Questions and Answers About Lightning
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
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 freezing.
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 cloud-to-ground lightning.
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. 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 plumbing fixture.
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 Rule!
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 object.
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.
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