Lightning is a gigantic electric spark in the sky. Most lightning that people see extends between a cloud and the ground. But lightning more often occurs within a cloud, between a cloud and the air, and between two clouds. Lightning in the atmosphere may damage aircraft in flight. But it causes no harm on the ground. It is lightning striking the ground that can kill people and cause fires. On Earth, lightning occurs about 100 times per second. Lightning rapidly heats the air, creating a pressure wave that we hear as thunder.
Loading the player...Lightning during a thunderstorm
A lightning flash from a cloud to the ground consists of one or more strokes. Each stroke appears as a momentary brightening of a channel (path) between the cloud and the ground. The strokes are typically separated by tens of milliseconds. This separation explains why lightning often appears to “flicker.”
Flashes within clouds travel horizontally up to 6 miles (10 kilometers) or more. Typical cloud-to-ground flashes are 3 to 4 miles (5 to 7 kilometers) long. Roughly half of them hit the ground at more than one point. These points may lie many miles or kilometers apart.
Until the mid-1700’s, lightning remained a great mystery of nature. The ancient Greeks and Romans thought lightning was a weapon of the gods. Some African peoples believed individuals and places hit by lightning were cursed. For several centuries, people in Europe and America thought they could keep lightning away by ringing church bells. This practice resulted in the deaths of many people pulling the bells’ ropes.
Studies of electricity in thunderstorms can be traced back to May 10, 1752, in the village of Marly-la-Ville, France, near Paris. On that day, after receiving instructions from the French scientist Thomas-François Dalibard, a local man drew sparks from a tall iron rod with a storm nearby. The experiment had been proposed by the American scientist Benjamin Franklin. The test provided the first direct proof that thunderclouds contain electrical energy. Scientists in several countries later repeated the experiment. Franklin himself drew sparks from the string of a kite before learning of the success at Marly. Experimenters also launched conducting strings into clouds by balloon, mortar, and rocket.
In all these experiments, the electrical energy in the cloud caused the metallic rod or the conducting string to become polarized. That is, opposite electric charges built up at the opposite ends of the conductor. As the gap between the ground and the bottom end of the conductor narrowed, a spark discharged to the ground. In designing their experiments, Franklin and others did not consider the danger of a direct lightning strike.
Types of lightning
The most common source of lightning is the type of cloud known as a cumulonimbus or thundercloud. Lightning can be classified (1) by the origin and destination of the strike and (2) by its appearance.
Origin and destination.
The most common type of lightning is intracloud lightning, which occurs within a cloud. Intracloud lightning neutralizes positive and negative charges that have built up in a single cloud.
Charges that flow from a cloud to the air create cloud-to-air lightning. A flow of charges between two clouds results in the rare cloud-to-cloud or intercloud lightning.
Lightning between a cloud and the ground may develop downward or upward, depending on the direction in which the charges first flow. Downward lightning originates in the cloud. Upward lightning, which is relatively rare, starts from tall objects on the ground.
Over 90 percent of cloud-to-ground lightning transports negative charge to ground. The rest generally transfer a positive charge. Rarely, both positive and negative charges are transferred by the same flash.
Appearance.
People have given names to various visual aspects of lightning. Forked lightning is a flash with several visible branches. Streak lightning appears to illuminate a single jagged line. Ribbon lightning appears as parallel strips of light. It occurs when wind separates the individual strokes of a flash. Bead or chain lightning is a flash that breaks up into a dotted line as it ends.
Heat lightning, often seen on summer nights, seems to occur without thunder. Actually, the lightning strokes are so far from the observer that their accompanying thunder cannot be heard. Generally, the distance from the observer is beyond about 15 miles (24 kilometers).
Sheet lightning appears as an illumination of part of the sky. The flashes that produce sheet lightning are too far away to be seen or are hidden by clouds.
Ball lightning usually occurs after a cloud-to-ground flash. It appears as a glowing, fiery ball that floats for several seconds before disappearing. People have seen ball lightning floating along the ground and inside houses, barns, and even airplanes. Observers commonly describe it as red, yellow, or orange and between the size of an orange and that of a basketball. In 2014, Chinese researchers recorded ball lightning on video, the first time the event has been captured. There are many theories of how ball lightning forms, but none have been proved.
How lightning occurs
Lightning occurs when a cloud becomes electrically charged. Most researchers believe that charging results from collisions among various forms of water and ice within the cloud.
Charging.
Within a cloud, tiny pieces of ice rise in updrafts. These pieces may collide with heavier haillike pellets that are falling. Interactions between these particles, especially in the presence of small water droplets, transfer charges between the particles. At lower temperatures, the small ice crystals become positively charged and the heavier pellets become negatively charged. The oppositely charged particles become separated by distances of miles or kilometers due to updrafts and the downward force of gravity. The top of the cloud thus becomes positively charged. The bottom becomes negatively charged. Often, the cloud develops another small positive charge below the negative area.
Lightning strokes.
Each negative downward lightning flash typically consists of 3 to 5 component strokes, but upwards of 20 strokes may be produced. Each stroke consists of a leader and a return stroke. The leader travels downward. It creates a conducting path between the charged part of the cloud and the ground. It also distributes a negative charge from the cloud along this path. The return stroke follows the same path upward. It neutralizes the negative charge of the leader.
The leader of the first stroke spreads in brief stretches or “steps.” It is thus called a stepped leader. Between its tip and the ground, the electric potential difference or voltage probably measures tens of millions of volts.
When the stepped leader’s tip nears the ground, a short channel of opposite charge, often called a streamer, reaches up to connect the stepped leader to the ground, after which the first return stroke begins. This stroke is an electric pulse with a typical peak current of about 30 kiloamperes, measured at the ground. The return stroke effectively lowers the charge originally deposited along the leader’s channel to the ground.
The high-current return stroke rapidly heats the channel to a peak temperature of about 55,000 °F (30,000 °C). The heat creates air pressure in excess of 10 times greater than normal, causing the channel to swiftly expand and produce thunder.
Unlike the stepped leader’s tip, the tip of the next stroke’s leader appears to move continuously. Seen in slow motion, its tip resembles a downward-moving “dart.” This leader is thus called a dart leader. The dart leader follows the path of a preceding stroke.
Once the bottom of the dart leader connects to the ground, a subsequent return stroke is launched upward, neutralizing the leader charge. The current of this stroke typically has a peak output of 10 to 15 kiloamperes on the ground. Further dart leaders and return strokes may follow.
The study of lightning
Triggered lightning.
Scientists can trigger lightning by launching a small rocket trailing a thin grounded wire toward a charged cloud. Experiments with triggered lightning provide insight into natural lightning processes. They are also a useful tool to study the interaction of lightning with various objects and systems.
Emissions.
Scientists have observed several emissions related to lightning. Both stepped leaders and dart leaders produce bursts of X rays. These bursts can have energies at least twice that of a typical medical X ray.
Scientists have also observed intense flashes of highly energetic gamma rays associated with thunderstorm activity. These bursts are called terrestrial gamma-ray flashes (TGF’s). Thunderstorms also produce positrons, the antimatter equivalent to electrons. Scientists are not sure how these emissions originate.
Jets, sprites, and elves.
For many years, pilots and other people have reported lightninglike channels, pillars of light, and diffuse, upward flowing lights above thunderclouds. Scientists first reported evidence for these events in 1990. Many of them appear to extend between the cloud tops, about 12 miles (20 kilometers) above the ground, and roughly 35 miles (55 kilometers) above the ground.
The three most observed types of such events are: (1) blue jets, (2) red sprites, and (3) elves. Blue jets shoot upward from a cloud top in the form of a blue cone that dissipates before reaching the ionosphere. Rarely, jetlike events called gigantic jets shoot all the way into the ionosphere. Red sprites are brightest between 25 and 55 miles (40 and 90 kilometers) of altitude. They often have faint bluish strands extending downward from 30 miles (50 kilometers) to as low as 12 miles (20 kilometers). Red sprites generally develop in association with larger positive lightning flashes. Elves occur around a height of 55 miles (90 kilometers). They appear as short-lived circles of light that rapidly expand across the ionosphere from a point above a lightning flash.
Extraterrestrial lightning.
Lightning also occurs on other worlds. For example, space probes have observed lightninglike discharges on Jupiter and Saturn. Scientists believe the electric currents in Jupiter’s lightning to be many times greater than in Earth lightning.
Protection from lightning
People install metallic rods called lightning rods on building tops. Lightning rods connect to buried metallic rods called ground rods. A lightning rod intercepts lightning that would otherwise hit the building. The lightning safely flows through wires to a ground rod.
Almost no place is absolutely safe from lightning, but some places are safer than others. Lightning safety experts suggest these guidelines for personal safety:
(1) Large, enclosed structures tend to be safer than smaller or open structures. The risk for lightning injury depends on whether the structure incorporates special lightning protection, the type of construction materials used, and the size of the structure.
When inside a building, avoid using the telephone, running water, or touching conductive surfaces exposed to the outside. These surfaces include metal door or window frames, cable television wiring, electrical wiring, telephone wiring, and plumbing.
(2) In general, fully enclosed metal vehicles—such as cars, trucks, buses, and fully enclosed farm vehicles—provide good shelter from lightning, as long as the windows are rolled up. Avoid contact with metal or conducting surfaces outside or inside the vehicle.
(3) Avoid being in or near high places. Also avoid open fields, isolated trees, rain or picnic shelters, baseball dugouts, communications towers, flagpoles, light poles, metal or wood bleachers, metal fences, golf carts, and bodies of water.
See also Franklin, Benjamin (Experiments with electricity); Lightning rod; Thunder; Thunderstorm.
