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.
Most lightning moves sideways within clouds. Sometimes it jumps to the ground. This is what people see. Negative-to-ground lightning is the most common form. In usually appears as a single streak or un upside down tree with the trunk at the top and the branches at the bottom.
In normal negative-to-ground lighting: 1) Strong negative charges build up in a cloud when the moist air inside becomes unstable. 2) Negatively charged particles (electrons) flow toward the ground in 50 meter increments through an invisible channel called a step leader. Just before reaching the ground, the negative charge causes positively charged particles to gather in the Earth below it. 3) The positive charge jumps to complete the electrical circuit, causes flashes of lightening."
Lightning ranges in length from 500 feet to more than two miles. Its width varies form ⅓ inch to a foot at the strokes peak. Lightning can reach temperatures of up to 54,000 degrees F. The speed of the pilot stream speed is 0.05 the speed of light. The return streamer speed is 10 percent of the speed of light.
A bolt of lighting produces 200,000 amps of electricity and generated higher temperatures than the sun. The entire visible discharge may last from 1/500th of a second to as long a 1.6 of a second. The duration of the discharge depends on the number of return strokes. The average number of return streamers is around six or seven.
Lightning Strikes
Lightning usually strikes the highest point of ground. It follows the path of least resistance and the highest point is the shortest distance between the ground and the cloud. Tall trees are often the largest and highest objects, which is why they are often hit by lightning and people are told not to stand under them during thunderstorms. Trees are poor conductors. They often put up so much resistance that an electrical charge bursts out and strike anything nearby, including people.
Most people who are struck by lightning survive. Some are badly brined. Others have their eyebrows singed off. Aircraft and automobiles are well insulated the metal in them are good conductors. Thus the people inside them are usually safe in storms.
In cities, lighting often strikes the tallest buildings, which are often outfit with copper lightning rods connected to the structural steel of the building which guides the electric current harmlessly to the bedrock. Before September 11th the World Trade Center towers were struck regularly by lighting. After the terrorist attacks there there were worries that more people would be hurt by lightning.
Causes of Lightning
"Lightning is generated in cold upper layers of air when particles of partly frozen water and ice crystal collide inside large clouds. These collisions generate positive and negative electrical charges, the same way that walking across a carpet on a dry day can build up static charges and result in a shock when a person touches a doorknob."
"In general, negative charges sink to the middle or lower parts of the cloud, while positive charges rise to the upper levels...When air can no longer insulate these opposite charge, an avalanche of runaway electrons initiates a lightning bolt."
"A small discharge called a pilot streamer, moves towards the Earth carrying a negative charge. A stronger current, called a stepped leader, follows and ionizes the air in its path. The stepped leader moves in a series of jagged spurts, each about 150 feet long. When the pilot streamer hits the Earth, a high-current return streamer leaps from the ground toward the cloud. It travels along the path of ionized air created by the stepped leader. This is the part of the lightning strike that produces the brilliant flash that we see."
"As a charge in one cloud center is dissipated, a negative charge from an adjacent charge center moves in to replace it. A dart leader from the second charge center moves to the ground along the original current channel. This produces another return streamer which also travels up to the cloud. Discharge continues until all negative charges on the cloud have been drained off.”
More Complicated Look at 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 percent of lightning occurs within the storm cloud.
Types of Lightning
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.
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 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.
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.
Reverse Lightning and Other Kinds of Strange Lightning
Reverse lightning occurs when positively particles form the cloud meet electrons from the ground. This type of lighting is more common than once thought. It is sometimes created by skyscrapers. This kind of lightning often appears tree-shaped with the trunk at the bottom and the branches at the top.
In positive-to-ground lightning, positive charges first run from the cloud to the ground, creating a channel through which electrons flow from the ground back up to the cloud. This kind of lightning tends to carry more charge, last tens of second longer and be less branched than negative-to-ground lighting
Blue jets are extremely energetic fields of charged particles that rise up to 30 miles from the tops of clouds. After they occur lightning stops for several seconds. Red sprites are striated glowing ribbons that rise high above the altitude that jets fly and reach the ionosphere, lasting 3 to 10 milliseconds. They happen only over areas of reverse lighting. Eces are thin , expanding doughnuts of light found above sprites after a lightning pulse. Trolls are propagated waves of energy that appear to come back out of cloud tops and hook up with sprites. No one really knows what they are.
Intense busts of lighting are often observed in a place 20 minutes or so before a tornado strikes. Lightning can also be a predictor of hailstorms and a major source of ozone in the upper atmosphere and have a strong influence on greenhouse gases such nitrogen oxide and water vapor.
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.
Thunder is caused by the ionization of the air and gases by electrical charge. Lightning causes the air heat up to as high 55,000 degrees F (30,500 degrees C), over five times hotter than the sun, causing the air to expand at supersonic speeds, The shockwaves produced by the expansion weakens into sound waves, which generates the sound of thunder.
Because sound travels more slowly than light you can tell how far the lightning is by counting the seconds. Five seconds represents a distance of about a mile.
Studying Lightning
The mechanism behind lightning are poorly understood. It involves complex interactions involving electricity and magnetic fields that are too remote and quick to observe with conventional instruments.
Thus far scientists have never been able to find a source of electricity in a cloud powerful to produce a bolt of lighting. They have only found one about a tenth of what is thought to be large enough to produce the necessary charge.
Scientist studying lightening have sent gliders into thunderstorms, set off rockets to attract lighting and created artificial lighting that lasts for a millionth of a billionth of second with a laser mounted in a trailer.
The presence of strange X-rays observed during lightning has lent some to support to the “runaway breakdown” theory which argues that lightning bolts are cosmic-rays, energetic particles that originate from outer space. According to this theory a cosmic ray accelerates electrons to high energies and these trigger chain reactions that cause lightning.
Image Sources: World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA), Wikimedia Commons
Text Sources:World Meteorological Organization; National Oceanic and Atmospheric Administration (NOAA), New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.
Last updated January 2012