Introduction
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),
Annual Flash Rate Maps (NASA) 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.
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.
Annual Flash Rate Maps (NASA)
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.
Charge Separation
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.
Field Generation
In the world of electricity, opposites attract and insulators inhibit. As the charges separate within the cloud, an
electric field is generated between its top and base. The greater the magnitude of separation, the stronger the
field, and the stronger the attraction between the charges.
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 right). 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 even if the tree line is close by.
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:
Positive 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 understan
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.
1. 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.)
2. With nearby lightning strikes the thunder will sound like a loud bang, crack or snap and its duration will be very short.
3. 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.
4. 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.
This 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 - Frequently Asked Questions
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 rainshower 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 average annual per capita strike rate in the United States is around 1 in 600,000. However, this DOES NOT mean your odds of being struck are 1 in 600,000. 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.
What should I do if I'm caught out in the open during a thunderstorm and no shelter is nearby?
There are NO SAFE PLACES outdoors during a lightning storm. To minimize your chance of being struck, crouch down on the balls of your feet (a position known as "The Lightning Crouch") and cover your ears. The Lightning Crouch will minimize your chances of being struck, but it WILL NOT guarantee your safety.
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 INTO 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 they lightning a preferred pathway into the ground, not unlike a flood spillway system.
Simson College (IA)