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LIGHTNING FAQs

Lightning, as best we understand, is a channel of negative charge, called a stepped leader that zigzags 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 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.

The answer is both. Cloud-to-ground lightning comes from the sky down, but the part you see comes from the ground up. A typical cloud-to-ground flash lowers a path of negative electricity (that we cannot see) towards the ground in a series of steps. Objects on the ground generally have a positive charge. Since opposites attract, an upward streamer is sent out from the object about to be struck. When these two paths meet, a return stroke zips back up to the sky. It is the return stroke that produces the visible flash, but it all happens so fast - in about one-millionth of a second - so the human eye doesn't see the actual formation of the stroke.

No, it is not possible to have lightning without thunder. Thunder is a direct result of lightning. However, it IS possible that you could not hear the thunder because it was too far away. Sometimes it is called "heat lightning" because it occurs most often in the summer.

Recent research from Vaisala-GAI's LDAR and LDAR II lightning detection networks show that lightning can travel 60 miles or more. The longest bolts start at the front of a squall line and travel horizontally back into clouds trailing 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, scientists are learning more every day and these numbers may change.

A cloud flash is lightning that occurs inside the cloud, travels from one part of a cloud to another, or from the cloud to the air.

A stepped leader is a stream of weakly charged particles that flows from the cloud – it moves towards the ground, starting and stopping, and sometimes branching, trying to find the path of least resistance.

Yes, lightning is always produced by a thunderstorm. Lightning causes thunder, and you can't have a thunderstorm without thunder. Thunderstorms are the only weather condition strong enough to carry water droplets to the upper parts of the atmosphere where they will freeze and become charged, i.e., because thunderstorms have an updraft.

Energy from lightning heats the air anywhere from 18,000 degrees Fahrenheit to up to 60,000 degrees Fahrenheit.

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.

Dry lightning is cloud-to-ground lightning without any rain nearby. The NOAA Storm Prediction Center routinely forecasts dry lightning because this kind is more likely to cause forest fires.

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.

Schematic of thunderstorm showing location of different types of lightning
larger image

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.

» ANIMATION OF CG STRIKE

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.

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