Lightning Basics
What is lightning?
Lightning is a gigantic electrostatic discharge (the same kind of electricity that can shock you when you touch a doorknob) between the cloud and the ground, other clouds, or within a cloud. Scientists do not understand yet exactly how it works or how it interacts with the upper atmosphere or the earth 's electromagnetic field.
Lightning is one of the oldest observed natural phenomena on earth. It has been seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, in large hurricanes, and obviously, thunderstorms.
What causes lightning?
The creation of lightning is a complicated process. We generally know what conditions are needed to produce lightning, but there is still debate about exactly how lightning forms.The exact way a cloud builds up the electrical charges that lead to lightning is not completely understood. Precipitation and convection theories both attempt to explain the electrical structure within clouds. Precipitation theorists suppose that different size raindrops, hail, and graupel get their positive or negative charge as they collide, with heavier particles carrying negative charge to the cloud bottom. Convection theorists believe that updrafts transport positive charges near the ground upward through the cloud while downdrafts carry negative charges downward. What follows is a summary of what we know.
Thunderstorms have very turbulent environments - strong updrafts and downdrafts occur often and close together. The updrafts carry 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. At the same time, downdrafts are transporting hail and ice from the frozen upper parts of the storm. When these particles collide, the water droplets freeze and release heat. This heat 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 key process occurs involving electrical charge: negatively charged electrons are sheared off the rising particles and collect on the falling particles. The result is a storm cloud that is negatively charged at its base, and positively charged at the top.
Opposite charges attract one another. As the positive and negative areas grow more distinct within the cloud, an electric field is created between the oppositely-charged thunderstorm base and its top. The farther apart these regions are, the stronger the field and the stronger the attraction between the charges. But we cannot forget that the atmosphere is a very good insulator that inhibits electric flow. So, a HUGE amount of charge has to build up before the strength of the electric field overpowers the atmosphere's insulating properties. A current of electricity forces a path through the air until it encounters something that makes a good connection. The current is discharged as a stroke of lightning.
While all this is happening inside the storm, beneath the storm, positive charge begins to pool within the surface of the earth. 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 about 75-80% of lightning occurs within the storm cloud.
» More About ELECTRICAL CHARGE DISTRIBUTION
Lightning types
GROUND FLASHES
There are two categories of ground flashes: natural (those that occur because
of normal electrification in the environment), and artificially initiated or
triggered. Artificially initiated lightning includes strikes to very tall structures,
airplanes, rockets and towers on mountains. Triggered lightning goes from ground
to cloud, while "natural" lightning is cloud to ground.
Terms used to describe ground flashes include forked lightning, which shows branching to the ground from a nearly vertical channel; ribbon lightning, when the horizontal displacement of the channel by the wind appears as a series of ribbons; and bead lightning, when the decaying channel of a ground flash will sometimes break into a series of bright and dark spots. Ball lightning is a luminous sphere whose physics is not well understood.
GROUND FLASHES | |
Natural |
Triggered |
Cloud-to-ground lightning (CG's)
A channel of negative charge, called a step
leader, will zigzag
downward in roughly 50-yard segments in a forked pattern. This step
leader is invisible to the human eye, and shoots to the ground in less time
than it takes to blink. As it nears the ground, the negatively charged
step
leader is attracted to a channel of positive charge reaching up, a
streamer,
normally through something tall, such as a tree, house, or telephone pole.
When the oppositely-charged leader and streamer connect, a powerful electrical
current begins flowing. A return stroke of bright luminosity travels about
60,000 miles per second back towards the cloud. A flash consists of one
or perhaps as many as 20 return strokes. We see lightning flicker when
the process rapidly repeats itself several times along the same path. The
actual diameter of a lightning channel is one-to two inches.
A typical cloud-to-ground flash is a negative stepped leader that travels downward through the cloud, followed by an upward traveling return stroke. The net effect of this flash is to lower negative charge from the cloud to the ground. Less common, a downward traveling positive leader followed by an upward return stroke will lower positive charge to earth. These positive ground flashes now appear to be linked to certain severe storms and are the focus of intense research by scientists.
CLOUD FLASHES
Cloud flashes sometimes have visible channels that extend out into the air
around the storm (cloud-to-air or CA), but do not strike
the ground. The term sheet lightning or intra-cloud
lightning (IC) refers to lightning embedded within a cloud that
lights up as a sheet of luminosity during the flash. A related term, heat
lightning, is lightning
or lightning-induced illumination that is too far away for thunder to be
heard. Lightning can also travel from cloud-to-cloud
(CC). Spider lightning refers to long, horizontally
traveling flashes often seen on the underside of stratiform clouds.
CLOUD FLASHES | |
In-Cloud |
Spider |
There are also additional types of electrical discharges associated with thunderstorms called transient luminous events that occur high in the atmosphere. They are rarely observed visually and not well understood.
» More About TRANSIENT LUMINOUS EVENTS
What causes thunder?
Lightning causes thunder. Thunder is the sound caused by rapidly expanding gases along a channel of lightning discharge. Energy from lightning heats the air to around 18,000 degrees Fahrenheit. This causes a rapid expansion of the air, creating a sound wave heard as thunder. An initial tearing sound is usually caused by the stepped leader, and the sharp click or crack heard at a very close range, just before the main crash of thunder, is caused by the ground streamer.
Thunder is rarely heard at points farther than 15 miles from the lightning discharge, but occasionally can be heard up to 25 miles away. At these distances, thunder is heard as more of a low rumbling sound because the higher frequency pitches are more easily absorbed by the surrounding environment, and the sound waves set off by the lightning discharge have different arrival times.
HOW DOES NSSL CONTRIBUTE?
NSSL works to develop an understanding of how storms produce lightning and what the lightning data reveals about the microphysics, kinematics, and severity of storms. This information is used to help develop methods for using lightning data to improve forecasting and nowcasting of storms and storm hazards.
In 1998, NSSL's Don MacGorman and Dave Rust wrote The Electrical Nature of Storms , a textbook discussion of atmospheric electricity and the electrical processes that occur in storms.
Field Observations
NSSL developed mobile atmospheric laboratories to collect data in
and around thunderstorms using balloons and other instruments. These
observations help scientists
understand how storms produce lightning. Electric field data from free-flying
instrumented balloons led to the discovery that some storms have an "inverted"
or reversed charge structure.
Lightning modeling
NSSL/CIMMS scientists simulated realistic cloud-to-ground lightning flashes
for the first time using a 3-D cloud model that generates complex precipitation
such as graupel, which is known to affect lightning production.