Lightning detection

Remote observation of lightning strikes

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 first such device was invented in 1894 by Alexander Stepanovich Popov. It was also 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 along with an analysis of the characteristic frequencies emitted by lightning. Ground based systems can use triangulation from multiple locations to determine distance, while mobile systems can estimate distance using signal frequency and attenuation. Space based detectors on satellites can be used to locate lightning range, bearing and intensity 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 (EUCLID), the Institute for Ubiquitous Meteorology (Ubimet) and by other organizations like electrical utilities and forest fire prevention services.

Each system used for lightning detection has its own limitations. These include

A single ground based lightning network must be able to detect a flash with at least three antennas to locate it with an acceptable margin of error. This often leads to the rejection of cloud to cloud lightning, as one antenna might detect the position of the flash on the starting cloud and the other antenna the receiving one. As a result, ground based networks have a tendency to underestimate the number of flashes, especially at the beginning of storms where cloud to cloud lightning is prevalent. Ground based systems that use multiple locations and time of flight detection methods must have a central device to collect strike and timing data to calculate location. In addition, each detection station must have a precision timing source that is used in the calculation. Since they use attenuation rather than triangulation, mobile detectors sometimes mistakenly indicate a weak lightning flash nearby as a strong one further away, or vice versa. Space based lightning networks suffer from neither of these limitations, but the information provided by them is often several minutes old by the time it is widely available, making it of limited use for real time applications such as air navigation.

Lightning detectors and weather radar work 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. Air is moving upward due to instability. Condensation occurs and radar detects echoes above the ground (colored areas). Eventually, the mass of raindrops is too large to be sustained by the updraft and they fall toward the ground. 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 farther 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 also be 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.

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. Strikefinder can detect and properly display IC (intracloud) and CG (cloud to ground) strikes, as well as being able to differentiate between real strikes and signal bounces reflected off the Ionosphere. 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).