Thunder and Lightning Storms

By: Isabel Sandigo

Introduction

All thunderstorms are dangerous. Every thunderstorm produces lightning. While lightning fatalities have decreased over the past 30 years, lightning continues to be one of the top three storm-related killers in the United States. In 2010 there were 29 fatalities and 182 injuries from lightning. Although most lightning victims survive, people struck by lightning often report a variety of long-term, debilitating symptoms.

Other associated dangers of thunderstorms include tornadoes, strong winds, hail and flash flooding. Flash flooding is responsible for more fatalities – more than 140 annually – than any other thunderstorm-associated hazard. Dry thunderstorms that do not produce rain that reaches the ground are most prevalent in the western United States. Falling raindrops evaporate, but lightning can still reach the ground and can start wildfires.

What are the Causes?

The causes of thunderstorms begin when layers of warm, moist air rise in a large, swift updraft to cooler regions of the atmosphere. There the moisture contained in the updraft creates clouds to form towering cumulonimbus clouds and soon enough it will begin to rain. Columns of cooled air then sink earthward, striking the ground with strong downdrafts and horizontal winds. At the same time, electrical charges accumulate on cloud particles. Lightning discharges occur when the accumulated electric charge becomes sufficiently large. Lightning heats the air it passes through so intensely and quickly that shock waves are produced; these shock waves are the sounds of thunder.

How Severe is the Disaster

Most lightning strikes cause damage through the large current flowing in the return stroke or through the heat that is generated by this and the continuing current. The exact mechanisms of how lightning currents cause damage are not completely understood, however. If lightning strikes a person, the stroke current can damage the central nervous system, heart, lungs, and other vital organs.

When a building or power line is struck by lightning or is exposed to the intense electromagnetic fields from a nearby flash, the currents and voltages that appear on the structure are determined both by the currents and fields in the discharge and by the electrical response of the object and its grounding system. In other words, if a lightning surge enters an unprotected residence by way of an electric power line, the voltages may be large enough to cause sparks in the house wiring or appliances. When such flashovers occur, they may short-circuit the alternating current power system, and the resulting power arc may start a fire. In such instances, the lightning does not start the fire directly, but it does cause a power fault (short circuit), and then the power currents do the damage. In the case of metals, large currents heat the surface at the air-arc interface and the interior by electron collisions with the metal lattice. If this heat is also great enough, the metal will melt or evaporate.

At least three properties of the return-stroke current can cause damage; these are the peak current, the maximum rate of change of the initial current, and the total amount of charge transferred. For objects that have a resistive impedance, such as a ground rod or a long power line, the peak voltage during a strike is proportional to the peak current produced of the lightning stroke and the resistivity of the struck object. For example, if a 100,000 ampere peak current flows into a 10-ohm grounding system, 1 million volts will be produced. A common hazard associated with the large voltages produced by lightning strikes is the re-direction of some of the energy (that is, a flashover) from the original target to an adjacent object. Such secondary discharges, or side-flashes, often cause damage comparable to that of a direct strike, and they are one of the main hazards of standing under or near an isolated tree (or any other tall object) during a thunderstorm. Such large voltages frequently cause secondary discharges or side-flashes to radiate outward from the object that is struck to another object nearby. One form of a side-flash can even occur in the ground near the point of lightning attachment.

For objects that have an inductive electrical impedance, such as the wires in a home electrical system, the peak voltage will be proportional to the maximum rate of change of the lightning current and the inductance of the object. For example, one metre of straight copper wire has a self-inductance on the order of one microhenry. The peak rate of change in the lightning current in a return stroke is on the order of 100,000 amperes per microsecond; therefore, about 100,000 volts will appear across this length of conductor for the duration of the change, typically 100 nanoseconds (billionths of a second).

The heating and subsequent burn-through of metal sheets, as on a metal roof or tank, are to a first approximation proportional to the total charge injected into the metal at the air-arc interface. Generally, large charge transfers are produced by long-duration continuing currents that are in the range of 100 to 1,000 amperes, rather than by the peak currents, which have a relatively short duration. The heat produced by long continuing currents is frequently the cause of forest fires. A typical cloud-to-ground flash transfers 20 to 30 coulombs of charge to the ground, and extreme flashes transfer hundreds and occasionally thousands of coulombs.

What Global Range of These Disasters?

Thunderstorms are known to occur in almost every region of the world, though they are rare in polar regions and infrequent at latitudes higher than 50° N and 50° S. The temperate and tropical regions of the world, therefore, are the most prone to thunderstorms. In the United States the areas of maximum thunderstorm activity are the Florida peninsula (more than 90 thunderstorm days per year), the Gulf Coast (70–80 days per year), and the mountains of New Mexico (50–60 days per year). Central Europe and Asia average 20 to 60 thunderstorm days per year.


How are Humans Affected by These Disasters

While thunder won't hurt you—lightning will! So it's important to pay attention when you hear thunder. Thunderstorms happen in every state, and every thunderstorm has lightning. Lightning can strike people and buildings and is very dangerous.

Thunderstorms affect small areas when compared with hurricanes and winter storms. The typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. Nearly 1,800 thunderstorms are happening at any moment around the world. That's 16 million a year!


Despite their small size, all thunderstorms are dangerous. Every thunderstorm produceslightning, which kills more people each year than tornadoes. Strong winds, hail, and tornadoesare also dangers associated with some thunderstorms.


Can we Predict or Control the Disaster?

Meteorologists combine these measurements with information from weather balloons launched to measure conditions at various heights in the atmosphere and geostationary satellites that sense moisture in the atmosphere and reveal the locations of clouds.

All of the weather and satellite data is fed intonumerical simulations run on supercomputers, which crunch the numbers and spit out a model of the atmosphere's behavior. Scientists compare that output with weather observations, and if it’s a good match, they use the model to make a forecast.

Once a storm is brewing, scientists begin monitoring it using radar. Radar energy is beamed off the precipitation inside clouds, and the strength of the reflected signal reveals the density of moisture, snow, hail or dust in the storm system. The frequency of the signal tells scientists whether the storm is moving toward the radar source or away from it.

If the storm is rotating, it could spawn a tornado. Because tornadoes are relatively small, localized features, meteorologists can't forecast them more than a few hours in advance.


How Can We be Safe

The best personal protection against lightning is to be alert to the presence of a hazard and then to take common-sense precautions, such as staying inside a house or building or inside an automobile, where one is surrounded by (but not in contact with) metal. People are advised to stay away from outside doors and windows and not to be in contact with any electrical appliances, such as a telephone, or anything connected to the plumbing system. If caught outdoors, people are advised to avoid isolated trees or other objects that are preferred targets and to keep low so as to minimize both height and contact with the ground (that is, crouch but do not lie down). Swimming pools are not safe during a lightning storm because water is a good conductor of electricity, and hence being in the pool effectively greatly multiplies the area of one’s “ground” contact.

The frequency with which lightning will directly strike a building in a particular region can be estimated from the building’s size and the average number of strikes that occur in the region. If a building is struck whenever a stepped leader comes within 10 metres (33 feet) of the exterior of the building, then a building that is 12 metres (39 feet) wide and 16 metres (52 feet) long (an area of 192 square metres, or about 2,000 square feet) will have an effective strike zone of 32 metres by 36 metres (an area of 1,152 square metres, or 12,400 square feet). In a region where an average of three cloud-to-ground lightning strikes occur per square kilometre annually, such a building will experience an average of 0.0035 direct strike per year, or one strike about every 290 years (1,152 square metres × 3 flashes per square kilometre × 10−6 metres per square kilometre). In a region where there is an annual average of five strikes per square kilometre, the same building will experience an average of 0.0058 direct strike per year, or one strike about every 174 years. These calculations indicate that, for the second example, an average of one of every 174 buildings of similar size will be directly struck by lightning in that region each year.Encyclopædia Britannica, Inc.Encyclopædia Britannica, Inc.Encyclopædia Britannica, Inc.Encyclopædia Britannica, Inc.

Structures may be protected from lightning by either channeling the current along the outside of the building and into the ground or by shielding the building against damage from transient currents and voltages caused by a strike. Many buildings constrain the path of lightning currents and voltages through use of lightning rods, or air terminals, and conductors that route the current down into a grounding system. When a lightning leader comes near the building, the lightning rod initiates a discharge that travels upward and connects with it, thus controlling the point of attachment of lightning to the building. A lightning rod functions only when a lightning strike in the immediate vicinity is already immanent and so does not attract significantly more lighting to the building. The down conductors and grounding system function to guide the current into the ground while minimizing damage to the structure. To minimize side-flashes, the grounding resistance should be kept as low as possible, and the geometry should be arranged so as to minimize surface breakdown. Overhead wires and grounded vertical cones may also be used to provide a cone-shaped area of lightning protection. Such systems are most efficient when their height is 30 metres (98 feet) or less.

Protection of the contents of a structure can be enhanced by using lightning arresters to reduce any transient currents and voltages that might be caused by the discharge and that might propagate into the structure as traveling waves on any electric power or telephone wires exposed to the outside environment. The most effective protection for complex structures is provided by topological shielding. This form of protection reduces amounts of voltage and power at each level of a system of successive nested shields. The partial metallic shields are isolated, and the inside surface of each is grounded to the outside surface of the next. Power surges along wires coming into the structure are deflected by arrestors, or transient protectors, to the outside surface of each shield as they travel through the series, and are thus incrementally attenuated.


Thunderstorm Structure

''Structure of a thunderstormWhen the atmosphere becomes unstable enough to form large, powerful updrafts and downdrafts (as indicated by the red and blue arrows), a towering thundercloud is built up. At times the updrafts are strong enough to extend the top of the cloud into the tropopause, the boundary between the troposphere (or lowest layer of the atmosphere) and the stratosphere.Click on the icons along the left-hand side of the figure to view illustrations of other phenomena associated with thunderstorms.''

Electrical Charges

''When the electrical charges become sufficiently separated in a thundercloud, with some regions acquiring a negative charge and others a positive, a discharge of lightning becomes likely. About one-third of lightning flashes travel from the cloud to the ground; most of these originate in negatively charged regions of the cloud.''

Evolution of a gust front

''Evolution of a gust front(Left) During a thunderstorm a large column of cold air, originating high in the thundercloud, can descend rapidly to form a gust front. (Right, inset) Fed by the main downdraft, the gust front flows in a turbulent layer along the ground and can extend far from the main body of the storm. A gust front is often felt by observers as a sudden cool wind arriving well in advance of a storm.''

Thunderstorm Micro burst

''Thunderstorm microburst. The air that forms the microburst is initially “dammed” aloft by the strength of the storm's updraft then cascades downward in a high-velocity, narrow column (less than 4 km, or 2.5 miles, in diameter). (Right, inset) Microbursts are very dangerous to aircraft and can create great damage on the ground. In the absence of observers, microburst damage can often be distinguished from that of a tornado by the presence of a “starburst” pattern of destruction radiating from a central point.''

Tornadic Thunderstorm


''(Left) Tornadic thunderstormThe rotating updraft that produces the tornado extends high into the main body of the cloud.(Right) Anatomy of a tornadoAir feeds into the base of a tornado and meets the tornado's central downflow. These flows mix and spiral upward around the central axis. The tornado's diameter can be much greater than that of the visible condensation funnel. At times the tornado may be hidden by a shroud of debris lifted from the ground.''

Hail producing thunderstorms

''Hail-producing thunderstorm(Left) A hailstone can travel through much of the height of the storm during its development and may make multiple vertical loops. (Right) Most hailstones are formed by accretion around a nucleus (spherical embryo). Peculiarly shaped hailstones are generally the product of multiple stones fusing together.''

Sound

''(Top) As shown in the chart, the elapsed time between seeing a flash of lightning and hearing the thunder is roughly three seconds for each kilometre, or five seconds for each mile. (Bottom) An observer's relative distance from the main lightning channel and its secondary branches determines whether thunder is heard to start with a sudden clap or a softer rumbling.''