What causes lightning?
Lightning originates around 15,000 to 25,000 feet above sea
level when raindrops are carried upward until some of them
convert to ice. For reasons that are not widely agreed upon, a
cloud-to-ground lightning flash originates in this mixed water
and ice region. The charge then moves downward in 50-yard
sections called step leaders. It keeps moving toward the ground
in these steps and produces a channel along which charge is
deposited. Eventually, it encounters something on the ground
that is a good connection. The circuit is complete at that time,
and the charge is lowered from cloud to ground.
The return stroke is a flow of charge (current) which
produces a luminosity much brighter than the part that came
down. This entire event usually takes less than half a second.
What causes thunder?
Thunder is caused by lightning. The bright light of the
lightning flash caused by the return stroke mentioned above
represents a great deal of energy. This energy heats the air in
the channel to above 50,000 degrees F in only a few millionths
of a second! The air that is now heated to such a high
temperature had no time to expand, so it is now at a very high
pressure. The high pressure air then expands outward into the
surrounding air compressing it and causing a disturbance that
propagates in all directions away from the stroke. The
disturbance is a shock wave for the first 10 yards, after which
it becomes an ordinary sound wave, or thunder.
Thunder can seem like it goes on and on because each point
along the channel produces a shock wave and sound wave.
How far away from a storm can lightning strike?
It's not clear what the maximum possible distance might be.
Lightning has been known to strike more than 10 miles from the
storm in an area of clear sky above.
How long can a lightning bolt be?
Recent research from Vaisala-GAI's LDAR and LDAR II lightning
detection networks show that lightning can travel 60 miles or
more. They find the longest bolts originate in the front of a
squall line and travel 62 miles horizontally back into the
trailing stratiform region behind the squall line. The longest
bolt they have seen to date was 118 miles long in the Dallas-Ft.
Worth, TX area. Since 3-D lightning measurements are relatively
new, however, scientists are learning more every day and these
numbers may change.
Where does lightning usually strike?
Lightning comes from a parent cumulonimbus cloud. These
thunderstorm clouds are formed wherever there is enough upward
motion, instability in the vertical, and moisture to produce a
deep cloud that reaches up to levels somewhat colder than
These conditions are most often met in summer. In general,
the US mainland has a decreasing amount of lightning toward the
northwest. Over the entire year, the highest frequency of
cloud-to-ground lightning is in Florida between Tampa and
Orlando. This is due to the presence, on many days during the
year, of a large moisture content in the atmosphere at low
levels (below 5,000 feet), as well as high surface temperatures
that produce strong sea breezes along the Florida coasts. The
western mountains of the US also produce strong upward motions
and contribute to frequent cloud-to-ground lightning. There are
also high frequencies along the Gulf of Mexico coast westward to
Texas, the Atlantic coast in the southeast US, and inland from
the Gulf. Regions along the Pacific west coast have the least
Flashes that do not strike the surface are called cloud
flashes. They may be inside a cloud, travel from one part of a
cloud to another, or from cloud to air.
Can lightning be detected?
Since the 1980s, cloud-to-ground lightning flashes have been
detected and mapped in real time across the entire US by several
In 1994, the networks were combined into one national network
consisting of antennas that detect the angle from ground strike
points to an antenna (direction-finder antenna), that detect the
time it took for them to arrive at an antenna (time-of-arrival
method), or a combination of both detection methods. The network
is operated by Global Atmospherics, owned by
manufactures environmental and industrial measurement products.
Flashes have also been detected from space during the past
few years by an optical sensor. This experimental satellite covers the earth
twice a day in tropical regions. The satellite also detects
flashes that do not strike the ground, but cannot tell the
difference between ground strikes and cloud flashes.
How many flashes are there?
Over the continental 48 states, an average of 20,000,000
cloud-to-ground flashes have been detected every year since the
lightning detection network covered all of the continental US in
1989. In addition, about half of all flashes have more than one
ground strike point, so at least 30 million points on the ground
are struck on the average each year in the US. Besides
cloud-to-ground flashes, there are roughly 5 to 10 times as many
cloud flashes as there are to ground.
What types of damage can lightning cause?
Cloud-to-ground lightning can kill or injure people by direct or
indirect means. The lightning current can branch off to a person
from a tree, fence, pole, or other tall object. It is not known
if all people are killed who are directly struck by the flash
itself. In addition, flashes may conduct their current through
the ground to a person after the flash strikes a nearby tree,
antenna, or other tall object. The current also may travel
through power or telephone lines, or plumbing pipes to a person
who is in contact with an electric appliance, telephone, or
Similarly, objects can be directly struck and this impact may
result in an explosion, burn, or total destruction. Or, the
damage may be indirect when the current passes through or near
it. Sometimes, current may enter a building and transfer through
wires or plumbing and damage everything in its path. Similarly,
in urban areas, it may strike a pole or tree and the current
then travels to several nearby houses and other structures and
enter them through wiring or plumbing.
How to stay safe when lightning is around: use the 30-30
The best defense is to plan ahead and avoid exposure to
lightning when a thunderstorm occurs. Know where safe shelter is
located and leave enough time to reach safe shelter before your
danger level is high. Don't be an isolated tall object, and
don't be connected to anything that may be an isolated tall
NSSL's scientists and collaborators did a study to find out
how close is too close. They found that 80% of the next
lightning strikes in a storm are within 2 to 3 miles of each
other in Florida, but as far as 6 miles from each other in
Oklahoma. Use the 'flash-to-bang' method to find the distance to
lightning. Safe shelter must be reached by the time a flash is
within 30 seconds flash-to-bang. In most cases, then, when you
can hear thunder you are no longer safe. Lightning safety is
also considered at:
But there is often blue sky in some direction while lightning
is occurring nearby, and it may not be raining, so pay much more
attention to the lightning than the rain. A particularly
difficult situation is the first flash from a storm--watch for a
storm that is growing quickly, such as when a storm is becoming
very dark at its base or is growing very tall. An equally
dangerous situation is when a storm appears to be finished, and
only light rain and/or occasional thunder are heard, but the
cloud overhead continues to be fairly dark. The most common
situation for a lightning death or injury in Florida was found
NOT to be in the heaviest rain area with lots of flashes, but
after or before the time when rain and lightning was the most
intense. So, the weak storm without too many flashes, at the
edge of a larger storm, or early or late in the life of a storm
is most dangerous.
The best shelter is a substantial building that has plumbing
and wiring--in other words, one that is used or lived in by
people for a major portion of the day. A very unsafe building
for lightning has only a roof and some supports, but no wiring
or pipes extending into the ground. A vehicle with a metal roof
provides good shelter, and is much better than being in the open
or in an ungrounded building, but is not as good as being in a
building that is grounded by wires and pipes.