- Lesson on Lightning
-
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.
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.
 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.
 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.
 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 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.
Take it to the MAX! The
Lightning Process: Keeping in Step
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 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 understand.
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 and hurricanes COMBINED. 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 |
Where NOT 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. |
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 |
What NOT 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.). |
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!
The 30/30 rule
 Any
lightning safety plan should incorporate the 30/30 Rule. The 30/30 Rule
states that people should seek shelter if the "Flash-To-Bang"
delay (length of time in seconds between a lightning flash and its
subsequent thunder), is 30 seconds or less, and that they remain under
cover until 30 minutes after the final clap of thunder.
A 30 second lead time is necessary prior to a storm's arrival because of
the possibility of distant strikes. A 30 minute wait after the last
thunder is heard is necessary because the trailing storm clouds still
carry a lingering charge. This charge can and does occasionally produce
lightning on the back edge of a storm, several minutes after the rain has
ended.
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
shows many people are unaware of how far lightning can strike from its
parent thunderstorm. DO NOT wait for the rain to start before seeking
shelter, and do not leave shelter just because the rain has ended.
Safety Guidelines
For YOU!
 Plan
Ahead! Make sure you get the latest weather forecast before going out. Get
it here!
Carry a NOAA
weather radio (found at most electronics stores) 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.
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.
If lightning is seen and the time delay to its subsequent thunder is 30
seconds or less, or if thunderclouds are building overhead, the group
should implement the 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. 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.
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.
Remember the time needed for proper evacuation increases proportionally to
the number of people involved. As evacuation time increases, the distance
at which lightning is considered a threat must also increase. It is
suggested that people be in safe shelter when an approaching storm is six
miles from the event location. The event organizers should decide how
close the lightning can get before warning "alarms" are issued.
This will depend on storm motion, direction, and the time it will take to
get people into safe shelters.
Example:
You are a safety coordinator for an outdoor high
school reunion. Your weather monitor has been listening to the
NOAA Weather Radio and observing the local NWS Doppler Radar loop
on a laptop computer for the past hour.
You are informed of a storm 30 miles west of your location, moving
east at 30 mph. You know it will take approximately 10 minutes to
get people into a safe location. At what point should issue the
"lightning alarm"?
First, you know that people should already be in safe shelter when
lightning is within 6 miles of the event site. If the storm is
moving east at 30 mph, then it is moving half a mile per minute.
In the 10 minutes it will take to reach safe shelter, the storm
will move 5 miles [1/2 (mile/min) x 10 min = 5 miles].
Based on the storm motion and the time it will take people to get
to shelter, the warning should be issued when the storm is 11
miles (5 miles + 6 miles = 11 miles) away from the event site. In
this circumstance, if the storm maintains its strength and motion,
then the warning should be issued in about 40 minutes. |
|
- What is the difference between a thundershower
and a thunderstorm?
- What are my chances of being struck by
lightning?
- What should I do if I'm caught out in the open
during a thunderstorm and no shelter is nearby?
- Shouldn't I lie flat on the ground to get as
low as possible?
- How do I avoid having to use the Lightning
Crouch?
- Does lightning travel from the cloud to the
ground, or from the ground to the cloud?
- If lightning travels from the ground into the
cloud, why do photographs show branches of lightning descending
from the cloud?
- How far can lightning strike?
- Can lightning strike me while I'm indoors?
- Can I use my cell phone or cordless phone
during a storm?
- Can I be struck by lightning if I wear rubber
soled shoes?
- I have a boat with a tall mast. How can I
protect myself from lightning?
- I have a lightning rod on my house. Am I safe
from lightning?
- Should I install a lightning protection
system on my home or business?
|
- 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 rain shower is strong enough to produce lightning, even just one single
bolt, it's called a thunderstorm. Top
 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. Top
 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. Top
 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. Top
- 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. Top
- 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. Top
- 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. Top
 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. Top
- 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. Top
- 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. Top
 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. Top
- 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. Top
 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. Top
- 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. Top
|
More
about thunderstorm development
|
weather.gov 
){kind=link}
){kind=link}
){kind=link}
){kind=link}