# lightning

Lightning is an electrical discharge, it can occur during thunderstorms, volcanic eruptions, nuclear reactions, forest fires where the dust creates a static charge or it can be triggered in a lab. Lightning is the sudden release of built-up charge stored in an electric field, though exactly what triggers it remains a mystery. There are many different types of lightning:

* cloud-to-ground, this is the archetypal lightning bolt, one that arcs out of the sky and strikes the ground with a flash of light.

* cloud discharge, this is lightning that occurs within a thundercloud, between two thunderclouds, or from a thundercloud to the air. Cloud discharges are certainly more common than the cloud-to-ground variety: 10 or more cloud flashes may occur before the first one that strikes the earth.

* ball lightning, there are not many scientific documentations such as videos, or other recordings of ball lightning, so experts have had to rely on eyewitness accounts, which have been numerous. Judging from such accounts, balls of lightning are typically between a golfball and a basketball in size, about as bright as a 60-watt light bulb, and often red, orange, or yellow in color. Observed shooting through the air, across the ground, and or even into houses, they are fleeting, generally lasting a few seconds before vanishing gradually or abruptly.

*blue jet, blue jets shoot upward from the tops of thunderclouds. This remoteness, and the fact that they last but a few hundred milliseconds at most, perhaps accounts for why they were not discovered until 1994. The color of sapphires, they are cone-shaped in structure and extend for many miles. Like sprites and elves, blue jets provide a mechanism for energy transfer from lightning and thunderstorms to regions of the atmosphere between thunderclouds and the lower ionosphere.

*red sprites, red sprites occur above large thunderstorm systems and are generally associated with larger positive cloud-to-ground flashes far below. They are most luminous very high up in the atmosphere, between altitudes of about 25 and 55 miles. Yet even at their most luminous, they are very hard to see, in part because they last for only a few thousandths of a second. Red in color and often bearing faint bluish tendrils extending downward, sprites come in several shapes, designated by colorful names like “carrot,” “angel,” and “columniform.”

*elves, like celestial halos, elves are circles of light that appear some 50 miles or more above thunderstorms. Triggered by lightning flashes far below, these ephemeral discs spread out radially across the bottom of the ionosphere in the briefest instant, expanding up to hundreds of miles in diameter in less than a millisecond. Experts believe elves are caused by lightning processes that accelerate electrons in the lower ionosphere.

* volcanic lightning, lightning-like discharges are sometimes observed during volcanic eruptions, with no thunderstorm anywhere nearby. Hundreds or even thousands of feet in length, these bolts can flash to the ground or remain entirely within the ash cloud above the volcano. Here, lightning flashes during an eruption of Japan’s Sakurajima Volcano in 1991.

How lightning initially forms is still a matter of debate. Scientists have studied root causes ranging from atmospheric perturbations, wind, humidity, atmospheric pressure, to the impact of solar wind and accumulation of charged solar particles. Ice inside a cloud is thought to be a key element in lightning development, and may cause a forcible separation of positive and negative charges within the cloud, thus assisting in the formation of lightning.

Of course, most lightning occurs inside thunderclouds but there are three parts to a storm – above, within and below a storm, and while scientists have been able to measure above and below a storm, it is still a mystery what happens within the strom – within the cloud. Several ideas have been suggested, including colliding raindrops, localized regions of concentrated charge, and avalanches of high-energy electrons initiated by cosmic rays from outer space.
The exact arrangement of charge in the clouds has not been determined, but one model hypothesizes that the upper regions of the clouds have a strong positive charge, the center has a strong negative charge, and the lower regions have a weak positive charge. This is based on the idea that heavier and larger particles tend to gain a negative charge while lighter particles tend to gain a positive charge in collisions. The charged particles then separate due to the differences in size and density, moving to certain levels of the cloud system. This model has been demonstrated consistently in laboratory simulations of the inside of a thunderhead. The amount of total charge and polarity is also affected by the temperature in the layer of the cloud, the content of the water particles, and several other conditions. This theory is the most widely accepted, although it is just one of many which attempt to explain the properties of the charge buildup in electrical storms.

The thunderstrom configeration of a positive charge above negative charge, is called a positive dipole. Wilson (1920) proposed a current flowing from the tops of thunderstorms to the upper atmosphere to supply the ‘fair weather current’ – fair weather is when the electrical state of the lower to middle atmosphere is in quasi-static equilibrium, meaning that the charge moving into a region equals the charge leaving the region – a simplified definition of fair weather would mean no thunderstorms around. Above the tops of the storms, a net positive current flows towards the electrosphere. Blakeslee (1989) discovered conduction currents averaging 1.7 A, with a maximum of 3.7 A.

Thunderclouds are a consequence of atmospheric instability and develop as warm, moist air near the earth rises and displaces the colder, denser air above. Thunderclouds are large atmospheric heat-engines with water vapour as the primary heat-transfer agent. They increase the local stability of the atmosphere and are believed to maintain the atmosphere’s electrical potential relative to the earth.

Earth has a magnetic field that permeates the atmosphere and extends above the atmosphere into space. The ionosphere is the region in the upper atmosphere where there are enough electrons and ions to make the atmosphere a reasonably good conductor. Solar radiation at extreme ultraviolet frequenceies is absorbed into the ionosphere through the process of photoionization. In fair weather the electrical state of the lower to middle atmosphere is in quasi-static equilibrium (the charge moving into a region equals the charge leaving the region).

Coulumb (1795) discovered that air is conductive. Peltier (1842) stated that the Earth is negativeley charged. Finally, Wilson (1920) completed the circuit concept by proposing that “a thunder-cloud or shower-cloud is the seat of an electromagnetic force which must cause a current to flow through the cloud between Earth’s surface and the upper atmosphere.” Positive and negative ions move in opposite directions under the influence of an electric field, so current flows in the atmosphere whenever an electric field is present. In fair weather atmosphere the relationship between currents and electric fields is given by Ohm’s law:

$\mathbf{J} = \sigma \cdot \mathbf{E}$
where J, the current density, equals conductivity times the electric field.

In order to get a conventional spark, the kind that a spark plug makes, the electric field needs to surpass the conventional breakdown field, the point at which air loses its insulating properties and becomes capable of conducting electricity. For air, this is about 70,000 volts per inch at sea level. Thunderstorms can also generate big voltages.The voltages produced by the resulting charge separation are impressive, sometimes exceeding 100,000,000 volts.

The leader of a bolt of lightning can travel at speeds of 60,000 m/s, and can reach temperatures approaching 30,000 °C, 54,000 °F.

Lightning produces current that can be divided into those that directly transfer charge to Earth via ground flashes and those that are internal to the strom generator via cloud flashes. Livingston and Krider (1978) estimated that ground strikes produced an average current density of 3 nA m-2, resulting in a current of 3.5 A. Krehbiel (1981) estimated each cloud flash involves about 50-100 nA m-2 or a total current of 0.1-0.7 A.

There are over 16 million lightning storms every year.

At any time there are 1,000 thunderclouds continuously in progress over the surface of the earth. Solar heating warms the surface of the earth with a thermal input of 1kW m-2 and as the earth rotates around the sun new thunderclouds from in the subsolar area so that a wave of thunderstorms move westward every day.

Geological evidence of thunderstorms dates back 250 million years and scientists belive that thunderstorm activity has been taking place since the devlopment of the earth’s atmosphere – in fact lightning played a significant role in the modification of the early atmosphere and to the origin of life on the planet.

Active thunderclouds can extend from 3km in diameter to greater than 50km. Distrubances have lasted more than 48 hours and moved more than 2,000Km. The distance (in kilometres) to a lightning flash may be estimated by dividing the time delay (in seconds) between the flash and the thunder by 3. (If you hear thunder where the time delay is less than 30 seconds, you should find shelter.)

Because scientist are not yet sure how lightning gets started, I have included below one theory that speculates that incoming cosmic rays from outer space serve as the trigger.

Sources:

http://www.pbs.org/wgbh/nova/sciencenow/3214/02.html

The Electrical Nature of Storms,  Donald MacDormand – Oxford University Press, USA

Lightning,  Rudolf Golde