Lightning is one of the oldest observed natural phenomena on earth. At the same time, it also is one of the least understood. While lightning is simply a gigantic spark of static electricity (the same kind of electricity that sometimes shocks you when you touch a doorknob), scientists do not have a complete grasp on how it works, or how it interacts with solar flares impacting the upper atmosphere or the earth's electromagnetic field.
Lightning has been seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, and in large hurricanes. However, it is most often seen in thunderstorms. In fact, lightning (and the thunder that results) is what makes a thunderstorms.
At any given moment, there can be as many as 2,000 thunderstorms occurring across the globe. This translates to more than 14.5 million storms each year. NASA satellite research indicated these storms produce lightning flashes about 40 times a second worldwide.
This is a change from the commonly accepted value of 100 flashes per second which was an estimate from 1925. Whether it is 40, 100, or somewhere in between, we live on an electrified planet.
How Lightning is Created
The conditions needed to produce lightning have been known for some time. However, exactly how lightning forms has never been verified so there is room for debate. Leading theories focus around separation of electric charge and generation of an electric field within a thunderstorm. Recent studies also indicate that ice, hail, and semi-frozen water drops known as graupel are essential to lightning development. Storms that fail to produce large quantities of ice usually fail to produce lightning.
Forecasting when and where lightning will strike is not yet possible and most likely never will be. But by educating yourself about lightning and learning some basic safety rules, you, your family, and your friends can avoid needless exposure to the dangers of one of the most capricious and unpredictable forces of nature.
This image shows the global average annual occurrence of lightning at a resolution of ½° by ½°. (Very large 300 dpi version - 1440k) The data was obtained from the following two space-based sensors: The Lightning Imaging Sensor (LIS), is used to detect the distribution and variability of total lightning (cloud-to-cloud, intracloud, and cloud-to-ground lightning) that occurs in the tropical regions of the globe. The LIS is a science instrument aboard the TRMM Observatory, which was launched on November 28, 1997 from the Tanegashima Space Center in Japan. The Optical Transient Detector (OTD) is a solid-state optical sensor similar in some ways to a TV camera. It is uniquely designed for the job of observing and measuring lightning from space.
Thunderstorms have very turbulent environments. Strong updrafts and downdrafts occur with regularity and within close proximity to each other. The updrafts transport small liquid water droplets from the lower regions of the storm to heights between 35,000 and 70,000 feet, miles above the freezing level.
Meanwhile, downdrafts transport hail and ice from the frozen upper regions of the storm. When these collide, the water droplets freeze and release heat. This heat in turn keeps the surface of the hail and ice slightly warmer than its surrounding environment, and a "soft hail", or "graupel" forms.
When this graupel collides with additional water droplets and ice particles, a critical phenomenon occurs: Electrons are sheared off of the ascending particles and collect on the descending particles. Because electrons carry a negative charge, the result is a storm cloud with a negatively charged base and a positively charged top.
The electric field within a thunderstormIn the world of electricity, opposites attract and insulators inhibit. As positive and negative charges begin to separate within the cloud, an electric field is generated between its top and base. Further separation of these charges into pools of positive and negative regions results in a strengthening of the electric field.
However, the atmosphere is a very good insulator that inhibits electric flow, so a TREMENDOUS amount of charge has to build up before lightning can occur. When that charge threshold is reached, the strength of the electric field overpowers the atmosphere's insulating properties, and lightning results.
The electric field within the storm is not the only one that develops. Below the negatively charged storm base, positive charge begins to pool within the surface of the earth (see image left). This positive charge will shadow the storm wherever it goes, and is responsible for cloud-to-ground lightning. However, the electric field within the storm is much stronger than the one between the storm base and the earth's surface, so most lightning (~75-80%) occurs within the storm cloud itself.
How Lightning Develops Between The Cloud And The Ground
A moving thunderstorm gathers another pool of positively charged particles along the ground that travel with the storm (image 1). As the differences in charges continue to increase, positively charged particles rise up taller objects such as trees, houses, and telephone poles.
A channel of negative charge, called a "stepped leader" will descend from the bottom of the storm toward the ground (image 2). It is invisible to the human eye, and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink your eye. As the negative leader approaches the ground, positive charge collects in the ground and in objects on the ground.
This positive charge "reaches" out to the approaching negative charge with its own channel, called a "streamer" (image 3). When these channels connect, the resulting electrical transfer is what we see as lightning. After the initial lightning stroke, if enough charge is leftover, additional lightning strokes will use the same channel and will give the bolt its flickering appearance.
Tall objects such as trees and skyscrapers are commonly struck by lightning. Mountains also make good targets. The reason for this is their tops are closer to the base of the storm cloud. Remember, the atmosphere is a good electrical insulator. The less insulation the lightning has to burn through, the easier it is for it to strike. However, this does not always mean tall objects will be struck. It all depends on where the charges accumulate. Lighting can strike the ground in an open field the even if the tree line is close by.
The Lightning Process: Keeping in Step
Lightning can be divided into two types:
- Flashes with at least one channel connecting the cloud to the ground, known as "cloud-to-ground" discharges (CG); and
- Flashes with no channel to ground, known as "in-cloud" (IC), "cloud-to-cloud" (CC), or "cloud-to-air" (CA).
The lightning process is more or less the same for both types.
The stepped leaderA typical CG lightning strike initiates inside the storm. Under the influences of the electric field between the cloud and the ground, a very faint, negatively charged channel called a "stepped leader" emerges from the storm base and propagates toward the ground in a series of steps about 50 meters (160 feet) in length and 1 microsecond (0.000001 seconds) in duration.
In what can be loosely described as an "avalanche of electrons", the stepped leader usually branches out in many directions as it approaches the ground, carrying an EXTREMELY strong electric potential: about 100 MILLION volts with respect to the ground and about 5 coulombs of negative charge.
Between each step there is a pause of about 50 microseconds, during which the stepped leader "looks" around for an object to strike. If none is "seen", it takes another step, and repeats the process until it "finds" a target.
It takes the stepped leader about 50 milliseconds (1/20th of a second) to reach its full length, though this number varies depending on the length of its path. Studies of individual strikes have shown that a single leader can be comprised of more than 10,000 steps!
Stepped leader inducing streamers As the stepped leader approaches the ground, its strong, negative charge repels all negative charge within the immediate strike zone of the earth's surface, while attracting vast amounts of positive charge. The influx of positive charge into the strike zone is so strong that the stepped leader actually induces electric channels up from the ground known as "streamers".
When one of these positively charged streamers connects with a negatively charged stepped leader (anywhere from 100 to 300 feet (30 to 100 meters) above the surface of the earth), the following steps occur in less than 100 microseconds.
Connection with the ground is made The electric potential of the stepped leader is connected to the ground and the negative charge starts flowing DOWN the established channel.
The return stroke, what we see when lightning flashes An electric current wave, called a "return stroke", shoots UP the channel as a brilliant pulse. Behind the wave front, electric charge flows up the channel and produces a ground current. It takes the current about 1 microsecond to reach its peak value, which averages around 30,000 amperes.
The "return stroke" produces more than 99% of a lightning bolt's luminosity and is what we see as lightning. The stroke actually travels FROM the ground INTO the cloud, but because the strike takes place so quickly, to the unaided eye is appears the opposite is true.
Dart leader generally uses the same channel created by the stepped leaderAfter the return stroke ceases flowing up the channel, there is a pause of about 20 to 50 milliseconds. After that, if enough charge is still available within the cloud, another leader can propagate down to the ground. This leader is called a "dart leader" because it uses the channel already established by the stepped leader and therefore has a continuous path.
Dart leaders give lightning its flickering appearance and normally are not branched like the initial stepped leader. Go to the lightning stroke animationNot every lightning flash will produce a dart leader because sufficient charge to initiate one must be available within about 100 milliseconds of the initial stepped leader. Click on the image at right to see a lightning stroke animation.
The dart leader carries additional electric potential to the ground and induces a new streamer from the ground. The dart leader's peak current is usually less than the initial stepped leader and its return stroke has a shorter duration than the initial return stroke. As additional dart leaders are produced, their peak currents and return stroke durations continue to decrease.
Dart leaders and their return strokes don't necessarily have to use the same cloud-to-ground channel that was burned by initial stepped leader. If a dart leader takes a different path to the ground, the lightning will appear to dance from one spot to another. This is known as "forked lightning".
The Positive and Negative Side of Lightning
The previous section describes what is called "negative lightning", because there is the transfer of negative charge from the cloud to the ground. However, not all lightning forms in the negatively charged region under the thunderstorm base.
Some lightning originates in the cirrus anvil or upper parts near the top of the thunderstorm, where a high positive charge resides. Lightning that forms in this region follows the same scenario as previously described, but the descending stepped leader will carry a positive charge while its subsequent ground streamers will have a negative charge. These bolts are known as "positive lightning" because there is a net transfer of positive charge from the cloud to the ground.
Positive lightning makes up less than 5% of all strikes. However, despite a significantly lower rate of occurrence, positive lightning is particularly dangerous for several reasons. Since it originates in the upper levels of a storm, the amount of air it must burn through to reach the ground usually much greater. Therefore, its electric field typically is much stronger than a negative strike. Its flash duration is longer, and its peak charge and potential can be ten times greater than a negative strike; as much as 300,000 amperes and one billion volts!
Some positive strikes can occur within the parent thunderstorm and strike the ground beneath the cloud. However, many positive strikes occur near the edge of the cloud or strike more than 10 miles away, where you may not perceive any risk nor hear any thunder.
Also, positive flashes are believed to be responsible for a large percentage of forest fires and power line damage. Thus, positive lightning is much more lethal and causes greater damage than negative lightning. Some interesting properties of positive lightning:
Bolt from the Blue photo, copyright by Al MollerPositive lightning can be the dominate type of cloud-to-ground during the winter months and in the dissipating stage of a thunderstorm.
- Positive lightning has been identified as a major source for the recently discovered sprites and elves. Sprites and elves are most likely lightning discharges but occur from 18-60 miles (30-95 km) in altitude, well above the parent thunderstorm.
- Positive lightning is usually composed of one stroke (negative lightning typically consists of two or more strokes)
Finally, there is bipolar lightning, lightning that actually changes its polarity (positive becoming negative or vice versa). It is no less dangerous than any other type of lightning but shows that we live on a complex planet with many aspects we do not fully understand.
The Sound of Thunder
Regardless of whether lightning is positive or negative, thunder is produced the same way. Thunder is the acoustic shock wave resulting from the extreme heat generated by a lightning flash. Lightning can be as hot as 54,000°F (30,000°C), a temperature that is five times the surface of the sun! When lightning occurs, it heats the air surrounding its channel to that same incredible temperature in a fraction of a second.
Like all gases, when air molecules are heated, they expand. The faster they are heated, the faster their rate of expansion. But when air is heated to 54,000°F in a fraction of a second, a phenomenon known as "explosive expansion" occurs. This is where air expands so rapidly that it compresses the air in front of it, forming a shock wave similar to a sonic boom. Exploding fireworks produce a similar result.
When lightning strikes a shock wave is generated at each point along the path of the lightning bolt. (The above illustrations show only four points.)
|With nearby lightning strikes the thunder will sound like a loud bang, crack or snap and its duration will be very short.||As the shock wave propagates away from the strike center, it stretches, diminishes, and becomes elongated. Then other shock waves from more distance locations arrive at the listener.||At large distances from the center, the shock wave (thunder) can be many miles across. To the listener, the combination of shock waves gives thunder the continuous rumble we hear.|
In addition, the temperature of the atmosphere affects the thunder sound you hear as well as how far away you can hear it. Sound waves move faster in warm air than they do in cool air. Typically, the air temperature decreases with height. When this occurs, thunder will normally have an audible range up to 10 miles (16 km).
However, when the air temperature increases with height, called an inversion, sound waves are refracted (bent back toward the earth) as they move due to their faster motion in the warmer air. Normally, only the direct sound of thunder is heard. But refraction can add some additional sound, effectively amplifying the thunder and making it sound louder.
Graphic showing how refraction of sound leads to louder thunderThis is more common in the winter as thunderstorms develop in the warm air above a cooler surface air mass. If the lightning in these "elevated thunderstorms" remains above the inversion, then most of the thunder sound also remains above the inversion. However, much of the sound waves from cloud-to-ground strikes remain below the inversion giving thunder a much louder impact.
Lightning is the most underrated weather hazard. On average, only floods kill more people. Lightning makes every single thunderstorm a potential killer, whether the storm produces one single bolt or ten thousand bolts.
In the United States, lightning routinely kills more people each year than tornadoes or hurricanes. Tornadoes, hail, and wind gusts get the most attention, but only lightning can strike outside the storm itself. Lightning is the first thunderstorm hazard to arrive and the last to leave.
Lightning is one of the most capricious and unpredictable characteristics of a thunderstorm. Because of this, no one can guarantee an individual or group absolute protection from lightning. However, knowing and following proven lightning safety guidelines can greatly reduce the risk of injury or death. Remember, You are ultimately responsible for your personal safety, and should take appropriate action when threatened by lightning.
While no place is 100% safe from lightning, some places are much safer than others.
Where to Go
The safest location during a thunderstorm is inside a large enclosed structure with plumbing and electrical wiring. These include shopping centers, schools, office buildings, and private residences.
If lightning strikes the building, the plumbing and wiring will conduct the electricity more efficiently than a human body. If no buildings are available, then an enclosed metal vehicle such as an automobile, van, or school bus makes a decent alternative.
Where NOT to Go
Not all types of buildings or vehicles are safe during thunderstorms. Buildings which are not safe (even if they are "grounded") have exposed openings. These include beach shacks, metal sheds, picnic shelters/pavilions, carports, and baseball dugouts. Porches are dangerous as well.
Convertible vehicles offer no safety from lightning, even if the top is "up". Other vehicles which are not safe during lightning storms are those which have open cabs, such as golf carts, tractors, and construction equipment.
What To Do
Once inside a sturdy building, stay away from electrical appliances and plumbing fixtures. As an added safety measure, stay in an interior room.
If you are inside a vehicle, roll the windows up, and avoid contact with any conducting paths leading to the outside of the vehicle (e.g. radios, CB's, ignition, etc.).
What NOT to Do
Lightning can travel great distances through power lines, especially in rural areas. Do not use electrical appliances, especially corded telephones unless it is an emergency (cordless and cell phones are safe to use).
Computers are also dangerous as they usually are connected to both phone and electrical cords. Do not take a shower or bath or use a hot tub.
Lightning Safety Plan
A lightning safety plan should be an integral part of the planning process for any outdoor event. Do not wait for storm clouds to develop before considering what to do should lightning threaten! An effective plan begins LONG before any lightning threat is realized. You can't control the weather, so you have to work around it!
Detailed weather forecasts are accurate only out to seven days at best, but outdoor events often are planned many months in advance. Because of this limitation, every outdoor event coordinator should consider the possibility of lightning, especially if the event is scheduled during the late spring to early autumn months. The key to an effective lightning safety action plan lies in your answers to the following questions:
- Where is the safest lightning shelter?
- How far am I (or the group I am responsible for) from that location?
- How long will it take me (or my group) to get there?
Knowing the answers to these questions will greatly reduce your chances of being struck by lightning, provided you know them BEFORE thunderstorms threaten! When Thunder Roars, Go Indoors
Studies have shown most people struck by lightning are struck not at the height of a thunderstorm, but before and after the storm has peaked. This is because lightning can strike as far as 10 miles from the area where it is raining and many people are unaware of how far lightning can strike from its parent thunderstorm.
Therefore, if you can hear thunder, you are within striking distance. Seek safe shelter immediately. Remember this lightning safety rule...When thunder roars, go indoors and stay there until 30 minutes after the last clap of thunder. DO NOT wait for the rain to start before seeking shelter, and do not leave shelter just because the rain has ended.
With common sense, you can greatly increase your safety and the safety of those you are with. At the first clap of thunder, go to a large building or fully enclosed vehicle and wait 30 minutes after the last clap of thunder before you to go back outside.
Plan Ahead! Make sure you get the latest weather forecast before going out.
Carry a NOAA weather radio (found at most electronics stores in the United States) or a portable radio with you, especially if you will be away from sturdy shelter (such as boating, camping, etc.). This way you will always be able to get the latest forecast. At the very least, the reception of an AM radio will have static created by lightning. So if you hear the static, keep an eye to the sky as a thunderstorm may be nearby.
If thunderstorms are expected and you go ahead with your planned outdoor activity, have a lightning safety plan in place. Upon arriving on-site, determine how far away your shelter is in case lightning threatens. Remember to account for the time it will require to get to your safe location. If storms threaten or the sky begins to darken, monitor the sky for lightning.
If lightning is seen and the time delay to its subsequent thunder is 30 seconds or less, or if thunderclouds are building overhead, implement your lightning safety action plan without delay!
Remember the "Flash to Bang" method to estimate lightning from your location - If you see lightning, count the number of seconds until you hear thunder. Divide the number of seconds by five to get the distance the lightning is away from you. For example, if you see lightning and it takes 10 seconds before you hear the thunder, then the lightning is 2 miles away from you (10 divided by 5 = 2 miles, too close!).
Do not resume outdoor activities until 30 minutes after the last thunder clap.
For Small Groups
Plan Ahead! Make sure someone in the group gets the weather forecast before going out and make your lightning safety action plan known by all members in the group.
Designate one of the members to monitor NOAA weather radio or a portable radio. This way you will always be able to get the latest forecast. If you have a wireless device that is internet capable, you can also obtain that information. If your wireless device can also display graphics, you can also view the local NWS Doppler radar to determine location of thunderstorms. The address for Anywhere/Anytime Weather from the NWS is www.srh.weather.gov.
If thunderstorms are expected and you go ahead with your planned outdoor activity, have a lightning safety plan in place. Upon arriving on-site, determine how far away your shelter is in case lightning threatens. Remember to account for the time it will require to get to your safe location. If storms threaten or the sky begins to darken, make sure someone in the group continuously monitors the sky for lightning and listening for thunder.
As soon as you hear thunder, seek safe shelter immediately. Do not wait. The group should implement the lightning safety action plan without delay! You are in danger of being struck by lighting. Do not resume outdoor activities until 30 minutes after the last thunder clap.
For Large Groups
Plan Ahead! Make sure the event organizers responsible for safety get a good weather forecast before the event begins and make your lightning safety action plan known and used by all event organizers.
Safety organizers should monitor NOAA weather radio (found at most electronics stores), a portable radio, or local cable, radio or TV broadcasts.
Since it may take considerable time to evacuate people to a safe location, personal observation of the lightning threat may not be adequate, especially for fast moving lightning storms. Hand held or portable lightning detectors should be made available so that lightning can be observed significant distances from the event site. Event organizers should know how long it will take to get people to safe shelter.
With large groups of people, safe locations must be identified beforehand, along with a means to route people to these locations. Event organizers might consider placing lightning safety tips on programs, score cards, etc. Lightning safety placards set up in strategic locations can be an effective means of raising awareness and communicating the lightning threat to the attending audience.
Frequently Asked Questions about Lightning
What is the difference between a thundershower and a thunderstorm?
Technically, there is none. In general, the term "thundershower" tends to denote a fairly weak storm with light to moderate rainfall and low levels of lightning activity. However, there are no defined parameters that distinguish between a thundershower and a thunderstorm. In fact, in order to avoid confusion, we in the National Weather Service do not use the term "thundershower". If a shower is strong enough to produce lightning, even just one single bolt, it's called a thunderstorm.
What are my chances of being struck by lightning?
This is a seemingly simple question, but there is no single answer that fits everyone. The odds of being struck vary from person to person because they depend on several factors. The most significant are:
- Geographical location and climatology
- Diurnal and annual climatology
- Personal lifestyle/hobbies
Where there is a lot of lightning, there is an increased chance of being struck. The central Florida peninsula from Tampa Bay to Cape Canaveral has the highest lightning concentration in the United States. More than 90% of the lightning in this area occurs between May and October, between the hours of noon and midnight.
During this time of day and year, people in Central Florida who spend a large portion of their lives outdoors (e.g. construction workers, park rangers, golfers, campers etc.) are more likely to be struck than anywhere else in the country. On the other hand, thunderstorms are uncommon in the Pacific northwest, and are virtually unheard of during the winter months. People in this region who spend much of their lives indoors (e.g. shopkeepers, librarians, bowlers, billiard players, etc.) might win the lottery before they were struck by lightning. It is impossible to assign one single probability to every person in every situation.
|Odds of Becoming a Lightning Victim|
|U.S. 2000 Census population as of 2008||300,000,000|
|Number of deaths actually reported: 60 Number of injuries reported: 340||(total) 400|
|Estimated number of actual U.S. deaths: 60 Estimated number of actual injuries: 540||(total) 600|
|Odds of being struck by lightning in a given year (reported deaths + injuries)||1/750,000|
|Odds of being struck by lightning in a given year (estimated total deaths + injuries)||1/500,000|
|Odds of being struck in your lifetime (Est. 80 years)||1/6250|
|Odds you will be affected by someone being struck (Ten people affected for every one struck)||1/625|
What should I do if I'm caught out in the open during a thunderstorm and no shelter is nearby?
There are no safe place outdoors during a lightning storm. Don't kid yourself--you are NOT safe outside. Following these tips will not prevent you from being struck by lightning, but may slightly lessen the odds.
If camping, hiking, etc., far from a safe vehicle or building, avoid open fields, the top of a hill or a ridge top. Keep your site away from tall, isolated trees or other tall objects. If you are in a forest, stay near a lower stand of trees. If you are camping in an open area, set up camp in a valley, ravine or other low area. Remember, a tent offers NO protection from lighting. If you are camping and your vehicle is nearby, run to it before the storm arrives.
Stay away from water, wet items such as ropes and metal objects, such as fences and poles. Water and metal are excellent conductors of electricity. The current from a lightning flash will easily travel for long distances.
Shouldn't I lie flat on the ground to get as low as possible?
NO! Lying flat on the ground was once thought to be the best course of action, but this advice is now decades out of date. When lightning strikes the earth, it induces currents in the ground that can be fatal up to 100 feet away. These currents fan out from the strike center in a tendril pattern, so in order to minimize your chance of being struck, you have to minimize BOTH your height AND your body's contact with the earth's surface.
How do I avoid having to use the Lightning Crouch?
Be aware of your situation and plan ahead. If you going to be involved in an outdoor activity, make sure you know what the forecast is, ESPECIALLY if you live in a lightning prone area. If storms are forecast, have a plan of action that you can enact quickly to reduce your chances of being struck.
Does lightning travel from the cloud to the ground, or from the ground to the cloud?
An entire lightning strike employs both upward and downward moving forces. However, the return stroke of a lightning bolt travels from the ground to the cloud and accounts for more that 99% of the luminosity of a lightning strike. What we see as lightning does indeed travel from the ground into the cloud.
If lightning travels from the ground into the cloud, why do photographs show branches of lightning descending from the cloud?
In photographs, it may appear that lightning is descending from the cloud to the ground, but in reality, the return stroke is so brilliant that as it travels up the strike channel, it illuminates all of the branches of the stepped leader that did not connect with a streamer.
How far can lightning strike?
Almost all lightning will occur within 10 miles of its parent thunderstorm, but it CAN strike much farther than that. Lightning detection equipment has confirmed bolts striking almost 50 miles away, but these are very rare.
Can lightning strike me while I'm indoors?
Yes! If a bolt strikes your house or a nearby power line, it CAN travel into your house through the plumbing or the electric wiring! If you are using any electrical appliances or plumbing fixtures (INCLUDING telephones and computers), and a storm is overhead, you are putting yourself at risk! FACT: About 4-5% of people struck by lightning are struck while talking on a corded telephone.
Can I use my cell phone or cordless phone during a storm?
Yes. These are safe to use because there is no direct path between you and the lightning. Avoid using a corded telephone unless it's an emergency.
Can I be struck by lightning if I wear rubber soled shoes?
Absolutely! While rubber is an electric insulator, it's only effective to a certain point. The average lightning bolt carries about 30,000 amps of charge, has 100 million volts of electric potential, and is about 50,000°F. These amounts are several orders of magnitude HIGHER than what humans use on a daily basis and can burn through ANY insulator (even the ceramic insulators on power lines!) Besides, the lightning bolt may just have traveled many miles through the atmosphere, which is a good insulator. Your ½" (or less) of rubber will make no difference.
I have a boat with a tall mast. How can I protect myself from lightning?
Install a lightning protection system and make sure your insurance covers lightning damage. Lightning tends to strike the tallest object and boats on the open water fit this profile to a tee. Lightning protection systems WILL NOT prevent your boat from being struck, but they can help minimize the damage.
I have a lightning rod on my house. Am I safe from lightning?
Lightning rods will not prevent your building from being struck. They actually INCREASE it by making your house TALLER. The purpose of the lightning rod is to direct the current from the lightning to the ground along a preferred path instead of to the house. However, this works only if the rod is connected to the ground with heavy gauge wire.
Should I install a lightning protection system on my home or business?
It depends. Do you have electrically sensitive equipment and do you think your building may be struck? Contrary to some popular beliefs, lightning protection systems DO NOT prevent lightning. Instead, they actually bank on the assumption that your building will be struck. They help mitigate damage by giving the lightning a preferred pathway into the ground, not unlike a flood spillway system.
Image by André Karwath