Lightning detection

Lightning detector at the Kennedy Space Center in Florida.

A lightning detector is a device that detects lightning produced by thunderstorms. There are three primary types of detectors: ground-based systems using multiple antennas, mobile systems using a direction and a sense antenna in the same location (often aboard an aircraft), and space-based systems.

The device was invented in 1894 by Alexander Stepanovich Popov. It also was the first radio receiver in the world.

Ground-based and mobile detectors calculate the direction and severity of lightning from the current location using radio direction-finding techniques together with an analysis of the characteristic frequencies emitted by lightning. Ground-based systems use triangulation from multiple locations to determine distance, while mobile systems estimate distance using signal frequency and attenuation. Space-based lightning detectors, on artificial satellites, can locate range, bearing and intensities by direct observation.

Ground-based lightning detector networks are used by meteorological services like the National Weather Service in the United States, the Meteorological Service of Canada, the European Cooperation for Lightning Detection, and by other organizations like electrical utilities and forest fire prevention services.

Limitations

Each system used for lightning detection has its own limitations.[1] These include:

Lightning detectors vs. weather radar

A thunderstorm life cycle and associated reflectivities from a weather radar
Distribution of electric charges and lightning strikes in and around a thunderstorm

Lightning detectors and weather radar are used together to detect storms. Lightning detectors indicate electrical activity, while weather radar indicates precipitation. Both phenomena are associated with thunderstorms and can help indicate storm strength.

The first image on the right shows the life cycle of a thunderstorm:

The cloud must develop to a certain vertical extent before lightning is produced, so generally weather radar will indicate a developing storm before a lightning detector does. It is not always clear from early returns if a shower cloud will develop into a thunderstorm, and weather radar also sometimes suffers from a masking effect by attenuation, where precipitation close to the radar can hide (perhaps more intense) precipitation further away. Lightning detectors do not suffer from a masking effect and can provide confirmation when a shower cloud has evolved into a thunderstorm.

Lightning may be also located outside the precipitation recorded by radar. The second image shows that this happens when strikes originate in the anvil of the thundercloud (top part blown ahead of the cumulonimbus cloud by upper winds) or on the outside edge of the rain shaft. In both cases, there is still an area of radar echoes somewhere nearby.

Aviation use

Large airliners are more likely to use weather radar than lightning detectors, since weather radar can detect smaller storms that also cause turbulence; however, modern avionics systems often include lightning detection as well, for additional safety.

For smaller aircraft, especially in general aviation, there are two main brands of lightning detectors (often referred to as sferics, short for radio atmospherics): Stormscope, produced originally by Ryan (later B.F. Goodrich) and currently by L-3 Communications, and the Strikefinder, produced by Insight. Lightning detectors are inexpensive and lightweight, making them attractive to owners of light aircraft (particularly of single-engine aircraft, where the aircraft nose is not available for installation of a radome).

Personal lightning detectors

A StrikeAlert personal lightning detector.

One type of lightning detector is the battery-operated personal lightning detector. Similar in size to a pager, personal lightning detectors are popular among golfers, campers, law enforcement, sports officials, and other persons who work or recreate outdoors. Personal lightning detectors function by detecting the electromagnetic pulse emitted by a lightning strike. By measuring the strength of the detected EMP, the device can then estimate how far away the detected strike was. When exposed to multiple detected strikes, some personal lightning detectors can even calculate and extrapolate the direction of the storm's movement relative to its position (approaching, departing, or stationary).

Although personal lightning detectors are well able to detect nearby lightning, they are quite basic in functionality when compared to professional lightning detectors. For example, they cannot tell where a lightning strike was located or from which direction the lightning is approaching, only that lightning is in the area. Also, since a personal lightning detector is triggered by EMPs, interference from other EMP-emitting devices (such as electronic equipment, appliances, fluorescent lights, and even car engines) can sometimes result in either false alarms or missed strikes. This interference often has the additional effect of preventing personal lightning detectors from functioning properly while indoors.

Professional-quality portable lightning detectors

Ray counter in a Museum Patio

Inexpensive portable lightning detectors as well as other single sensor lightning mappers, such as used on aircraft, have limitations including detection of false signals and poor sensitivity, particularly for intracloud (IC) lightning. Professional-quality portable lightning detectors improve performance in these areas by several techniques which facilitate each other, thus magnifying their effects:

However, since RF signals and light pulses rarely occur simultaneously except when produced by lightning, RF sensors and light pulse sensors can usefully be connected in a “coincidence circuit” which requires both kinds of signals simultaneously in order to produce an output.[2] If such a system is pointed toward a cloud and lightning occurs in that cloud, both signals will be received; the coincidence circuit will produce an output; and the user can be sure the cause was lightning. When a lightning discharge occurs within a cloud at night, the entire cloud appears to illuminate. In daylight these intracloud flashes are rarely visible to the human eye; nevertheless, optical sensors can detect them. Looking through the window of the space shuttle in early missions, astronauts used optical sensors to detect lightning in bright sunlit clouds far below. This application led to development of the dual signal portable lightning detector which utilizes light flashes as well as the “sferics” signals detected by previous devices.

The improvements described above significantly extend the detector’s utility in many areas:

Lightning range estimation

When an RF lightning signal is detected at a single location it is possible to determine its direction using a crossed-loop magnetic direction finder, but it is difficult to determine its distance. Attempts have been made using the amplitude of the signal, but this does not work very well because lightning signals have considerable variation in intensity. Thus, using amplitude for distance estimation, a strong flash appears to be nearby and a weaker signal from the same flash – or from a weaker flash from the same storm cell – appears to be farther away. You can tell where lightning will strike within a mile radius using this device, electrons in the air help aid this machine in the acurracy of this prediction.

To understand this aspect of lightning detection it is necessary to know that a lightning “flash” generally consists of several strokes, a typical number of strokes from a CG flash is in the range 3 to 6 but some flashes can have more than 10 strokes. [10]:18 The initial stroke leaves an ionized path from the cloud to ground and subsequent “return strokes”, separated by an interval of about 50 milliseconds, go up that channel. The complete discharge sequence is typically about ½ second in duration while the duration of the individual strokes varies greatly between 100 nanoseconds and a few tens of microseconds. The strokes in a CG flash can be seen at night as a non-periodic sequence of illuminations of the lightning channel. This can also be heard on sophisticated lightning detectors as individual staccato sounds for each stroke, forming a distinctive pattern.

Single sensor lightning detectors have been used on aircraft and while the lightning direction can be determined from a crossed loop sensor, the distance can not be determined reliably because the signal amplitude varies between the individual strokes described above, [10]:115 and these systems use amplitude to estimate distance. Because the strokes have different amplitudes, these detectors provide a line of dots on the display like spokes on a wheel extending out radially from the hub in the general direction of the lightning source. The dots are at different distances along the line because the strokes have different intensities. These characteristic lines of dots in such sensor displays are called “radial spread”. [11] These sensors operate in the very low frequency (VLF) and low frequency (LF) range (below 300 kHz) which provides the strongest lightning signals: those generated by return strokes from the ground. But unless the sensor is close to the flash they do not pick up the weaker signals from IC discharges which have a significant amount of energy in the high frequency (HF) range (up to 30 MHz).

Another issue with VLF lightning receivers is that they pick up reflections from the ionosphere so sometimes can not tell the difference in distance between lightning 100 km away and several hundred km away. At distances of several hundred km the reflected signal (termed the “sky wave”) is stronger than the direct signal (termed the “ground wave”). [12]

The Earth-ionosphere waveguide traps electromagnetic VLF- and ELF waves. Electromagnetic pulses transmitted by lightning strikes propagate within that waveguide. The waveguide is dispersive, which means that their group velocity depends on frequency. The difference of the group time delay of a lighting pulse at adjacent frequencies is proportional to the distance between transmitter and receiver. Together with the direction finding method, this allows to locate lightning strikes by a single station up to distances of 10000 km from their origin. Moreover, the eigenfrequencies of the Earth-ionospheric waveguide, the Schumann resonances at about 7.5 Hz, are used to determine the global thunderstorm activity.[13]

Because of the difficulty in obtaining distance to lightning with a single sensor, the only current reliable method for positioning lightning is through interconnected networks of spaced sensors covering an area of the Earth’s surface using time-of-arrival differences between the sensors and/or crossed-bearings from different sensors. Several such national networks currently operating in the U.S. can provide the position of CG flashes but currently cannot reliably detect and position IC flashes. [14] There are a few small area networks (like Kennedy Space Center's LDAR network, one of whose sensors is pictured at the top of this article) that have VHF time of arrival systems and can detect and position IC flashes. These are called lightning mapper arrays. They typically cover a circle of 30–40 miles diameter.

See also

References

  1. ↑ Richard Kithil (2006). "An Overview of Lightning Detection Equipment". National Lightning Safety Institute. Retrieved 2006-07-07.
  2. ↑ Brook, M., and N. Kitagawa (1960). "Electric-Field Changes and the Design of Lightning-Flash Counters". Journal of Geophysical Research 65 (7): 1927–1930. Bibcode:1960JGR....65.1927B. doi:10.1029/JZ065i007p01927.
  3. 1 2 MacGorman, Donald R.; Rust, W. David (1998). The Electrical Nature of Storms. Oxford University Press, NY. ISBN 0-19-507337-1.
  4. 1 2 Williams, Earle R. (1995). "Meteorological aspects of thunderstorms". In Volland, Hans. Handbook of Atmospheric Electrodynamics, Vol. 1. CRC Press, Boca Raton. ISBN 0-8493-8647-0.
  5. 1 2 Williams, Earle R. (1985). "Large scale charge separation in thunderclouds". Journal of Geophysical Research 90 (D4): 6013. Bibcode:1985JGR....90.6013W. doi:10.1029/jd090id04p06013.
  6. ↑ Yoshida, Satoru; Takeshi Morimoto; Tomoo Ushio & ZenIchiro Kawasaki (2009). "A fifth-power relationship for lightning activity from Tropical Rainfall Measuring Mission satellite observations". Journal of Geophysical Research 114. Bibcode:2009JGRD..114.9104Y. doi:10.1029/2008jd010370.
  7. ↑ Vonnegut, Bernard; Moore, C.B. (1957). "Electrical activity associated with the Blackwell-Udall tornado". Journal of Meteorology 14 (3): 284–285. Bibcode:1957JAtS...14..284M. doi:10.1175/1520-0469(1957)014<0284:EAAWTB>2.0.CO;2.
  8. ↑ Vonnegut, Bernard and James R. Weyer (1966-09-09). "Luminous phenomena in nocturnal tornadoes". Science 153 (3741): 1213–1220. Bibcode:1966Sci...153.1213V. doi:10.1126/science.153.3741.1213. PMID 17754241.
  9. ↑ Rutledge, S.A., E.R. Williams and T.D. Kennan (1992). "The down under Doppler and electricity experiment (DUNDEE): Overview and preliminary results". Bulletin of the American Meteorological Society 73 (1): 3–16. Bibcode:1992BAMS...73....3R. doi:10.1175/1520-0477(1992)073<0003:TDUDAE>2.0.CO;2.
  10. 1 2 Uman, Martin A. (1987). The Lightning Discharge. Academic Press, N.Y. ISBN 0-12-708350-2.
  11. ↑ WX-500 Stormscope Series II Weather Mapping Sensor User’s Guide (PDF). BF Goodrich Avionics Systems, Inc. 1997. pp. 4–2, 4–7.
  12. ↑ Golde, Rudolf H. (1977). Lightning, Vol. 1. Academic Press, N.Y. p. 368. ISBN 0-12-287801-9.
  13. ↑ Volland, H. (ed): "Handbook of Atmospheric Electrodynamics", CRC Press, Boca Raton, 1995
  14. ↑ Murphy Martin J., Demetriades, Nicholas W.S., Cummins, Kenneth L., and Ronald L. Holle (2007). Cloud Lightning from the U.S. National Lightning Detection Network (PDF). International Commission on Atmospheric Electricity, 13th International Conference on Atmospheric Electricity, Beijing.

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