Lightning is one of the most awesome displays in nature.It is a discharge of electricity, a giant spark, that occurs in mature thunderstorms. Lightning is actually aflow of electrical current through the air. As electrical currentflows through the air within the lightning channel, atmospheric gases are heatedto a temperature as high as 30,000°C (54,000°F), which is 5 times hotter than the surface of the sun. The hot gas emits a flash of light (lightning) andexpands explosively to create the sound we hear as thunder.
Lightning can be deadly. Over the last 30 years inthe United States an average of 53 people are killed each year by lightning.Probably about 10 times that many people are struck and injured each year, but surviveto tell the story. One contributing reason for the large number of people struck each year is that lightning victims frequently are struck just before or after the occurrence of precipitation at their location. Many people apparently feel safe from lightning when not experiencing rain. Remember that thunderstorms do not haveto produce rain at all.
Lightning is responsible for over 30% of all power outages. Here in the western UnitedStates lightning starts about half of all forest fires. Lightning caused fires are a particular problem at the beginning of the thunderstorm season in Arizona. At this time the air underneath thunderstorms is still relatively dry. Rain falling from a thunderstorm will often evaporate before reaching the ground (virga). Lightning then strikes dry ground, starts a fire, and there isn't any rain to put out or at least slow the spread of the fire. In fact strong downdraft and gust front winds candevelop with virga, which can act to fan the flames and intensify fires. The occurence of lightning with no rain is called dry lightning.
Electric Charge and Charge Separation sets the Stage for Lightning
A few basics about electric charge will be described before applying it to lightning. The most fundamentalunits of electrical charge are the electrons and protons that make up atoms. Each electron is said to have oneunit of negative electrical charge, while each proton is said to have one unit of positive charge. Most atomsand molecules are electrically neutral because they contain equal numbers of electrons and protons. A neutralparticle becomes ionized when an electron is removed from it. After ionization, there are now two differentcharged particles: a negatively charged electron and a positively charged ionized particle, which has one moreproton than it has electrons. When considering charged particles, like charges repel each other, while oppositelycharged particles are electrically attracted to each other.
An everyday example of charge separation can happen when combing your hair. As the comb rubs against your hair,the comb acquires a postive charge and your hair acquires a negative charge since some electrons move from thecomb to your hair. This causes your hair to stick up. Since each hair is negatively charged, they repel each other, and the individual hairs move to be as far from each other as possible. If you wave the postively charged comb just above yourhair, you will see the negatively charged hair is attracted toward the postively charged comb. Another example of charge separation happenswhen you rub your feet on a carpet. The carpet becomes positively charged (electrons removed), while you become negativelycharged (electons deposited from carpet to you). After acquiring a negative charge in this way, you may experience a slight"shock" just before touching an electrical conductor, like a doorknob. Electric current (flow of electrons) is readily ableto flow through a conductor, and the shock that you sense is the movement of electrons from your body to the conductor.
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Charge separation occurs in thunderstorms to set the stage for lightning. The exact mechanisms of charge separation within cloudsremains an area of active research. One possible mechanism is described here. The figure above is representative of a typicalsummertime thunderstorm. Thunderstorms are usually quite tall vertically and thus there is a wide range of temperature withinthe cloud. A large region in the middle of the cloud, where the air temperature is below freezing (lower than 0°C), but warmer than -40°C,contains a mixture of supercooled water droplets and ice crystals. Supercooled water is liquid water that is colder than the typical freezing temperature of water. This is the region of the cloud where precipitation forms and is also where electrical charge is separated. Collisions betweenparticles within the cloud separate the electrical charge needed for lightning. When temperatures are colder than -15°C (above the dotted line in the figure below), graupel becomes negatively charged after colliding with a snow (or ice) crystal. The ice crystal is positively charged and is carried up toward the top of the cloud by the updraft winds. Graupel particlesare particles that grow by acretion of supercooled liquid droplets, i.e., frozen particles that grow by collions with supercooledliquid droplets forming layers or irregular clumps of ice. In situations where the graupel particles get very large, hail will fall. Graupel may by thought ofas small hail. Most thunderstorms will contain graupel, though most of the time the particles will not become big enough to become hail.The larger, heavier, and negatively charged graupel particles tend to accumulate in the middle of the cloud as the smaller, lighter, andpositively charged ice crystals are more easily carried upward by thunderstorm updrafts.To confuse the situation, at temperatures warmer than -15°C (but still below freezing), the polarities are reversed in the charge separation process. The important point is that we know charge gets separated within thunderstorms and we know the typical structure of the charge separationbased on measurements. This charge separation must happen for lightning to take place.A large volume of positive charge builds up in the top of the thunderstorm, while a layer of negative charge accumulates in the middle of the cloud. Some smaller volumes of positive charge are often found below the layer of negative charge as well. Notice that positive charge also forms on the ground directly underneath the thunderstorm. This is called an induction charge becauseit is induced by the layer of negative charge in the middle of the cloud. The Earth's ground surface is a good conductor of electricity, whichmeans that electrons can easily move around in response to nearby centers of charge (more generally in response to electric fields).Electrons on the ground are repeled by the negativecharge region overhead and thus flow away from the area, leaving a positive induction charge on the ground.
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Lightning Happens
When the electrical attractive forces between these charge centers gets high enough lightning occurs, which is a flow ofelectrical current through the air. Most lightning (2/3 rds) stays inside the cloud and travels between the main positive charge center near the top of the cloud and a large layer of negative charge in the middle of the cloud; this is intracloud lightning (Pt. 1 in figure above). About 1/3 rd of all lightning flashes strike the ground. These are called cloud-to-ground discharges (actually negative cloud-to-ground lightning, Pt. 2 in figure).Thus lightning is a flow of electrical current (flow of electrons) through the air from a negative to a positive charge center.As electrical current flows through a material, we say the material is conducting electricity. Some materials are verygood conductors of electricity, i.e., it is easy for current to flow through. Some examples of good electrical conductorsare metals (which is why we make wires with metal), water, and the Earth's ground surface. The fact the ground surface is a goodconductor is the reason why an induction charge forms underneath thunderstorms, as the electrons in the ground materialare repeled by the negative charge in the cloud above and move away. By the way all conducting objects on the ground willacquire a positive induction charge, including people. The attractive force between the negative charge center in the middle of the cloudand the positive induction charge on a person standing on the groundmay cause a person's hair to stick up.Incidentally, if you are ever caught out in a thunderstorm and your hair sticks up, be wary, cloud-to-ground lightningis probably about to strike somewhere nearby.Other materials are poor conductors orelectricity or electrical insulators, for example glass and air, because it is difficult to make current flow through them. Airis such a poor conductor of electricity that a huge charge build-up is required before current (lightning) will flow throughthe air. This is why lightning is so powerful. Nothing happens until a large enough charge build-up happens, then a massivecurrent will flow through the air as lightning. In a sense, air resists the flow of electrical current (lightning) until thecharge build-up becomes too much to hold back. Cloud-to-ground lightning acts to neutralize the charge differences betweenthe cloud and ground, however, an active thunderstorm will continue to separate charge setting the stage for more lightning.Positive polarity cloud to ground lightning (Pt. 3 in figure) accounts for a few percent of lightning discharges. Upward lightning is the rarest form of lightning (Pt. 4 in figure). We'll look at both of these unusual types of lightning later.
A complicated and very rapid sequence of events occurs prior to a cloud-to-ground lightning strike. This material willbe briefly covered here. You are not expected to understand the details of this process. Once there is sufficient chargebuild-up, electrons begin surging away from the negative charge region of the cloud in branching surges called "stepped leaders." A developing channel, which will become the lightning channel through which current will flow, makes its way down toward the ground in 50 meter jumps that occur every 50 millionths of a second or so. Every jump produces a short flash of light (think of a strobe light dropped from an airplane that flashes periodically as it falls toward the ground). This sketch shows what you'd see if you were able to photograph the stepped leader on moving film. Every 50 microseconds or so you'd get a new picture of a slightly longer channel displaced slightly on the film. Thesebranching steps are of course happening much faster than we are able to observe in real time. When a stepped leader gets closeto the ground, a strong electrical attraction develops between the negative stepped leader and the positive charges on the ground.Several positively charged sparks, called upward leaders, develop and move upward toward the stepped leader. Typically,one of these will intercept the stepped leader and close the connection between negative charge in the cloud and positive charge on the ground.The connection between the stepped leader and the upward discharge creates a "short circuit" between the charge in the cloud and the charge in the ground. Lightning will then flow through this channel. See this summary figure for the development of cloud-to-ground lightning.About half of all cloud-to-ground lightning strikes end at thispoint. In about 50% of cloud-to-ground lightning strikes, there will be several distinct bursts of current that flow throughthe same lightning channel. This happens slow enough that your are able to detect this as flickering strokes of lightning. It is also possible for the negative stepped leader to connect to more than one upward leader. In these cases thecloud-to-ground lightning channel will have multiple attachment points to objects on the ground.
Lightning rods (invented by Benjamin Franklin) make use of the upward connecting discharge. Lightning rods are madeof a conducting material and stick up above the structure they are meant to protect. Picture of a house with lightning protection.The basic idea is for lightningto strike the lightning rod and not nearby structures. The lightning rod safely carries the current to the ground throughthick, insulated wires. The sharp tipped points on top of lightning rods are designed to produce strong upward leaders, whichin most cases will intercept the cloud-to-ground lightning strike.Lightning rods do work and have changed little since their initial development in the 1700s. Most of the newer buildings on campus are protected with lightning rods. However, lightning rods do not provideabsolute protection. The path taken by downward stepped leaders is erratic and not well understood. Even thougha lightning rod is statistically the most likely object to be struck in a given area, there have been caseswhen lightning has struck a building (or other objects nearby) that was equipped with a properly installed lightning rod.
Unusual Types of Lightning
Most lightning strikes originate in the negative charge center in a thunderstorm as described above. Occasionally a lightning stroke will travel from the positive charge region in the top of the thunderstorm cloud to ground. Thistype of lightning is called "positive lightning" and accounts for about 5% of all cloud-to-ground lightning strikes. See thisfigure depicting a common cloud-to-ground lightning strike on the left and a positive cloud-to-groundlightning strike on the right. Positive strikes can happen when the postively charged top of a cumulonimbus cloud (called theanvil) moves away from the lower part of the cloud. There is now a region where positive charge sits above the ground surface. Thisresults in a negative induction charge on the ground as electrons are attracted to the positive charge in the cloud overhead andmove toward it. Positive strikes are most common near the end of storms when it is most common for the anvil to get pushed away fromthe middle and bottom portions of the cloud. Positive lightning is powerful and typically carries more current than normal cloud-to-groundlightning. They sometimes produce an unusually loud and long lasting clap of thunder. Positive strikes can happen when the postively charged top of a cumulonimbus cloud (called theanvil) moves away from the lower part of the cloud. There is now a region where positive charge sits above the ground surface. Thisresults in a negative induction charge on the ground as electrons are attracted to the positive charge in the cloud overhead andmove toward it. Positive strikes are most common near the end of storms when it is most common for the anvil to get pushed away fromthe middle and bottom portions of the cloud. Positive lightning is powerful and typically carries more current than normal cloud-to-groundlightning. They sometimes produce an unusually loud and long lasting clap of thunder ... the type that can rattle windows.
An even rarer form of lightning is known as "upward triggered lightning." In this case, the lightning starts at the groundand travels upward. Upward lightning is generally only initiated by mountains and tall objects such as a skyscraper or a tower.For example, the Empire State Building is struck many times every year by lightning and often it's lightning that was initiated by the building itself. Upward triggered lightning is initiated by an upward leader, which surges upwardfrom objects on the ground often forming upward directed branches as it moves toward the cloud. Picture of upward-triggered lightning initiated by a tall tower atop Mount San Salvatore, near Lugano, Switzarland.
Lightning is possible where sufficient charge separation and build-up occurs. While thunderstorms are the most common way forlightning to form, lightning has also been observed in dust storms and volcanic erruptions. The 2010 erruption of Eyjafjallajokull in Iceland has provided some interesting photographs of volcanic-induced lightning.
Thunder and Distance to a Lightning Strike
Lightning is powerful. A lot of energy is released during a lightningstrike. Here we will consider the three forms of energy listed below:
- Some of the energy is in the form of visible radiation, which is what wesee.
- Some of the energy goes into heating and expanding the air, whichgenerates sound waves that we hear as thunder.
- Other energy is releasedas longer wavelength radiation (that we cannot see) called radio waves.
One way to estimate how close lightning is to your location is to measurethe time between when you see the lightning strike and when you hear the thunder. The sound of thunder results from the heating of air withinthe lightning channel, which causes the air to expand outward, generating thesound we hear as thunder. Sound travels much slower than light, so you see lightning first,then hear the thunder. The speed of sound near the Earth's surface isabout 1 mile per 5 seconds. So for example, if you see lightning, thenhear the thunder 10 seconds later:
Distance to Lightning = (speed of sound) x (time to hear thunder)
Distance to Lightning = (1 mile / 5 sec) x (10 sec) = 2 miles
Lightning Safety
Lightning is a serious weather hazard. Here are some lightning safety rules that you should keep in mind.
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Stay away from tall isolated objects during a lightning storm, as these are most likely to be struck. You can be hurt or killed just by being close to a lightning strike even if you're not struck directly. Lightning currents often travel outward along the surface of the ground (or in water) rather than going straight down into the ground. Just being close to something struck by lightning puts you at risk. Atree may even explode when it is struckby lightning and the flying debris can cause injuries. This happens because the lightning current flows throughthe water-conducting sapwood below the bark, heating it, until the built up steam pressure causes thetrunk to burst.
Unlike a tornado, remaining in your car is generally a safe place to wait out a thunderstorm.An automobile with a metal roof and body provides good protection from lightning. The lightning current will travel through the metal and around the passengers inside. The rubber tires really don't play any role at all. You shouldn't use a corded phone or electrical appliances during a lightning storm because lightning currents can follow wires into your home. Cordless phones and cell phones are safe. It is also a good idea to stay away from plumbing as much as possible (don't take a shower during a lightning storm, for example). Vent pipes that are connected to the plumbing go up to the roof of the house which puts them in a perfect location to be struck.
Research studies have shown that about 95% of cloud to ground discharges strike the ground within 5 miles of a point directly below the center of the storm. That's a 10 mile diameter circle and covers the area of a medium size city.This means that 5% of cloud-to-ground lightning strikes happen more than 5 miles away from the center of the thunderstormthat produced the lightning. When this happens, it seems that the negative stepped leader branches out horizontally for some distance away from the cloud before making a turn toward the ground, eventually resulting in a surprise lightning strike. Most thunderstorm cells are smaller than a 5 mile radius, some much smaller. So it is possible for a cloud-to-ground lightningstrike to occur a considerable distance away from the parent storm, where most people would feel relatively safe.Thus the old adage "lightning struck out of the clear blue sky" actually does happen sometimes.
To best ensure your safety, the experts suggest following the 30/30 Rule.
- When you see lightning, count the seconds until you hear thunder. If this time is 30 seconds or less, quickly go inside a substantial building.If such a building is not available, a metal-topped vehicle is the next best choice.
- Wait at least 30 minutes after seeing the last lightning or hearing the last thunder before going back outside.
Learn to reduce your lightning risk through outdoor and home lightning safety education. A very good source of information about lightning safety and medical problems associated with being struck is provided on the National Weather Service Lightning Safety Page
Lightning Prediction and Detection
The liklihood that cloud to ground lighting is about to occur can be estimatedby measuring the electric field build up in the air. The Electric Fieldis a measure of the strength of the charge build up. When a certain thresholdis reached, lightning is about to hit somewhere (see figure). Electric field measuringequipment is used at many outdoor events (e.g., golf tournaments, state fairs, etc.)to allow authorities to give warning that people should take cover. Note that while the electric field instrumentation can provide information that tells uslightning is about to strike somewhere in the area, there is no way to predict exactly where the lightning will hit. In addition no warning system is 100% reliable.
Cloud-to-ground lightning thathas already occurred can be located using an instrument called a lightning direction-finder, which works by detecting the radio waves produced by lightning. Radio waves are a type of longwave raditation. This type of radiationis able to travel long distances along the surface of the Earth. Specialized magneticdevices detect the signature radio waves generated by lightning.A network of these magnetic deviceshas been set up all around the United States as part of the National Lightning Detection Network. The location of each cloudto ground strike is pinpointed using triangulationby noting the the direction from which the radio waves arrive and the time the signal was detected (see figure). This informationis desplayed on maps showing the time and location of all detected cloud to ground lightning strikes This is valuable in showing the general motion of lightning producing storms and the density of lightning strikes, butit can not be used to predict when and where a newly developing storm willfirst produce lightning.Recent cloud to ground lightning displays. The previous link will notwork if you are connected from off campus.
