The lightning flash is one of the most visually remarkable, yet scientifically elusive, of all atmospheric phenomena. Though the earliest lightning observations go back thousands of years, perhaps the biggest breakthrough in our understanding of the lightning flash came in the mid 18th century, when Benjamin Franklin suggested—and later proved in his famous kite experiment—that lightning was an electrical phenomenon. Such a discovery set the stage for subsequent research into lightning formation, lightning detection, lightning safety, and even the use of lightning as a source of energy.
To the weather enthusiast, lightning offers incredible visual displays, sometimes referred to as Nature's fireworks. To the scientist, lightning detection provides a wealth of information on atmospheric processes, thunderstorm formation, and interactions between the land surface and the atmosphere, and it can be used to inform forecasters and hazard mitigation specialists. Unfortunately, lightning is also a deadly phenomenon and can cause severe damage to built structures, trees, and airplanes, as well as disrupt power grids and ignite fires.
An understanding of lightning formation begins with an understanding of electrical charge in the atmosphere. In the absence of clouds and major weather systems (i.e., “fair-weather” conditions), Earth's atmosphere contains a preponderance of positively charged ions (an ion is simply a charged particle), while Earth's surface contains a preponderance of negatively charged ions. This global-scale distribution of charge is largely the result of thunderstorms, which act to remove negative charge from the atmosphere and deposit it on Earth's surface. Since air is a poor conductor of electricity, an exceptionally large electrical field must be established for lightning to occur. Under fair-weather conditions, Earth's electrical field is relatively weak, but in a thunderstorm environment, the strength of the electrical field increases by many orders of magnitude until an electrical current can move freely between the cloud and another conducting surface.
The most popular theory for the development of electrical charge in thunderstorms is the noninductive charging mechanism. Since thunderstorms grow to altitudes where temperatures are below freezing, they become composed of liquid water, ice crystals, and hailstones. Each of these particles has a slightly different arrangement of electrons, meaning that as they collide with each other in the turbulent air currents found within a thunderstorm, electrons will be transferred between them. This transfer of electrons results in an accumulation of positive charge near the upper portions of the thunderstorm (where ice particles are kept aloft) and an accumulation of negative charge in the middle and lower portions of the thunderstorm (where small, but heavier hailstones settle). Secondary charging mechanisms help strengthen the electrical field as electrons are continuously transferred between colliding thunderstorm particles. On the ground, an accumulation of positive charge builds as negative charge near the lower portion of the thunderstorm repels the negative charge on Earth's surface that existed under fair-weather conditions. When a lightning flash occurs between the cloud and the ground, negative charge is transferred back to the ground.
There are three primary types of lightning flashes: those that occur within the cloud (i.e., intracloud), between clouds (i.e., cloud to cloud), and between the cloud and the ground (i.e., cloud to ground). Approximately 80% of all lightning flashes occur either within the cloud or between clouds. Other types of optical and electromagnetic phenomena, both on the ground (e.g., St. Elmo's Fire) and in the upper atmosphere (e.g., sprites and jets), have been observed in association with lightning flashes.
To the naked eye, a lightning flash appears as a continuous flash of light. However, high-speed photography and laboratory experiments have revealed that a single lightning flash is actually composed of a series of flashes each lasting just a few millionths of a second. In a typical cloud-to-ground flash, negative ions (electrons) begin moving downward through the cloud toward the ground. To facilitate this process, electrons may branch out and take multiple paths downward—this creates the “forked” appearance of many lightning flashes. As the flow of electrons moves downward, a stepped leader is formed that attempts to establish a conductive channel for electrons to flow through and neutralize the charge difference between the cloud and the ground. As the channel nears the ground, a spark occurs on a conducting surface (e.g., the top of a tree), creating a conduit for electrons to flow through. Once the lightning channel is established, the current flows upward toward the cloud, creating a brilliant flash known as a return stroke. Depending on the strength of the electrical field, multiple return strokes may occur in an effort to dispel the negative charge from the cloud.
A less frequent type of cloud-to-ground flash is the positive flash, which transfers positive charge from the cloud to the ground. These flashes often originate in thunderstorms that are horizontally tilted, exposing the positively charged upper regions of the cloud to the ground. Positive flashes may also emanate from the sides of the cloud, striking the ground a considerable distance from the main body of the thunderstorm. This type of flash is often referred to as a “bolt from the blue,” since it occurs away from the thunderstorm under generally clear skies.
Prior to the advent of modern lightning detection equipment, the occurrence of lightning was typically inferred by the sound of thunder—the sound wave produced by a lightning flash as it rapidly heats and expands the air around it. Weather observers would mark the presence of thunderstorms in their logbooks if audible thunder was detected. The first attempts to measure the electrical field in the vicinity of thunderstorms took place in the early 1950s and used sferics, or radio waves, to measure electromagnetic radiation produced by the lightning flash.
One of the biggest shortcomings of early lightning detection methods was that they could not pinpoint the true location of the lightning channel. This changed in the 1970s with the development of magnetic direction finders. These ground-based instruments are able to “sense” the radiation emitted during the first few microseconds when a flash strikes the ground or a ground-based object. The first sensors became operational in 1979 and covered only small portions of the United States. In 1989, these regional networks merged into the present-day National Lightning Detection Network, based in Tucson, Arizona. This network consists of more than 100 sensors that record the time and location of all cloud-to-ground flashes, as well as other characteristics (e.g., magnitude of the electrical current, number of return strokes). Some regional lightning networks in the private sector continue to operate their own direction finders for specialized research and monitoring purposes. Lightning sensors have also been placed aboard satellites to provide global coverage of cloud-to-ground and intracloud lightning (a map produced by the National Atmospheric and Space Administration [NASA] showing the global distribution of lightning flashes can be found at http://virtualskies.arc.nasa.gov/research/tutorial/lightningMap.html).
Lightning is the leading cause of thunderstorm-related fatalities in the United States, killing an average of 100 people annually from 1959 to 2006. Since the 1930s, lightning fatalities and fatality rates have steadily declined due to a decreasing rural population, improvements in forecasts and warnings, better medical response to victims, and mitigation efforts through public education. Fatality rates are often much higher in less developed countries (6 fatalities per million population) in comparison with more developed nations (from 0.1 to 1 per million). People engaged in outdoor recreational or work-related activities are the most vulnerable to the hazard. Uniquely, males are far more likely to be killed by lightning than females, with males comprising nearly 85% of lightning fatalities in the United States. Victims struck directly or indirectly by lightning often suffer from injuries to the cardiac and neurological systems (cardiac arrest being the most common cause of death) as well as psychological effects that include neurocognitive deficits, memory problems, and even depression. The spatial distribution of lightning fatalities and injuries is commensurate with population and cloud-to-ground lightning densities. Casualties tend to occur most often during warm-season afternoons, when thunderstorms are at their climatological maximum and people are more likely to be outdoors.
Lightning routinely causes damage to personal, commercial, and public property and is a major source of property, agricultural, and casualty insurance losses. Though lightning losses are often isolated and rarely achieve the tallies reported for other weather hazards such as tornadoes and hurricanes, the cumulative insured losses from lightning are considerable, with an estimated $5 billion in insured losses annually. Lightning regularly disrupts electricity generation, transmission, and distribution. The hazard is a major cause of local and regional electrical and telecommunication blackouts and inflicts losses of millions of dollars annually on utility infrastructure and sales. Lightning-initiated wildland fires result in annual timber and property losses of millions of dollars, with millions of dollars more spent on fire prevention and suppression in the more developed countries. The National Interagency Fire Center estimates that approximately 12,000 wildland fires are initiated by lightning each year in the United States, affecting an average of 5 million acres annually.
The spatial and temporal distribution of lightning flashes and the utility of lightning data sets have been explored in a variety of geographical studies. The general goal of these studies is to inform researchers and those with lightning-oriented interests of the patterns of lightning within a range of meteorological and hazard scenarios. Geographers are in a unique position to provide this information using advanced visualization and mapping techniques (i.e., geographic information systems, or GIS) that allow inferences into mechanism, causation, and other process-based inquiries.
The spectrum of geographically based lightning studies is rather broad and contains many overlapping categories and components. Most of these studies employ a descriptive approach, where patterns in lightning frequency are mapped and explained across various space and timescales (i.e., a lightning climatology). Some studies use this information to explore the possible mechanisms for the resulting distributions and the processes and weather systems that drive them. Still other studies use lightning data to explore various properties of the atmosphere. Improvements in the spatial and temporal resolution of lightning data have allowed for such investigations from the macroscale down to the microscale. Examples include the use of lightning data to understand the role of aerosols and other small particles in thunderstorm composition, rainfall, and cloud electrification as well as the atmospheric processes occurring in severe thunderstorms, tropical cyclones, and winter storms (e.g., “thunder-snow”). Lightning data have been used in macroscale studies to examine the relationship between atmospheric circulation patterns and the frequency and intensity of convection in subtropical and midlatitude environments.
Among geographers who use lightning data sets, the role of land use in initiating and altering the frequency and character of lightning flashes has been a popular research topic in recent years. In particular, there has been a significant growth in the number and scope of studies examining the lightning climatology of urban areas, both in the United States and abroad (e.g., South America, Western Europe, Australia). These studies are part of a much larger initiative aimed at examining how the urban environment affects atmospheric processes across multiple scales. Relatively fewer studies have investigated the distribution of lightning over mountainous terrain and the relationships between slope angle, aspect, elevation, and lightning frequency. Continued improvements in lightning detection networks and a longer time series will likely lead to more studies of lightning over complex terrain. Additionally, the continued development of satellite-based lightning sensors will provide a longer time series of lightning coverage over the oceans and higher latitudes. This information will be of value to studies examining the global distribution of lightning and its trends over time, particularly in the context of global and regional climate change. Continued development of GIS technologies will be of tremendous benefit to climatological studies of lightning as well as studies exploring the relationships between lighting and other aspects of the Earth system (e.g., land cover, demographics) across space and time.
Natural Hazards and Risk Analysis, Thunderstorms, Vulnerability, Risks, and Hazards, Wildfires: Risk and Hazard
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