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Forest Fire in Canada > Forest Fire Facts and Questions Frequently Asked Questions About LightningIntroductionLightning is one of the most spectacular meteorological phenomenons and the most common severe weather event to affect people directly. This Frequently Asked Questions (FAQ) web site is intended to provide the reader with a basic knowledge of lightning and answer some commonly asked questions about lightning.
The Earth's Electrical StructureWhat
is the Earth's charge? What
charges the Earth? How many lightning flashes occur each day? If there is a fair weather electric field strength of 100 V/m, why
can't I set up two plates 10 centimetres (cm) apart, which could act like
a battery and power my Walkman? Thunderstorm Structure What is a thundercloud? How
are thunderclouds charged? Is there an association between lightning activity and radar echoes? Charge Generation in ThundercloudsHow do clouds get charged? For the gravitational theory to work, there must be some charge exchange
process between particles of different sizes. Charge can be exchanged
between particles in various states by inductive and non-inductive processes.
The most promising is the non-inductive exchange between ice crystals
and hailstones, referred to as the ice-ice process. Theories of thundercloud charge generation are still very speculative. The favorability of one process over another has fluctuated over time due to the inadequate number of laboratory experiments and scarcity of useful field observations. One clear conclusion is that there is no unique mechanism to generate the required charge under all conditions. For example, the ice-ice process, presently the most favored, does not explain warm cloud lightning, albeit a not too frequent event. As research develops, the most likely explanation will lie in a combination of factors. The Lightning FlashWhy does lightning occur? Is all lightning the same? Intracloud (IC) flashes, redistributing the charge within the cloud, account for over half the lightning flashes in northern latitudes (Uman and Krider 1989). Cloud-to-cloud and cloud-to-air flashes are less common. Aside from aviation, these three types of flashes have little effect on people. Cloud-to-ground (CG) flashes are very common and have been well documented. They exchange charge between the cloud and ground. These flashes affect people greatly, causing injury and death, disrupting power and communications, and igniting forest fires. Because of these effects, the cloud-to-ground flash has been the topic of much research. The cloud-to-ground lightning flash can lower positive (+CG) or negative (-CG) charge, depending on the source of the flash. This can be determined by the polarity of the stroke's current. Characteristics of negative and positive cloud-to-ground flashes are summarized below. Table 1. Some characteristics of positive and negative cloud-to-ground flashes
Ground-to-cloud flashes (those that originate from the ground) occur as well, as observed from large buildings such as the Empire State Building, but they are not normally distinguished from CG flashes in studies. Does lightning go up or down? To properly answer whether lightning goes up or down one must look at the processes involved in a lightning flash. The negative cloud-to-ground lightning flash can be broken down into three stages. The stepped leader, the return stroke, and the dart leader. The stepped leader is a small packet of negative charge that descends from the cloud to the ground along the path of least resistance. In its path, the leader leaves a trail of ionized gas. It moves in steps, each typically tens of metres in length and microseconds in duration. After a step, the leader pauses for about 50 microseconds, then takes its next step. The leader charge packet sometimes breaks up to follow different paths, giving lightning its forked appearance. As the stepped leader approaches the ground, electrons on the surface retreat from the leader creating a region of positive charge. Corona discharges (dielectric breakdowns in the air, also known as St. Elmo's Fire) are released from tall objects on the surface and reach out to the approaching leader. When the downward moving leader connects with a surface corona discharge, a continuous path between the cloud and the ground is established and a powerful return stroke is triggered. The return stroke rapidly moves as a wave upwards into the cloud following the ionized trail of the stepped leader, stripping the electrons from its path. After the return stroke, the lightning flash may end or, if enough charge in the cloud is collected, a dart leader may come down from the cloud following a direct path to the surface. In turn, the dart leader triggers a second return stroke. A single lightning flash can comprise several return strokes. The average number of return strokes in a lightning flash is 3 or 4, each stroke typically separated by 40 to 80 milliseconds. Are
lightning flashes positive or negative? Several studies have concentrated on the characteristics of the positive flash, but results are inconclusive due to the number of observations. The percentage of positive flashes appear to increase with latitude and with the height of local terrain. Also, positive flashes are more common in winter storms. The apparent cause of this is the lower freezing level, which places the positive charge center closer to the ground, thus increasing the likelihood of a flash. Positive flashes are more common in stratiform clouds, while negative flashes tend to occur in areas of strong convection. Also, thunderstorms that predominantly consist of negative flashes in their early stages often end with positive discharges as the storm matures and the anvil spreads out. A popular theory is that horizontal wind shears force a tilting of the dipole axis providing a route for the positive flash, but this has yet to be shown conclusively. Lightning DetectionHow is lightning detected? How are lightning maps made? Both methods work on the principle of detecting the electromagnetic signature of a lightning flash. The antenna's bandwidths are from 1 kilohertz (kHz) to 1 megahertz (MHz). The direction finder can discriminate cloud-to-ground flash from other forms of lightning or noise by the electromagnetic signature. When the stepped leader reaches the ground, the return stroke is triggered, producing a sharp voltage rise. This telling factor distinguishes a cloud-to-ground flash from other electromagnetic noise. The magnetic direction finder (MDF) senses the electromagnetic field radiated by a lightning flash using two erect, orthogonal wire loop antennas and a horizontal flat plate antenna. The radiated field of a lightning flash induces a current in the loops. The voltage signal measured in the loops is related to the flash's generated magnetic field strength by the cosine of the angle between the loop antenna and the direction to the flash. By comparing the voltage signals from the two loops, a direction to the flash can be determined. The flat plate antenna is used to resolve the 180 degree ambiguity associated with the calculations. In turn, the direction finder sends the data of each registered lightning flash to a position analyzer. The position analyzer triangulates data from direction finders to locate the position of a lightning flash. If the flash is in line with or directly between two direction finders (called the baseline), the position analyzer uses the ratio of the signal strengths as well. The time of arrival (TOA) detector consists of an array of four antennae. The direction of a lightning flash is determined by comparing the time each antennae senses the flash. Recent developments have allowed both systems to be merged into a single sensor called IMPACT. What is the National Lightning Detection Network? For further information on the US National Lightning Detection Network, go to the Vaisala-GAI web site. For information on the Canadian Lightning Detection System, go to the Environment Canada web site. Suggested ReadingFor this list, I have included general texts and magazine articles that provide a good background to the topic of lightning and atmospheric electricity. Most should be available at a college or university library. Most of the texts are showing their age, but clearly the most up-to-date and comprehensive books are by Dr. Uman. His book, All About Lightning, is a wonderful book written at perhaps the junior high school level. It answers specific questions at a general level, is full of pretty pictures, and is cheap. Uman's other book, The Lightning Discharge, is the definitive, academic publication on the subject. It covers all aspects of lightning and provides extensive references throughout. Chalmer, J.A. 1967. Atmospheric electricity. Pergamon Press, New York, NY. Golde, R.H., ed. 1977. Lightning Vol. 1. Physics of lightning. Academic Press, London. UK. Malan, D.J. 1963. Physics of lightning. The English Universities Press Ltd., London. UK. Mason, B.J. 1971. The physics of clouds. Clarendon Press, Oxford. Uman, M.A. 1969. Lightning. McGraw Hill, New York, NY. Uman, M.A. 1986. All about lightning. Dover Publications, Inc., New York, NY. Uman, M.A. 1987. The lightning discharge. Academic Press, Orlando, FL. Viemeister, P.E. 1972. The lightning book. MIT Press, Cambridge, MA. Williams, E.R. 1988. The electrification of thunderstorms. Scientific American 259(5). Literature CitedLopez, R.E.; William, D.O.; Ortiz, R.; Holle, R.L. 1990. The lightning characteristics of convective cloud systems in Northeastern Colorado. Pages 727-731 in 16th Conference on Severe Local Storms and Conference on Atmospheric Electricity, October 22-26, 1990, Kananaskis Village, Alberta. American Meteorological Society, Boston, MA. MacGorman, D.R.; Rust, W.D; Taylor, W.L. 1983. Cloud-to-ground lightning in tornadic storms on 22 May 1981. Pages 197-200 in 13th Confernece on Severe Local Storms, Tulsa, OK. American Meteorological Society, Boston, MA. Malan, D.J. 1963. Physics of lightning. The English Universities Press Ltd., London. UK. Mazur, V.; Rust, W.D.; Gerlach, J.C. 1983. Lightning flash density and storm structure. Pages 207-210 in 13th Confernece on Severe Local Storms, Tulsa, OK. American Meteorological Society, Boston, MA. Mazur, V.; Rust, W.D; Gerlach, J.C. 1985. Evolution of lightning flash density and reflectivity structure in a multicell thunderstorm. Pages 363-367 in 14th Conf on Severe Local Storms, Indianapolis, IN. American Meteorological Society, Boston, MA. Uman, M.A.; Krider, E.P. 1989. Natural and artificially initiated lightning. Science 246: 457-464. Kerry Anderson
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Last updated:
2005-08-31
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