What is Lightning?
|Photo from: NOAA
At any given moment, there are approximately 1,800 thunderstorms occurring over
the Earth. It is estimated that 100 lightning flashes occur each
second somewhere on the Earth, adding up to nearly 8 million lightning
flashes per day. During your lifetime, you have a one in 600,000
chance of being struck by lightning.
The climate of northern Arizona provides us with a three month period when thunderstorms
occur almost every day.
Typically forming in the late morning and lasting throughout the afternoon, the vivid
display of lightning can be a beautiful sight to see. But Arizona
thunderstorms have the potential to cause injury and death to people unfortunate enough to be
struck by lightning.
Since 1959, an average of 86 people die each year nationally as a result of lightning
strikes. In Arizona, an average of four people are killed by lightning each year. While the
number of deaths for Arizona may seem low by national standards, nearly all these deaths
occur in our short three month thunderstorm season.
All lightning is dangerous and even the weakest thunderstorms produce lightning. Most
people in recent years have been killed by lightning while swimming, golfing, or hiking.
But they have also been killed doing less dangerous activities, like talking on the
telephone, playing soccer or baseball, fishing on a lake, taking a shower, or loading
laundry in a clothes dryer. Lightning starts over 15 thousand fires to structures, and burns
down about 2 million acres of forest each year in the United States.
How is Lightning Formed?
Lightning is a chaotic and dangerous aspect of weather.
Lightning occurs most frequently during thunderstorms, but has also
been observed during volcanic eruptions, extremely intense forest
fires, and surface nuclear detonations. In a thunderstorm, lightning
is created as a discharge of built up energy due to the separation of
positive and negative charges which are generated inside the thunderstorm.
|Photo from: NASA
The formation of ice in a cloud appears to be very important in the
development of this charge separation and ultimately of lightning.
Inside a thunderstorm, these ice particles vary in size, from small
ice crystals to larger hailstones. Owing to the rising and sinking air
associated with thunderstorms, these particles collide frequently
inside the cloud. These collisions within the
thunderstorm cause these particles to build up electric charge. Due to the
different rates of rising and falling within a thunderstorm,
a separation of electrical charge takes place. As the
thunderstorm grows, intense electrical fields can develop within it. A
large positive charge forms in the frozen upper part of the cloud and two charge
regions - a large, negatively charged region and a smaller positively
charged region - form in the lower portion of the cloud. The ground
normally maintains a small negative charge with respect to the
atmosphere, but when a thunderstorm drifts overhead, the negative
charge at the cloud base induces a positive charge on the ground below
the storm. The positive ground current follows the movement of the
cloud like a shadow and concentrates on elevated objects, such as
trees, buildings, and higher portions of terrain, in an attempt to
establish a current to equalize the charges between cloud base and
ground. Air, however is a good insulator, and the electrical
potential between cloud and ground must build up to levels of tens to
hundreds of millions of volts before the insulating properties of the
air break down and an ionized conductive channel is established for
the current to flow between the two charges. If you have ever had your
hair stand on end while under a thunderstorm, you were in this
positive ground current, and could have become a lightning target.
Lightning is usually initiated within the thunderstorm cloud when a
faint, negatively charged channel called the stepped leader
emerges from the base of the cloud and propagates toward the ground in
a series of steps of about 1 microsecond in duration and 150-300 feet
in length. The stepped leader reaches from cloud base to ground in
about a hundredth of a second. As the stepped leader approaches the
ground, streamers of positive charge rush upward from objects on the
ground. When one of the streams contacts the leading edge of the
stepped leader, the lightning channel is opened, negative charge
starts flowing to the ground, and a return stroke, lasting
about a tenth of a second, propagates through the channel as a bright
luminous pulse. Sometimes, following the initial return stroke, one or
more additional leaders may propagate down the decaying lightning
channel at intervals of about a tenth of a second. These leaders,
called dart leaders, are not stepped or branched
like the original leader, but are more or less direct and continuous.
Like the stepped leader, however, they initiate return strokes. These
return strokes are what we call lightning.
|Photo from: NASA
Not all lightning forms in the negatively charged area low in the
thunderstorm cloud. Some lightning forms in the cirrus anvil at the
top of the thunderstorm. This area carries a large positive charge,
and lightning from this area carries that positive charge to a
negative charged area on the ground. This type of lightning stroke is particularly
dangerous for several reasons. It frequently strikes away from the
rain core, either ahead or behind the thunderstorm. It can strike as
far as 5 or 10 miles from the storm in areas most people wouldn't
consider to be risky for lightning. The other problem is that positive
lightning typically has a longer duration, which results in more electrical
charge being transferred to the ground. This can allow for easier ignition
of fires and an increased risk to an individual
What is Thunder?
Thunder is the sound produced by rapidly expanding gases along a
lightning discharge channel where air is instantaneously heated to
temperatures near 10,000 degrees Celsius. The shock wave that is
created by this heating is what we hear as thunder.
How can You Tell How Far a Lightning Strike is From You?
Thunder can typically be heard as far as 10 to15 miles away from a
thunderstorm. Since light travels faster than sound, the further you
are from a thunderstorm, the more time will pass from when you see a
lightning strike and hear the thunder. The distance to a lightning
flash in miles can be determined by dividing the seconds that pass
between the lightning stroke and the thunder by five. When you
see a lightning bolt, start counting one-one thousand, two-one
thousand, etc. until you hear the thunder. Take that number and divide
it by 5 to get the distance in miles between you and the lightning.
|Lightning Safety Tips
- If you hear thunder from a storm, you are in danger of being
struck by lightning. Move indoors.
- A vehicle offers good protection from lightning as long you do not
touch any metal. If you are caught in your vehicle during an
electrical storm, stay there unless you are in danger from a flash
flood or tornado.
- Do not use a cord telephone unless it is an emergency. Lightning
easily travels through phone lines.
- Do not use any plug-in appliances during a thunderstorm. Computers, air conditioner
compressors and refrigerators are susceptible to relatively small
power surges. Unplug them if possible.
- Do not take a shower or bath during a thunderstorm. Lightning
travels easily through metal pipes.
- As soon as you hear thunder, stop outdoor activities such as swimming or playing an outdoor sport, and seek shelter. Many
recreational organizations mandate stoppage of play when a
thunderstorm is nearby, and with very good reason.
- If someone is struck, start first aid immediately, including CPR
if necessary. A lightning victim does not carry an electrical charge,
and most victims can be saved if treated immediately.
|Photo from: NASA
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