Frequently
Asked
Questions
About
Lightning
Does lightning travel from the ground up?
There are two ways of looking at
lightning's direction:
-
Upward streamers: Lightning starts in a cloud when there is an updraft
reaching subfreezing temperatures. A cloud-to-ground flash starts at
around 15,000 feet during summer in the US. It travels toward the ground
in 50-yard steps - this is the downward "leader". At the ground, there
are small, upward "streamers" reaching upwards toward the electrified
cloud. When the downward leader connects with the upward streamer, the
bright light that we call lightning is visible, and moves upward. The
way to think of it is that charge is lowered but current flows upward.
This happens in a split second, so it's difficult to see.
-
True ground-to-cloud lightning: In some cases, the cloud area where
lightning usually originates (15,000+ feet high) can be so low, like in
a winter storm, that very tall natural (mountains) or man-made objects
(towers or skyscrapers) sort of interfere with the initial charge moving
downward toward the ground. Instead, the very tall object actually
triggers the lightning, and the strike with its steps, forks, and
branches truly appears to go upward. This is an unusual lightning event
that we don't see every day, but there are photos that document it.
Can lightning be harnessed as an alternate power source?
Capturing the energy of lightning is intriguing, but not
yet feasible for several reasons. First, no one knows exactly where to
collect lightning as it does not naturally strike specific places more
than a few times a year. Next, conventional storage devices for
electricity--like a battery--do not accommodate the magnitude and duration
of a powerful flash that lasts only fractions of a second. Finally, about
half of the energy of a lightning strike dissipates into light, acoustic
energy, and electromagnetic waves. There are ongoing studies to explore
the use of laser beams to control lightning. It appears possible that a
laser beam can be sent into a cloud that's about ready to generate
lightning to "trigger" lightning--and possibly collect its energy in an
appropriate storage device.
How many people are struck by lightning?
In the United States, lightning kills 75 to 100 people each year.
Lightning safety experts estimate that there are 10 times as many
lightning-related injuries as deaths, totaling several hundred to a
thousand injuries each year. Some of these injures are reported to
authorities; many are not. To find out more, visit the National Weather
Service Natural Hazard Statistics or the Lightning Strike and Electric
Shock Survivors International.
Is it safe to use the telephone during thunderstorms?
Corded
telephones are not safe and should not be used during thunderstorms. The
usual way that current enters a telephone is through the wire.
Cloud-to-ground flashes tend to hit tall objects such as utility poles.
When a pole is struck, its current enters a building through the wiring,
then to the phone, and then straight to your head. Cell phones and
cordless phones are safer, but be sure to stand away from the cordless
phone's base as a strong current can possibly arc a few
feet from the base to the handset.
There is still a risk of ear damage from loud static and "pops" associated
with cell phone and cordless phone use during thunderstorms.
Do areas with underground utilities and underground irrigation systems
attract more lightning strikes?
No.
Cloud-to-ground lightning travels downward from 15,000 to 20,000 feet high
in a typical summer thunderstorm cloud. When a cloud-to-ground flash comes
to ground, it does not "pick" its target until it is within about 30 yards
of the ground. The University of Florida has an ongoing study of the
impact of lightning on underground utilities.
How can I protect my home computer, TV, and other electronics from
lightning?
Lightning
damage to home electronics usually occurs when lightning strikes to nearby
utility poles or wires, then enters the building through power, phone, and
TV wires. For direct or indirect hits, the only sure way is to pull the
power, phone, and cable plugs on sensitive electronics
before
thunderstorms threaten. Never touch
wires during a thunderstorm, even to unplug your equipment. People have
been electrocuted while unplugging their electronics during thunderstorms.
Better your computer than you!
What are "positive" and "negative" strikes?
The
sign--or polarity--of a lightning strike depends on what type of charge is
lowered to ground. A negative cloud-to-ground flash means that negative
charge is lowered to ground. In this case, excess negative charge occurred
aloft in a thunderstorm, often at about 20,000 feet above sea level, and
negative charge was brought down to the ground by the flash to try to
equalize the electrical imbalance. Usually, the layering of charge is such
that negative charge builds up closer to the ground than positive charge,
so negative charge lowers more readily to the earth's surface. About 95%
of all cloud-to-ground flashes worldwide are negative. In the case of
positive charge, the positive charge is brought to ground, but this
usually happens near the end of storms, in winter storms, and on the edge
of large thunderstorm complexes. Although less frequent, positive strokes
generally contain more current and are more apt to catch more people by
surprise.
Should I keep all windows in my home closed to
keep lightning from traveling into my house?
It
doesn't make much difference if windows are open or closed when lightning
is occurring. It's the window's frame that matters when it comes to
lightning. If the window frame is metal, lightning current can easily
travel through it and can injure a person touching it. So, it is a bad
idea to be touching or closing them during a thunderstorm. Of course,
lightning is Mother Nature's warning sign of severe weather. Lightning may
not come through your window, but during a thunderstorm's high winds or
microbursts, a patio chair can. So, stay away from windows. Also, it used
to be recommended that windows should be opened to "equalize pressure"
when a tornado is approaching, this has been proven wrong. Close your
windows and doors at the first signs of approaching thunderstorms.
Are headsets safe to use during
thunderstorms?
No,
is not safe to use any devices that have can conduct electricity from
outside the building to inside the building. That includes telephone
headsets, stereo headsets, TV video games, and your computer's keyboard.
Wiring is made to conduct electricity to you, including lightning's strong
electrical current.
Can we use the energy in lightning as an alternate power source?
Capturing
the energy of lightning is intriguing, but not yet feasible for several
reasons. First, no one knows exactly where to collect lightning as it does
not naturally strike specific places more than a few times a year. Next,
conventional storage devices for electricity--like a battery--do not
accommodate the magnitude and duration of a powerful flash that lasts only
fractions of a second. Finally, about half of the energy of a lightning
strike dissipates into light, acoustic energy, and electromagnetic waves.
There are ongoing studies to explore the use of laser beams to control
lightning. It appears possible that a laser beam can be sent into an
electrified cloud to trigger lightning which could then be, in theory,
captured.
Is marine life killed or injured when lightning strikes in an ocean?
When
lightning hits open water, the current is able to travel through the water
quite well. How far away the dangerous current travels is not known, but
it's probably on the order of tens of yards. At and very near the strike
point--within a few yards--current enters the body of a living
organization, resulting in injury or death. At greater distances, the
effects are less, so that the current might stun an organism but not kill
it. There are few reports of massive fish kills. It may be that there are
not too many fish in any single volume in the affected water, they may
recover quickly, or maybe larger fish eat the incapacitated fish.
Why is knowing where lightning strikes important?
At
any time, 2,000 thunderstorms are occurring around the world, producing
lightning that strikes the earth up to 100 times every second. Every time
it reaches the earth's surface, it makes an enormous impact, causing power
and telecommunications outages, forest and building fires, damage to
aircraft and sensitive electronics, and deaths. The better we can pinpoint
lightning, the better we can reduce its negative impact. We know that over
20 million flashes strike the ground in the United States every year. And
most of them hit more than one point on the ground. While the problems
caused by lightning often happen to one person or to one building at a
time, it adds up to cause as much damage and as many casualties as any
other storm hazard.
Several reasons why lightning
has become more important in recent years. There are more
risk-conscious about natural hazards, especially those that interrupt our
power supplies and communications systems. Second, lightning is being used
as a valuable tool to help forecasters predict other hazardous weather
conditions, like tornadoes and flash floods, to warn the public. Third,
major advances in understanding and technology have made it possible to
detect, locate, and track lightning over the whole countries.
How to lightning detection sensors detect electromagnetic signals from
lightning?
A
sensor detects a cloud-to-ground flash when five specific criteria are
satisfied. These criteria come from the time series of the electromagnetic
signal that is produced by a typical cloud-to-ground flash over a small
portion of a second. When the sensor observes that these features have
been satisfied, it declares that a cloud-to-ground flash has been
detected. This report is combined with similar information from several
other sensors. The data from multiple sensors are then used to calculate
the position of the flash by complex geometry and trigonometry at the
National Lightning Detection Network Control Center at the company's
facility in Tucson, Arizona.
How does a lightning produce electromagnetic effects?
When a
lightning flash occurs, it strips electrons from atoms in the air. This
process emits a large amount of electromagnetic energy as well as a great
deal of sound and light.
Does sheet lightning pose the same threat as bolt lightning?
The
term sheet lightning generally refers to long channels of lightning that
appear horizontal. This is often seen at the base of clouds in the 10,000-
to 15,000-foot range, and may be at the bottom of a thunderstorm's anvil
cloud. The horizontal--or sheet--portion of the flash may strike the
ground some distance away. Think of the flash as having an upside down "L"
shape, where there is a connection to ground and a long horizontal section
along the base of a cloud. Flashes have been observed by special lightning
detection systems that have one ground strike point that is connected to a
horizontal segment up to 54 miles (90 kilometers) long!
Why is the launch of the shuttle delayed because of bad weather, when
commercial airliners take off and land in thunderstorms and lightning?
The shuttle is quite different from a commercial airliner.
The shuttle launches straight up through any clouds that are there.
Airliners sometimes fly through clouds, but they can avoid the worst ones.
Rocket exhaust plumes stretching from the shuttle (or other rockets) to
the ground makes an excellent lightning rod and therefore lightning is
drawn especially to a flying rocket. This is very dangerous. The shuttle
has a fragile and critical thermal tile skin that must be intact for
re-entry. Lightning strikes would probably shatter thermal tiles. The
shuttle also uses extremely explosive hydrogen and oxygen for fuel. A
lightning strike on or near the launch pad or during flight could cause
this propellant to explode. An airliner uses a kerosene-like fuel which is
far less flammable. Airliners can land in the rain, but even they often
delay or divert to avoid severe thunderstorms.
How does lightning form?
Rapidly
rising air in a thunderstorm interacts with rapidly falling air within the
thunderstorm to create separately positive and negative charged areas
within the cloud. Air acts as an insulator, but when the charge builds up
to a level that exceeds its ability to act as an insulator, the result is
a spark we see as lightning. The lightning equalizes the positive and
negative charged areas.
Several types of lightning are common:
-
In-cloud lightning extends
from one charged region of a cloud to another
-
Cloud-to-cloud lightning extends between two clouds
-
Cloud-to-air lightning
extends from a cloud to the air, not touching the ground
-
Cloud-to-ground lightning stretches from a cloud to the ground
How far can lightning travel?
Typically,
lightning travels 10 miles or less. However, lightning has been observed
to travel 20 miles or more, often extending up to 10 miles away from the
cloud that formed the lightning.
How hot is lightning?
Within
the lightning bolt the air is superheated to temperatures exceeding
50,000°F, many times hotter than the surface of sun.
What causes thunder?
The air
in the lightning strike is heated very rapidly and expands, creating a
shock wave we hear as thunder. Thunder lasts a few seconds because we
first hear the shock wave from the portion of lightning closest to us and
then we continue to hear the shock wave from the lightning bolt farther
away from us that is reaching us at later times.
What causes a "bolt from the blue"?
Lightning
is known to travel 10 miles or more horizontally from a cloud before
striking the ground, but the lightning origin is always a thunderstorm. No
clouds may have been directly overhead, but the thunderstorm was probably
within 10 miles.
How can you tell how far away lightning is?
It
takes five seconds for sound to travel one mile. When you see a flash,
count the number of seconds between until you hear the thunder and then
divide by five to get the number of miles between the lightning and you.
National lightning safety experts agree a good rule of thumb is if you
count 30 seconds or less between the flash and thunder, you are in
immediate danger and should seek safety.
Do tires in a car or rubber soles on shoes protect you from lightning?
No. The
lightning has traveled many miles to reach you; the extra inch of rubber
offers no protection. The inside of a car is safer because the lightning
travels around the car's metal frame to the ground. Rubber soles on shoes
offer no protection from lightning.
How can I protect myself from lightning?
People
who spend much of their time outside are most at risk being struck by
lightning. Golfers, fishermen, and hikers are most prone to being struck.
Anyone who works outside, such as a construction worker, is also at risk.
You can avoid being struck by immediately seeking shelter inside a
substantial building or metal-topped vehicle when lightning is present.
Never seek shelter outside under a tree. It's better to be wet from rain
than dead or disabled by lightning. When inside and lightning is nearby,
do not use electrical appliances, including computers and video games
connected TV sets, telephones, or have contact with running water or water
pipes. Electricity from lightning can travel through phone wires, electric
wires, TV cables, and plumbing.
If I can't reach shelter how do I protect myself?
As a last
resort, make yourself as short as possible with as little contact with the
ground as possible. Lightning usually strikes the tallest objects such as
trees, utility poles, and metal objects. Do not seek shelter under trees,
utility poles, near metal fences or gates. Go to a low spot, but avoid
washes and places where water will collect, crouch down on the balls of
your feet and make yourself into as small a ball as you can. Do not lay
down or sit on the ground. Lightning currents can travel across the ground
so you want to make as little contact with the ground as possible.
Do people who are struck by lightning carry an electrical charge?
No.
People who are struck by lightning carry no charge and should be
immediately helped by those nearby. People who are struck by lightning may
have trouble breathing and their heart may have stopped. CPR should be
administered immediately.
Do all people die who are struck by lightning?
No.
Lightning kills 75 to 100 people every year in the United States; hundreds
more survive. Most lightning victims are injured, not killed. Lightning
safety experts estimate that for every person killed by lightning, about
10 are injured. Many lightning survivors report debilitating symptoms such
as memory and attention deficits, sleep disturbance, dizziness, inability
or order tasks, isolation, and depression. While some of these problems
may lessen with time, these or new difficulties may reappear years later.
How close to a thunderstorm can you be and remain safe?
In urban
areas, buildings and city noise reduce your ability to hear thunder until
it's only 10 miles away or less. Since lightning can easily travel 10
miles away from a thunderstorm, if you're in a city and can hear thunder,
you are at risk from being hit by lightning. In quiet, rural areas,
thunder can be heard from thunderstorms that are up to 15 or more miles
away. The best course of action is to remain alert to your weather
surroundings. Be aware of new thunderstorms forming near you as other
thunderstorms are leaving the area.
Where does the most lightning occur in the United States?
Year in
and year out, the hot spot for the most lightning per square mile is in
central Florida. Other lightning hot spots vary from year to year, but in
Gulf Coast areas of northwest Florida, Alabama, Mississippi, Louisiana,
and east-central Texas usually trail central Florida in strike density.
What's the difference between a flash, stroke, strike, and bolt?
In
lightning science vocabulary, lightning is broken down into several parts.
The initial downward discharge is called a "leader". The "return stroke"
that completes the electrical current is the bright, visual channel that
we see. Several return strokes can move up and down this channel many
times, creating the "flickering" effect. This entire event may last half a
second and is called a "flash". Outside the lightning science community,
most people commonly refer to the entire, half-second lightning event as a
"strike" or "bolt".
We had a lot of lightning in a recent storm, how can I
find out if the number is above or below the average?
Lightning
activity can vary widely with all the seasonal and local weather
conditions that cause lightning to occur. To determine your area's
lightning activities during a specific time period was typical, we'd need
to analyze similar storms in your specific location over the past 5 to 10
years by completing a
Regional Statistical Analysis report.
Is there much lightning in tornadoes and hurricanes?
Tornadoes
can have a wide variety of lightning. Some closely-observed tornadoes had
no thunder at all from the storm during the tornado. But usually, the
parent storm eventually produces a fair amount of lightning. Hurricanes
usually have very little or no lightning. The only significant lightning
is observed in the outer rain bands. Probably less than 10% have more than
a few cloud-to-ground flashes in the eye wall. Data on in-cloud flashes
during hurricanes is scarce. Why there is so little lightning in the core
of hurricanes has been the subject of conjecture. It probably has to do
with the strong horizontal winds not allowing much vertical mixing
compared to a more isolated storm.
Is lightning attracted to water or other objects on the ground?
A
cloud-to-ground flash usually begins in a cloud at 15,000 to 20,000 feet
above ground. It comes to earth in 50-yard steps. Until it's close to the
ground, it has no idea of what it will hit. Then, within 30 to 50 yards of
the ground, lightning decides what is the best connection to make with the
surface. Often, the best connection is the tallest object within the 30 to
50 yard area.
Is friction the cause of static buildup which leads to
lightning?
A
primary factor in lightning generation is an updraft, which can be
produced in a number of ways. An updraft may be caused by a sea breeze,
mountains, cold front, or other methods that produce instability in the
vertical. The updraft must reach levels colder than freezing at its top.
In the U.S. in summer, that's about 15,000 to 20,000 feet above sea level.
When the updraft carries liquid water to that height, some droplets turn
to ice. The hail or soft hail, called graupel, becomes heavier than the
water droplets, and starts to fall faster than the water, which may still
be going upward. So the hail is carrying one amount of charge down through
the raindrops that have another charge. This is the primary factor for
lightning formation. The exact way this happens is still not completely
understood. Basically, the more water droplets that are carried quickly up
to levels with sub-freezing temperatures, the more lightning is produced.
Is it necessary to evacuate a pool during lightning?
Yes.
There are a few cases in and around swimming pools every year in the U.S.
During the summer of 2000, two boys were killed at an outdoor pool in
Florida. Documented cases of indoor pool lightning victims are scarce. The
whole indoor or outdoor pool complex should be considered unsafe during
lightning unless proven otherwise. The open surface of the pool is one
factor, but the power, plumbing, and mechanical connections to the pool
are a very important factor, too. Water and electric lines are very good
conductors. The most likely scenario is for lightning to hit a utility
pole outside a pool. The current then travels through wires into the pool
area. Every pool has a pump, and the pump is designed to move water. That
connection can be the site of a transfer of lightning current from the
power system into water in the pool, then to wading pools, the shower
area, and anywhere there is standing water. Everyone in contact with water
in the complex could be a lightning strike victim. Lightning safety
experts recommend staying out of the water for 30 minutes after the last
flash or thunder. Studies found that waiting 30 minutes eliminates most
lightning risks as the storm leaves the area.
What are the odds of being a lightning victim?
With
about 1,000 lightning injuries and deaths each year in the United States
and a population of about 24o million, your the odds are 1 in 240,000 each
year. Over an 80-year lifetime, your chance of being a lightning victim
during your life is 1 in 3,000. You have about a 1 in 300 chance of
being affected by a lightning death or injury of a close friend or a
relative.
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