this information to produce a three-dimensional map of the discharge channel inside as well as outside the storm cloud.
Another important goal of the STEPS research was to investigate a phenomenon known as “positive CG flashes.” In almost all lightning strikes, negative charges flow from the cloud to the ground. However, scientists have noticed that during some thunderstorms, an unusually large percentage of CG flashes bring positive charges to the ground. Positive flashes have been observed since the 1980's by scientists monitoring the National Lightning Detection Network (NLDN), a network of lightning detectors throughout the United States operated by Global Atmospherics, Inc., of Tucson, Arizona. Storms dominated by positive CG flashes, or that switch from mainly negative to mainly positive CG flashes, are sometimes accompanied by other severe weather events, such as heavy rain, hail, high winds, and tornadoes. Scientists are seeking to learn whether there are any direct relationships between positive CG flashes and those other severe-weather events.
Lightning's Mysterious Costars
Some of the most intriguing lightning-related phenomena being studied are the mysterious colored lights—sprites, blue jets, and elves—sometimes seen high above thunderstorms. These displays, known collectively as Transient Luminous Events (TLE's) because they appear for no more than a few thousandths of a second, may be the same phenomena that observers on the ground have been reporting since the early 1900's. Scientists first directly observed TLE's in the late 1980's using highly sensitive cameras, but they still do not understand the significance of TLE's. By 2001, Walt Lyons, a physicist at the Yucca Ridge Field Station near Fort Collins, Colorado, and other scientists, had perfected techniques for capturing TLE's on film. Using these low-light techniques, researchers routinely record thousands of these events each year.
Sprites are faint flashes of light that occur just above the tops of thunderstorms, between about 40 and 100 kilometers (25 and 60 miles) above the Earth. Although they can take many shapes, sprites often resemble fuzzy red carrots, with small tails that change from red to faint blue as they extend downward. They occur both individually and in groups and appear to be related to especially highly charged positive CG lightning flashes. However, as of 2001, theories to explain these occurrences were not complete.
Sprites were first captured on film in 1989 by physicists John Winckler, Robert Franz, and Robert Nemzek, at the University of Minnesota, who were testing a low-light video camera on the ground. The black-and-white video showed two pillars of light extending about 30 kilometers (18 miles) above the top of a thunderstorm.
Blue jets are extremely fast-moving fountainlike cones of blue light that appear to extend above the tops of storm clouds up to about 50 kilometers (30 miles) above the Earth. They usually travel upward, away from the tops of storm clouds, at speeds of about 100 kilometers per second. Blue jets were discovered in 1994 by physicists Eugene M. Wescott and Davis D. Sentman, of the University of Alaska, while they were flying in a research plane near a hailstorm in Arkansas. So far, no clear relationship has been established between lightning flashes and blue jets. The jets have been observed only rarely, perhaps because they are more difficult to see from the ground. Some scientists have suggested that there may be an association between blue jets and hailstorms, but so few observations have been made that the evidence is inconclusive.
Elves are disks of light that appear from about 70 to 100 kilometers (45 to 60 miles) above the Earth. They were discovered in 1995 by scientists from the University of Tohoku in Japan, Stanford University in Stanford, California, and the Yucca Ridge Field Station in Colorado. Scientists think elves, which can be several hundred kilometers wide, appear when intense electromagnetic waves radiated by lightning flashes pass through Earth's upper atmosphere.
Looking Down On Thunderstorms
Although ground-based research has provided scientists with a wealth of data about lightning, satellites in orbit around the Earth give them the ability to look down into thunderstorms. The first such satellites were launched in the 1970's, but recently scientists have launched several such satellites with advanced sensors.
One of these satellites, the Fast On-orbit Recording of Transient Events (FORTE), was launched in 1997 into a polar orbit about 800 kilometers (500 miles) above Earth. FORTE is different from other satellites used for lightning observation in that it can detect, record, and analyze bursts of both visible light and VHF radio signals, emitted near the Earth's surface. The majority of those bursts are caused by lightning.
Scientists expect that data from FORTE will provide clues about the physics of lightning. The primary goal of FORTE is to study how the visible light from lightning flashes is related to (VHF) radio-wave emissions from the same flashes. Typically, it has been difficult to determine whether a lightning flash was a cloud flash or a CG flash solely from visible-light data from satellites. Early results from FORTE suggest that it may be possible to distinguish CG flashes from cloud flashes.
In 2001, scientists at NASA were planning another storm-observing satellite, the Lightning Mapping Sensor (LMS). It was to be placed in geosynchronous orbit, an orbit in which a satellite circles the Earth above the equator at a distance of 35,900 kilometers (22,300 miles), remaining over the same point at all times. Scientists will use the LMS to monitor lightning continuously and to detect where it occurs within about 8 kilometers (5 miles). Data from the LMS will then be compared with data from ground-based instruments. Scientists hope that the new information about the relationships between lightning as observed by satellites and other weather-related events will lead to better weather forecasts.
Lightning and Weather Prediction
Even with their powerful new research tools, scientists still find it difficult to determine definite cause-and-effect relationships among the complex atmospheric phenomena produced by thunderstorms. Nonetheless, buoyed by preliminary analysis of STEPS data, they are hopeful that there are some identifiable weather relationships that could lead to improved forecasts and warnings.
In June 2000, a team of researchers led by physicist Paul Krehbiel, of New Mexico Tech, were observing a storm near Goodland, Kansas, with radar and the LMA as it produced a small tornado. They noted that the storm had produced frequent cloud flashes but very few CG flashes for more than an hour before the tornado appeared. Then, at about the time the tornado formed, positive CG flashes began occurring.
Krehbiel and his team discovered a “hole,” or lack of lightning activity, about 9 kilometers (5 miles) in diameter on the west side of the storm. These observations were similar to those made by Krehbiel and his team while monitoring a storm in Oklahoma in 1998 that also spawned a tornado. In both cases, the regions in which there was no lightning activity appeared to be located near updrafts in the storm cloud and the tornadoes formed on the west edges of the holes.
As Krehbiel's team observed the lightning hole in June 2000, other scientists were monitoring the same storm using the NLDN. The NLDN detected a positive CG flash on the western edge of the lightning hole just before the tornado formed. In 2001, scientists continued to search for possible connections between positive CG flashes and tornadoes, but it was still unclear if there was any relationship between the two events.
As research continues, scientists are optimistic that data from research programs and satellites will lead to a better understanding of lightning and severe weather. If some of their preliminary findings on the workings of thunderstorms stand up to scrutiny, lightning data from the LMA and satellites could one day complement traditional weather radar data to provide improved warnings of severe weather. In 2001, the average warning time for the most dangerous tornadoes was about 18 minutes. Although that represented a tremendous improvement since the 1980's, made possible primarily because of advances in radar technology, scientists would like to do better. Some investigators think that the most valuable result of research on lightning and severe storms may be to increase this warning time even more, giving people in the paths of powerful storms an even greater opportunity to reach safety.