The lightning

A detector of the lightning is an apparatus which makes it possible to collect the electromagnetic wave generated by a flash coming from a storm. The detectors of the lightning are used by the weather services

Types and principles of detection

There exist various systems of detection of the lightning:
Mill with field
A mill with field is an measuring instrument of a static electric field. In meteorology, this instrument allows, thanks to the analysis of the electrostatic field above him, to announce the presence of a cloud electrically in charge translating the imminence of the lightning.
The principle consists in measuring the alternating voltage created on an electrode alternatively masked and exposed to the field to measure directional Antennes
The most sophisticated detectors of the lightning comprise an antenna with horizontal plate and two coil antennae placed orthogonally (with 90 degrees one of the other) in the vertical. A magnetogoniometer detects the electromagnetic field emitted by the love at first sight, this field inducing a current in the loops. There is a relationship between the tension of this signal and the amplitude of the magnetic field which is equivalent to the cosine of the angle between the coil antenna and the direction of the love at first sight.
A comparison between the amplitudes of the signals in the two loops makes it possible to determine the axis direction of the flash. There remains however an uncertainty on the direction because a cosine of X and (X + 180) degrees gives the same result: one knows in which axis the lightning comes but not yet from which direction. To solve that, the apparatus uses the data of the antenna with horizontal plate.
This system can distinguish between the discharges cloud-ground and the other forms from the lightning or the interference by the electromagnetic signature. Indeed, the flash reaching the ground produces a very characteristic sudden electric impulse.
To discover the position of the flash, it is then necessary to find the distance to the reception antenna. There exist two ways:
Network of reception antennas: by triangulation of the directions and times of arrival of a signal with at least three antennas, one can deduce the position. Three antennas are necessary since the lightning is not a specific signal but comes from any point between the beginning and the end of the flash. Two antennas can thus note directions of different origins and a third antenna is necessary to confirm the source inside a given ray of resolution.
System with single antenna and which can be mobile: one finds then the distance by the analysis of the frequency and the amplitude of the signal.
By satellite
The detection of the lightning by artificial satellite is carried out by sweeping the zone of vision for the detection of the luminous flashes produced by the storms. One uses for that of the geostationary satellites like the GOES and METEOSAT which are located at approximately 36000 km of the Earth. At this distance, one can neglect the thickness of the atmosphere and the position can be deduced in latitude and longitude directly. It is however necessary to pay attention to the parallax introduced while going towards the poles.
Limitations
Networks
Antennas belonging to a network of detection of the lightning in China. This network can detect the flashes in three dimensions in the storms
The lightning does not occur in a single point but passes from a point to the other in the cloud, or between clouds, or even between a cloud and the ground. The emitted electromagnetic signal can come from anywhere along this way. An antenna can note the direction of the beginning of the flash and a second the end of this one what wants to say that the lines of triangulation will never meet or at the bad place. Moreover, there is seldom only one flash at the time of storms and of the flashes close but emitted at slightly different times can be taken for the same flash with only the direction obtained by two antennas.
So that a data is accepted, it is necessary thus that at least three antennas can make of it the triangulation inside a given margin of error. The distance, it, is calculated by the coordinated time of arrival of the electromagnetic signal, and not of the sound, between the three antennas. Indeed, one knows the speed of light to which the signal moves and while going up in the direction from where the three antennas note the signal, one must arrive at the same time of emission with the point of crossing (more or less the resolution). The networks of surface give indications uninterrupted on the position of the lightning with a resolution of less than 1 km in general.
In the case of a love at first sight cloud-ground, that is relatively easy since the side distance traversed by the flash is not very important. However, in the case of flashes between clouds, an antenna can determine the direction as being that of the cloud source whereas the others give the direction towards the receiving cloud or some share of other along the trajectory of the flash. The distance between these two clouds being able to be out of the margin of error, the data will be often rejected. One estimates at only 10% the quantity of flashes cloud-cloud for which the problem is solved what decreases the effectiveness of the networks of detection of surface. As this type of flash is very prevail at the beginning of the storm, the user thus will have notification of the formation of a late storm.
Single antenna
The detectors of the lightning to single antenna will collect all the flashes and will give them a position. However, this type of system leaves with the assumption a relation between the frequency and the reduction the amplitude the radio signal with the distance from the transmitter to draw the distance to the antenna from it. However, the lightning does not follow necessarily this standard. The direction will be good but the error of position can be large. Moreover, one weak signal close to the antenna can be interpreted like a strong signal much further.
Satellite
The satellite can note more exactly the position of the flashes and does not suffer from the problem of discrimination of the source of the lightning being a single sensor. NASA estimates at 95% its rate of effectiveness. However, its sensors must carry out a complete sweeping of the field of view before sending information to a terrestrial relay. The data are thus available only all the 5 or 10 minutes. Certain users cannot accept this kind of time.

Detector of the lightning and weather radar


Life cycle of a storm with the simulated reflectivities of a weather radar color

distribution of the electric charges and the lightning in and around a storm.
The detectors of the lightning are used in conjunction with the weather radars to detect the formation, the position and the potential of threat of the storms. The image of right-hand side shows the training cycle of a cumulonimbus:
The unstable air undergoes the push of Archimedes
The steam contained in the air condenses in droplets of clouds then in precipitations
When the point of precipitation is larger than than the ascending current cannot support, the rain falls and produces a downward draft.
The weather radars can follow the evolution of precipitations in altitude and close to the ground but cannot say if there were presence of the lightning. The detector of the lightning will give this indication. The lightning also will occur between the anvil and the ground in front of the storm (second image), where the radar does not see precipitations, the detector of the lightning will be able to give this information. Finally, according to the wavelength used, the signal received by the weather radar can be attenuated by a strong precipitation and storms located behind this rain is likely to be masked. The detector of the lightning being less affected by that, it will be used as independent system of detection.
The owners of evolution of the reflectivities and the loves at first sight will give indications to the meterologists on the structure of the storm, its intensity and its potential of violent time
For aviation, the use of detectors of the lightning on board apparatuses makes it possible to avoid the storms. The apparatuses of line are generally provided with radars meterologic in addition to these detectors.
The lightning is a natural phenomenon of disruptive electrostatic discharge which occurs when static electricity accumulates between clouds of storm or such a cloud and the ground. The potential difference electric between the two points can go up to 100 million volts and produces a plasma during the discharge, causing an explosive expansion of the air by heat emission. While being dissipated, this plasma creates a flash of light and the thunder.
The lightning tends to strike the areas of high-altitude and the objects prominent. The thunder can resound of a dry cracking when the flash is close or thunder with far. Like the light travels more quickly than the sound, the flash is visible before the thunder is not audible.
Life cycle of a storm: strong upswing at the beginning and going down then. What creates the favorable conditions with the transport of the electric charges
The clouds of storm (cumulonimbus) create the weather conditions favorable to the accumulation of electric charges and consequently to the creation of a giant condenser:
A difference in important temperature between the bottom and the top of the cloud, inducing violent one displacements air volume
The presence of various particles like ice and dust which by effect triboelectric will facilitate the wrenching or the addition of electrons, according to the sign
Air (and all that it contains) being electrically charged, it is created in the cloud of the different zones with electric potential: negative at its base and positive at its top. It follows a very important electric field.
The electrification of the cloud of storm is based on two phenomena: gravitation and convection.

Gravitation

The drops of rain, let us hail them and the particles of grésil (of small grains of ice) fall by gravity to the bottom of the cloud, below the water drops and the crystals of ice of lower size which remain in suspension. When the large particles enter in collision with the crystals of ice at a temperature lower than a limit criticizes, around -15°C, the grains of grésil take care negatively, and positively if this temperature is higher than the aforementioned limit. As the grains fall more quickly than the crystals, they transport from the higher zones of the cloud, where the temperatures are lower than -15°C, of the negative charges downwards. The threshold of the -15°C exceeded, those become positive. One then obtains a tripolar structure of the cloud with a median layer charged negatively surrounded by two positive layers. However the shocks between particles are not alone at the origin of the electrification of the cloud.

Convection

The free ions in the atmosphere are collected by the droplets in the cloud which are then moved in the vertical currents created by the mechanism of the convection. This produced of accumulations of different loads according to altitude in the cloud.
On the one hand the cosmic rays strike the molecules of air located above the cloud and ionize them: these negative ions are fixed at the crystals and the droplets of the cloud and form a layer called screen layer in top of the cloud. In addition, the intense electric field in the vicinity of the pointed objects on the surface of the Earth produces a Corona discharge of positive ions: when the potential of the pointed object is sufficient, an intense electric field produces the excitation of the electrons bordering. Those enter then in collision with atoms neutral, which releases then from new electrons which go, in their turn, to create other electrons and so on, causing a chain reaction. It is the electron avalanche or ionization collision. The positive ions created are then pulled by the hot air rising by convection and thus take part in the electrification of the cloud. The positive sub-base of the cloud being rather fine, it is the negative layer which will have an influence on the Earth. Indeed, at the time of a storm this one takes care positively by influence.

Discharge


When this electrostatic field exceeds the dielectric limits of the air (variables according to the conditions of moisture and pressure), it follows the discharge of the lightning aiming to one rebalances electrostatic:
the tracer or precursor, transporting a weak electric charge, advances towards a zone of load opposed at a speed about 200km/s, thus creating an ionized channel. In the case of a negative discharge, this precursor progresses by jumps lengths proportional to the amplitude of the discharge. It is this phenomenon which the lightning conductors try to exploit.
The arc-backs start then successively they use the channel of the precursor to release the electric charges accumulated at a speed then being able to exceed 100000km/s.

Color

Along the traversed way, the gases are overheated and ionized (the temperature can reach there 30000°C, five times that of the surface of the sun) and thus form a conducting plasma, which explains the sudden emission of light that one observes. This luminous phenomenon is called flash. The color of this flash depends on several factors: density of current, the distance from the observer to the flash and various particles present in the atmosphere. However, in general, the color of the flash is white in an air dry, yellow in the presence of a great quantity of dust, red in the event of rain and blue in the presence of hail.

Frequency



World chart with the frequency of the lightning. It is noticed that the equatorial zones are those where the discharges are most frequent.
The lightning is the object of statistical studies because there are many differences in characteristics (amplitude, duration, many arc-backs) according to the flash (intra-cloud, cloud-ground, positive, negative).
50% of the loves at first sight have an intensity lower than 50000 has and 99% lower than 200000 A. Environ 60% of the discharges are will intra or inter-cloudy, one estimates at 32 million the number of flashes striking the ground each year in the world.
The frequency of the loves at first sight is defined starting from the level keraunic (many times where the thunder was heard in the year) and especially of the density of blasting (many loves at first sight to the km ² per annum). This last mode of quantification can be supplied with means of measurement, the detectors of the lightning: mill with fields, directional antennas and sensors by satellites.
One sees in the image of right-hand side that the rate of the lightning is generally connected to the latitude and the proximity of moisture. Thus the equatorial zones have the greatest densities, particularly the coastal areas. One should not be surprised about it since the storms which produce the lightning are generated by an instability of the atmosphere and a moisture of low level. Thus the equatorial zones are likely more to be hot and wet at the year that the polar zones.
Naturally, the conditions on a synoptic scale also organize the convection. It is not everywhere at the equator that the conditions are favorable to the formation of the storms. Thus, the zone of intertropical convergence, where the trade winds converge, gives rising necessary for the formation of rather continual storms but in the north and the south of this one there is a downward movement of the air which releases the sky.

Thunder

The lightning is accompanied by an acoustic wave, the thunder. This wave is generated by the brutal dilation of the air overheated by the electric arc. It can consist of a sharp snap or a deaf bearing according to the distance separating the listener from the lightning.

Other effects

The lightning can be accompanied, in the cases of strong discharges, secondary luminous phenomena in high-altitude. The brevity of these flashes, like their altitude (mésosphère and ionosphere), pushed back their discovery by the scientists at these last decades.

Outdistance

The speed of sound allows a good approximation of the distance which separates an observer from a flash. In the air, with atmospheric pressure and 15°C, the sound traverses 340,88 meters into 1 second. Thus, the duration which separates visual perception from a flash (practically instantaneous since the light moves with 300000 km ⁄ s) auditive perception of the thunder, makes it possible to calculate the distance which separates the observer from the flash.
Naturally, the pressure and the real temperature of the air will change this value but very little under the normal conditions. At the point of origin of the flash where a plasma is found, this variation is significant on a very short distance but that is negligible on the total way traversed by the sound. What can be more important in this approximation is the stability of the air. Indeed, the sound disperses under unstable conditions and carries further under stable conditions. This wants to say that it is very possible to see a flash without hearing the thunder and thus not to be able to calculate the distance to the storm. Thus, the estival storms occur in unstable air and there is a limit with the perception of the thunder. In the case of the winter storms occurring above a stable layer of inversion of temperature, the sound will be reverberated in altitude by this layer and will generally not be perceived on the ground but if it can penetrate it, it will carry very far.

Various types of the lightning

Intra-cloudy and inter-cloudy flashes
The provision of the electric charges in the storm, as explained before, creates potential differences between the top, the center and the base of the storm. When the potential is sufficiently large, the air between these various levels is not rather any more insulating and a breakdown occurs. The lightning then generated can occur between the various parts of the cloud or nearby clouds.
As these layers are more close in general between them than they are to it ground, this kind of flashes will be the first to be occurred. As the storm takes vertical extension and that the potential increases, the lightning cloud-ground will take the top without never being only. The change of proportion between the type inter ⁄ intra-cloudy and cloud-ground is thus an indication of the developmental stage of the cumulonimbus.
Flashes cloud-ground
There exist two types of the lightning cloud-ground: either descendant (top of the cloud towards the ground) or or ascending (ground towards base of the cloud). The type going down is most frequent but a swing of this type towards the ascending type is often indicative violent time because the cloud is then particularly developed. The ascending type also often occurs in front of the cloud itself, because it leaves the anvil what can surprise people who think themselves safe from seeing the storm in the distance. The ascending type is the most frequent in the case of love at first sight on structures great height (turn, pylon).

The lightning in ball.

The lightning in ball, or the globular lightning, is a phenomenon occurring sometimes with the impact (in very rare front cases). It is in general appeared as a luminous sphere of variable size (of the centimetre to several tens centimetres in diameter). The observations bring back various colors (white, reddish, sometimes yellow.) and a very different lifespan according to the cases, but generally at most a few seconds. Still today knowledge about it is rather fragmentary. It was tried many times to reproduce it in laboratory, such as for example according to the chemical theory imagined by the New Zealand researchers John Abrahamson and James Dinnis, without however bringing real explanation to the phenomenon. The first artificial fireballs would have been created by Brazilian scientists in this way:
An electric arc created between two electrodes vaporizes pure silicon.
While cooling, the silicon cloud contracts.
Silicon combines with oxygen in air. The chemical reaction releases from energy giving an estimated temperature of 1700°C to these fireballs which generally rotate just above the ground then disappear.

Dangers

There are approximately 2000 storms in the whole world at every moment. These storms produce between 30 and 100 flashes cloud-ground a second or approximately 5 million flashes per day.
The dangers of the lightning are defined by:
Direct effects (thermoelectric): the circulation of a very strong electric current overheats the matter and causes mechanical damage often very important, even spectacular. Each year, in France, approximately 2 million loves at first sight is recorded by the systems of detection, and nearly 250 bell-towers are more or less seriously damaged by the "fire of heaven" which also causes between 15000 and 20000 fires
Indirect effects (electromagnetic): the induced current of the lightning on the one hand a tension of common mode (U = IH + L dI/dt) and an electromagnetic field of an exceptional intensity. It follows the generation of very powerful parasitic electric impulses, which are mainly causes some in the damage according to the statistics. These parasites are indeed enough to degrade sensitive electronic materials (television sets, computers, etc) even if the flash is distant. If the flash is closer, the parasite can also destroy more resistant materials (lamps, engines, furnaces).
Conduction: Why the cows fear they it the lightning, the direct blasting of animals (or people) is very rare. However, when the lightning strikes the ground, the electric charges are dissipated in the ground whose electric potential becomes more or less important according to the nature of the ground (its resistivity) and the distance to the impact. The potential difference (tension) between two points is all the more important as the variation is large (amplitude of a step), for a given resistivity. The more important this tension is, the more the current which can then circulate by the lower extremities is important. This phenomenon is called tension of step, higher for a cow directed in the direction of the ray of a circle whose center is the impact, that for an human being.
The lightning is like exit of a perfect generator of current. One of the methods of protection is thus to facilitate the circulation of the electric charges towards the ground by means of nonfunctional drivers.
The lightning conductor will facilitate the way of the channel the lightning by effect of point. The lightning conductor will be effective on the condition of being in the presence of a love at first sight going down whose precursor advances by successive jumps, which is the case in 90% of the loves at first sight. It is, then, very important to ensure an electric continuity of great capacity to the ground.
This process does not guarantee the interception of an electric arc, which can fall right in the vicinity. For this reason, the significant industrial premises are equipped with many points and conducting ropes. It is also advised to carry out the interconnection of all the conducting parts present at the accesses (for example water pipelines) with this circuit of descent of the lightning.
A good device external of protection of an installation against the lightning consists of three components:
A device of capture, which can take several forms: tended wire, lightning conductors with stem or lightning conductors with priming device, drivers with a grid These devices must be dimensioned, according to the level of protection wished, by the method of the fictitious sphere deduced from the model electrogeometric of the lightning, so that an impact the lightning occurs preferentially on the device and not on the installation to be protected.
An earth electrode, made up of a network of naked and buried drivers, in intimate contact with the ground, which must make it possible to disperse "easily" the currents in the ground. With this intention, these drivers must have a low ground resistance, which makes it possible moreover to limit overpressures likely to appear on the external electric connections which penetrate in the installation to protect.
Drivers of descent, which ensure the junction between the device of capture and the earth electrode.
This whole of drivers must be inter-connected correctly and durably.
However, the installation of a lightning conductor does not take into account the indirect effects of the lightning on an installation. The flow of the current the lightning on the drivers of the device generates an intense impulse magnetic field which can even disturb to destroy certain components of the electrical installation of the building to be protected. Several solutions can be planned in order to limit these effects:
to move away the drivers from capture and descent from the installation to be protected, since the magnetic field radiated by a driver is inversely proportional to the distance compared to this driver
to multiply these drivers so as to divide the currents: one thus reduces the levels of fields near the drivers and if the current is well distributed around the installation to protect, one also obtains an effect of compensation of the magnetic field created by each driver
to increase the attenuation suitable for the structure of the installation, for example by an improvement of the electric continuity of reinforcement in the case of reinforced concrete constructions (welding of the crossings and iron overlappings) so as to constitute a better electromagnetic screen
to improve the equipotentiality of the metal masses of the installation to limit the potential differences induced, by inter-connecting the various conducting elements of the installation (metal, pipeline beams water, frame of the cupboards and electrical equipment) by means of bonding strips for example
to bring a treatment particular to the wiring of the installation: to place the cables closest to the metal masses (beams for example) or on metal cable shelves connected to the mass at their two ends, which makes it possible to reduce surfaces of the loops of masses and thus the parasitic tensions induced to the inputs ⁄ outputs of electrical equipment.
In the electrical communications, one prevents that the lightning falling on the electric lines is not propagated inside station while installing above the electric drivers of these lines of the earth wires, which in addition to their role of support of communication (they contain fiberoptics), play a part of lightning protection. Beyond this primary protection, the protection of electrical installations against the overpressures produced by the lightning on the active drivers of the electric connections is carried out by the use of components surge protectors (lightning protectors, spark-gaps with gas, thermisters, Transil diodes) the purpose of which are to short-circuit the parasitic impulses walking on on the electric connections by deriving the major part of energy from the impulse directly towards the ground. The good wiring of these components is essential with their effectiveness. The length and the position of the cables play a central role indeed.

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