RAYS / LIGHTNING / THUNDERS / GROUND

Reproduction of this text is authorized provided the holder is preserved and mentioned - Copyright Marcus Martins - PY4SM / PY2DD

CAUTION! Protect yourself! The rains are coming and BRASIL is the world champion in incidence of lightning!

Last summer the number of rays that hit BRAZIL was amazing and next summer the situation will be no different. This dramatically increases the risks for users of radios, telephones,
televisions , appliances, etc.  So we are addressing the issue . The increased incidence of lightning in BRAZIL during this last summer was staggering 150% over the previous year. 
As a result, studies conducted by the National Institute for Space Research have had more media coverage.

In fact we are world champions in rays !

About a hundred lightning strikes occur worldwide every second, which is equivalent to 9 million per day or 3 billion a year, occurring most of the continents in the tropics and during the summer. The extent and geographical position BRAZIL favor generators phenomena storms, making us the world champion in incidence of lightning. Annually, about 50 to 70 million rays reach the Brazilian soil, causing on average, the death of 200 people, injuring 1,000 and causing estimated damage at $ 500 million (fires, interruptions and fluctuations in the power grid, damage to systems telephony, etc.). The most affected region is the Amazon, followed by the Midwest and Southeast. Recently, there was a significant increase in those numbers BRAZIL. The reasons remain unclear, but the assumptions involve factors such as increasing pollution, soil sealing, broadcasting antennas proliferation (cell phone) and the end of the environmental phenomenon "La Niña", which resulted in an increase in average temperature in the country, resulting in increased number of summer storms. The severity of the problem led to Civil Defense Coordinator of the State of São Paulo to launch a manual with guidelines on how to prevent lightning. In ancient times, it was believed that the rays were punishments sent by angry gods. Only in the eighteenth century the phenomenon was scientifically explained by Benjamin Franklin (1706-1790) that in addition to political was also physicist and philosopher. Franklin enunciated the principle of charge conservation, discovered the electrical nature of lightning and invented the lightning rod. In the two centuries that followed, many researches have been made in the area of ​​meteorology and the phenomenon of generation of rays is now well known.

Storm clouds have height between 1.5 and 15 km, with very different internal temperatures. At the bottom, the temperature is close to the ambient (20 degrees C on average)while in the upper part can reach -50 degrees. This enormous temperature gradient generates very strong winds inside the clouds that, in turn, cause the separation of electrical charges due to friction between the ice particles existing at the top. Thus, the bottom of the clouds contains excess negative charges, while the upper, positive. By induction, the soil is no emergence of excess positive charges and settles a huge potential difference between cloud and ground, reaching millions of volts. Once defeated the insulating capacity of air between ground and cloud occur 30 to 40 successive electrical discharges approximately 0.01 seconds constitute a single radius.

Electric currents involved in this process vary from 10,000 to 200,000 amps, increasing the air temperature to up to 30,000 degrees Celsius, causing violent expansion and compression waves that can be audible a few kilometers away (thunder). The high currents and temperatures are responsible for fires, burns and deaths in accidents involving lightning. When a person is struck directly by lightning usually suffer instant death by carbonization. However, these cases are rare. Most often, the person is achieved indirectly by being at a distance less than 100 meters, may suffer cardiac arrest (35% of cases). Hundreds of people survive every year, after being indirectly hit by lightning, but unfortunately, many are left with severe sequelae (60% of survivors) such as heart problems, mental disorders and muscle paralysis.

The best form of protection is the surge arrester, consisting of a metal rod fixed to a high point and grounded through a thick thread. The region protected by this simple device has the shape of a cone whose diameter corresponds to twice the height from the ground to the top of the arrester. Because the electrical current always seek to drain the shortest way, the rays normally reach the highest points of a region. Thus the belief that lightning never reach twice the same place is absolutely false. A curious case is that of Ray Sullivan, a guard of US national parks, which has been hit seven times! Luckily survived all accidents, but with sequels. One should therefore avoid , during a storm , high places and open fields , swimming pools, beaches , soccer fields and isolated trees . In Brazil, one in four people killed by lightning was playing football .

Pools of users at risk even when lightning strikes the ground at a distance greater than 500 meters. This is because part of the current may flow for water pipes, traveling long distances to the pool. According to recommendations of the National Lightning Safety Institute (NLSI), water activities should be suspended and the people sent the safe haven, protected lightning rod, when a storm is closer than 10Km. This recommendation applies both for users of external and internal pools. But it was only after the discovery of electricity in the early 18th century, the electrical nature of Earth's atmosphere began to unravel. In 1708, William Wall, to see a spark out of a piece of amber electrically charged, she noted that she was like a lightning bolt. By mid-century, after the discovery of the first electrical properties of matter, it became evident that lightning should be a form of electricity, associated in some way with the storms. Benjamin Franklin was the first to design an experiment to try to prove the electrical nature of lightning. In July 1750, Franklin proposed that electricity could be drained in a cloud by a metal pole. If the mast was isolated from soil, and approach the same observer a grounded wire, a spark jump from the mast to the wire when an electrified cloud was near. Were this to happen, it would be proven that the clouds are electrically charged and, consequently, that lightning is an electrical phenomenon also. In May 1752, Thomas-François D'Alibard demonstrated that the suggestion of Franklin was right and that lightning were therefore an electrical phenomenon. In June 1752, Franklin conducted another experiment with the same purpose, his famous experiment with a kite. Instead of using a metal pole, he used a kite, since she could achieve higher altitudes and could be used anywhere. Again, sparks jumped from a switch placed at the end of the wire attached to kite toward your hand.

Also in 1752, LG Lemonnier repeated the experiment of Franklin with the metal mast, but instead of bringing a grounded wire, put a bit of dust to see if she would be attracted. He found that even when there were no clouds, a situation known as a condition of good weather, a weak electrification was in the atmosphere. It also found evidence that such electrification varied from night to day. In 1775, G. Beccaria confirmed the existence of diurnal variation of electrification in good weather condition and determined that the polarity of the electric charge in the atmosphere in these conditions was positive and she changed to negative when there were storms coming, in agreement with the observations of Franklin. In 1779, H. B. Saussure measured by the first load induced in a conductor immersed in the atmosphere. Its instrument, a precursor of the electrometer was to observe the separation between two small spheres suspended side by side by thin wires. In addition to confirming the results Beccaria Saussure discovered a variation of electrification in good weather condition as well as a variation with altitude. It believed that they could be explained by assuming that the air contained a positive charge.

In 1785, CA Coulomb discovered that air is conductive, noting that an isolated conductive object exposed to air gradually lost its charge. His discovery, however, was not understood at the time, since the gases were then regarded as insulators, and was completely forgotten. In 1804, P. Erman, in order to explain the observations Saussure, first suggested that earth must be negatively charged. In 1842, J. Peltier confirmed this idea and suggested that the load on the air should be originally from Earth, which in turn would becoming charged during their formation. In 1860, W. Thomson (aka Lord Kelvin) defended the idea that positive charges should exist in the atmosphere to explain their electrification in good time. He was also the first to recognize the electrification of the atmosphere as a manifestation of an electric field. In 1885, J. Elster and HF Geitel proposed the first theory to explain the structure of electrical storms. In 1887, W. Linss reached the same results obtained by Coulomb about 100 years ago, then the estimated earth lost almost all of its charge to the conductive atmosphere in less than an hour, unless the source loads were restored. This fact has given rise to what became known as the fundamental problem of atmospheric electricity, that is, as the negative charge of the earth is maintained. The first ideas to solve this problem only emerged in the following century. In 1889, HH Hoffert identified individual return strokes in a lightning into the ground using a primitive camera. In 1897, F. Pockels estimated first full output current of lightning into the ground, by measuring the residual magnetic field produced by lightning in basaltic rocks. Finally in 1899, J. Elster and HF Geitel discovered that the radioactivity is present in the atmosphere, establishing with it an explanation for the presence of ions in the atmosphere. The next discoveries about the atmosphere of electrification only emerged after the development of photographic and electrical instruments in the 20th century The basic physical laws to explain these findings are described by a set of equations known as Maxwell's equations, set out by JC Maxwell in 1865 .

The atmosphere of the earth can be divided into different regions based on different parameters. In terms of its temperature profile, the atmosphere is divided from the Earth's surface in the following regions: troposphere, stratosphere, mesosphere and thermosphere. The boundary between the troposphere and the stratosphere, where the temperature for decreasing and starts increasing with height is called the tropopause. The level of maximum temperature around 50 km (about 270 K) is called stratopause, and separates the stratosphere from the mesosphere. The level of minimum temperature around 80 km (about 180 K) is called the mesopause separates the mesosphere and the thermosphere. The temperature profile is variable with time and location, occasionally showing the troposphere thin layers within which the temperature increases with time, known as inversion. The height of the tropopause also depends on the time and place, in particular the geographical latitude. Below about 20 degrees latitude, it is typically located at about 15-18 km, whereas near the poles, it may at times be as low as 8 km. From the viewpoint of ion conductivity and the atmosphere can be divided into: lower atmosphere corresponding to the troposphere, middle atmosphere, corresponding to stratosphere and mesosfera and upper atmosphere, over 80 km, corresponding to termosphere.

The lower the average air atmosphere and are weakly conductive due to the presence of small concentrations of ions. In these regions, the ions are created by the ionization of neutral air molecules, generally molecules of nitrogen and oxygen, for primary and secondary cosmic rays and particles and radiation produced by the radioactive decay of substances in the soil, such as uranium and thorium, and air, such as radon gas. As a result of ionization of molecules, free electrons and positive ions are created. The electrons are then quickly connected to other neutral molecules producing negative ions. The production of ions by cosmic rays varies with altitude and latitude. The production of ions due to the decay of radioactive substances depends on soil characteristics. In particular, the oceans it is several orders of magnitude smaller than the continents. In general, the average ionization ratio (Production ion pairs) over land due to radioactive substances is predominant over that due to cosmic rays below 1 km. Over 1 km, the reason ionization is dominated by cosmic rays. The reason ionization is also dependent on weather conditions and geomagnetic and solar activity. Occasionally, the ionization created by energetic particles during high geomagnetic and solar activity periods can dominate on the ionization produced by cosmic rays above 20 km. Also the solar cycle of 11 years produces a variation in the ionization ratio in the atmosphere. This variation becomes more pronounced with increasing height and the increase of the geomagnetic latitude. After the ions are formed, they react with neutral molecules and relate the water vapor molecules of water always in the atmosphere, forming ion clusters. These clusters are relatively stable, and make up the majority of the ions molecular size, also called small ions. Examples of such ions are H 3 O + (H 2 O) and O2- (H2O) n. When ions are added small-particle aerosols they form large ions. In general, large ions are present in the atmosphere at lower concentrations than small ions, except in regions with high levels of pollution, where they can be more numerous.

During steady state conditions, the concentration of small ions in a moment and given location is the result of the balance between production (due to ionization) and the destruction of ions. Small ions are destroyed by recombination between them, and the combination with large ions and aerosol particles. The total average concentration of small ions on the continents and oceans are on approximately the same, and the order of 1000 cm-3, although the rate of ionization of the ocean is lower due to the absence of radioactive elements. This, however, is offset by the lower rate of destruction due to low concentration of aerosols. There are small positive ions to negative, and the difference is responsible for the existence of a net positive charge in the atmosphere. The existence of this net charge near the earth's surface means that an additional process should exist ions from the ionization process produces equal concentrations of negative and positive ions. One of these processes is called sporadic or corona discharge and is associated with large electric fields that occur near thunderstorms. As the electric field increases, the field around sharp objects reaches sufficient values ​​for the stiffness of the air break, producing small discharges in the atmosphere. As a result, a large number of ions of one polarity is injected into the atmosphere. Ions of one polarity can also be formed in the atmosphere near waterfalls (negative ions) and waves in the ocean (positive ions).

Unlike the lower atmosphere and the middle atmosphere, in the upper atmosphere exist beyond the negative and positive ions considerable amount of free electrons generated by the absorption of solar radiation by atoms and molecules. This process is called photoionization. Electrons can then join neutral molecules creating negative ions. The electrons and ions created by this process make the atmosphere a reasonable driver, forming a region called the ionosphere. Although it is possible to have areas of the ionosphere with excess negative or positive charges, such excess is very small compared to the total load so that the ionosphere can be considered neutral. In general, the number density of negative ions in the ionosphere is negligible and ionization can be described in terms of electron density. The electron density in the ionosphere varies considerably with the time of day, altitude, latitude, solar activity and other local effects. The greatest variation of electron density occurs throughout the day, depending on the variation of the solar radiation. At night, the photoionization is due to solar radiation scattered by hydrogen atoms of the outer layers of the atmosphere and is much lower than during the day.

Starting in the upper part of the ionosphere and extending upwardly magnetosphere is located, the region where the dynamics of the particles is governed by the Earth's magnetic field. Ions, protons and electrons originate in this region of the ionosphere and the solar wind, a stream of charged particles from the sun that reaches the Earth's atmosphere in the magnetopause, the upper limit of the magnetosphere. Inside the magnetosphere, charged particles are trapped by the magnetic field forming radiation belts around the Earth.

In the lower and middle atmosphere atmosphere, negative and positive ions move in response to electric fields. During their movement, they collide with neutral particles, which act so as to prevent movement. The ease of ions to move through the neutral particles is described by a factor called mobility, which depends on the mass and charge of the ions, neutral particles of density and temperature.

The atmosphere of the ability to conduct an electric current is expressed in terms of its conductivity. The conductivity in the lower atmosphere and middle atmosphere is isotropic, being given by the product of ion density, the charge of the ions and mobility. Only small ions contribute to the conductivity, since the mobility of large ions is several orders of magnitude smaller. In the lower atmosphere conductivities of negative and positive ions are exactly alike. The conductivity in the lower atmosphere and middle atmosphere increases with altitude. This variation is mainly due to the increased mobility with altitude as a consequence of decreased density in the atmosphere. The conductivity also varies with latitude due to the variation in the intensity of cosmic rays, and tends to be greater at higher latitudes. Near the Earth's surface, the conductivity presents variations in association with the presence of fog or pollution. In the upper atmosphere, the conductivity is anisotropic due to the fact the mobility of ions and electrons depend on the direction of the magnetic field. At 100 km altitude, the conductivity of air is 11 orders of magnitude greater than to that near the soil and approximately equal to the conductivity of the soil.

The conductivity of the atmosphere gives rise to a so-called relaxation time property, which is the time for the atmosphere shielding the load of an object immersed therein by a factor 0.37. The relaxation time, in good weather conditions, is given by the permittivity of air divided by conductivity. After about five times the relaxation time, the entire load of the object is shielded. Near the ground, the relaxation time is about 10 minutes. As the conductivity increases with altitude in the atmosphere, the relaxation time decreases. At 10 km, the relaxation time is about 1 minute.

The electric field of good weather is a consequence of the existence of a charge on the surface of the Earth and conductivity. Due to this charge is negative, the electric field is down. The continents, the average electric field is about 120 V / m. This corresponds to a surface load - 1.2 x 10-9 C / m2. Integrated on the surface of the Earth, this value results in a total load of 600 kD. An equal positive charge exists in the atmosphere. It is valid to note that this field is imperceptible in our lives. This is because virtually all things, including our body, being conductive compared to air.

Assuming an exponential increase in conductivity with altitude, it can be shown that the electric field decreases exponentially with altitude. At an altitude of 30 km, the electric field is as low as 300 mV / m. Integrating the electric field of the Earth's surface to the ionosphere results in a potential difference of approximately 200 kV. Near the ground the electric field shows wide variations caused by turbulent motions of charges in connection with the weather. Another mechanism for separating load near the surface is called the electrode effect. Due to the negative charge of the earth, negative ions in the atmosphere moving upwards. By not be substituted in the same proportion by negative ions generated by radioactive sources, forms a region of positive charges near the ground. In an atmosphere without spray, this region has a thickness of only a few meters, in which the electric field decreases by a factor of 2. The presence of aerosol makes this region is higher. On the water, this layer is also larger because of the absence of ions of radioactive sources.

The electric field of good weather has diurnal and seasonal variations. The typical diurnal variation of the field due to the universal time was first identified by the measures undertaken by the Carnegie ship in the 20's famous Carnegie curve is a result of electric field times the average values ​​taken over many days. Carnegie curve is very difficult to be reproduced in continental stations due to local processes such as convection currents and variations in aerosol concentrations. In general, fluctuations in load densities associated with these processes within the planetary layer has an effect on the electric field comparable to that of Carnegie curve. If local variations in continental stations are removed by medium, the electric field shows a dependency to universal time similar to that of Carnegie curve. The electric field of good weather also shows a seasonal variation. Although the variation follow the pattern of variation in universal time, there are small variations in the zone where the field is maximum, indicating changes in length of maximum storm activity. The average electric field also features seasonal variation with maximum values ​​in spring and summer in the northern hemisphere, indicating that there are more storms in these seasons in the northern hemisphere than in these same stations in the southern hemisphere. This, in turn, is a result of there being more land in the northern hemisphere.

The decrease of the electric field of good weather with the height must necessarily be accompanied by the presence of fillers in the atmosphere. If there are no load sources in the atmosphere, this variation is a direct effect of the variation of the conductivity with height. If the conductivity was uniform loads do not accumulate in the atmosphere and the electric field would be uniform. Almost every load in the atmosphere is below 30 km. Integrating the charge density on the surface atmosphere to the ionosphere (or, in practice 30 km), a total load of about 600 kD is obtained. The load on the earth's surface is also 600 kD to compensate for this load in the atmosphere.

In the atmosphere, under conditions of time, good constant current density is always present. Consequently, if there were a continuous source of load, after a certain time all the charge air would flow to the earth's surface canceling its load, so that an electric field does not exist in the atmosphere. This time was calculated to be much less than one hour. Therefore, the existence of an electric field in good weather conditions, or in other words, a constant current density implies that a continuous supply of loads must exist.

The origin of the current density in the atmosphere, which are always present and can be seen in any place, it was known in the early 20th century as the fundamental problem of atmospheric electricity. The first attempt to solve this problem was suggested by CTR Wilson in 1920. Wilson established the hypothesis, known as a theory of the spherical capacitor to the earth's surface, and an equipotential surface at some altitude should behave as a spherical capacitor plates. The equipotential layer was first called the electron cloud and was supposed to be located between 40 and 60 km. Later, she was considered to be coincident with the ionosphere. This spherical capacitor is charged to a potential difference of 200 kV. The loads between the plates would be moving toward the ground, constituting a leakage current. This current can be calculated by multiplying the current density in good weather conditions of the area of ​​the Earth's surface. This results in a total current discharge of the capacitor of about 1000 amperes. In order to keep the capacitor spherical loaded, the storm activity worldwide has been presumed to act as a current generator, and separating loads causing the transport of the positive charges for the earth's ionosphere at the same rate of 1000 Amperes.

Whereas there is something around 1000 storms always occurring, every storm would generate something like 1 Amp. If this theory is correct, there must be a relationship between the global storm activity (the intensity of the generator) and the electric field in good weather conditions. The similarity between the diurnal variation of the global storm activity and the Carnegie curve, with maximum and minimum values ​​about the same universal hours, was used to assign the diurnal variation of the electric field the global storm activity, being the strongest argument in for the theory of the spherical capacitor. The amplitudes of both the diurnal variations, however, are different. The amplitude of the variation of storm activity is about two times greater than the diurnal variation of the electric field. This difference in amplitudes is probably due to the variability of storms. The diurnal variation of storm activity has also been verified by radiation measurements produced by lightning, known as atmosférics or sferics. The theory of spherical capacitor can also be seen as a large electrical circuit including the surface of the Earth, the ionosphere and the atmosphere along with the storms. This circuit is generally known as global atmospheric electrical circuit. The theory is quasi static type, where the atmospheric electric field must be considered as a steady course (and not static) resulting from equilibrium between the process of generating the storm loads and loads annihilation process conditions in the regions of good weather. It can be applied to variation with periods longer than 10 minutes, which corresponds to a longer relaxation circuit which occurs near the ground.

The theory of spherical capacitor to the global atmospheric electric circuit, however, has several limitations. Although the conductivity of the ionosphere is quite high, it systematically increases with altitude, so that there is an equipotential layer. A more complete version of the global electrostatic atmospheric electrical circuit has been proposed without considering the existence of an upper layer equipotential. This fact makes possible the influence of ionospheric and magnetosféricos electric fields on the lower atmosphere, especially near the polar regions. Temporal variations in the global circuit are, however, predominantly associated with variations of storms, especially away from the polar regions. These variations may be related to changes in the total number of storms or variations in their characteristics. Temporal variations in the global circuit may also be associated with variations in the solar wind, through its modulation of the intensity of cosmic radiation. Variations in cosmic ray flux reaching the atmosphere can cause a considerable change in the electrical resistivity of the atmosphere above the storms, producing substantial changes in the circuit. The global circuit also presents annual and semi-annual variations in response to imbalances in areas with continents and oceans and semi-annual variations of air temperature in the tropics, presenting maximum values ​​at the equinoxes when the sun is centered on the equator.

Finally , it is possible that other generators not associated with storm may have a significant role in the overall circuit. Other electrified clouds, beyond the storm , is a possible candidate . However, due to the lack of a more precise knowledge of the electrical structure of these clouds, their importance remains elusive.

The radius is identified by two main features:

1 - thunder, the sound which is caused by the expansion of the air heated by the radius;

2 - Lightning, which is the bright light that appears in the way which the radius passed.

The rays occur because the clouds are loaded electrically. It is as if we have a large battery with a pole on the cloud and other pole on the ground. The "voltage" of this battery is applied between cloud and ground. If we connect a wire between the cloud and the ground will give a short-circuit the battery and pass a large electric current through the wire. The hell is this thread that binds the cloud earth. Under normal conditions, air is a good insulator of electricity. When we have a heavy cloud, the air between the cloud and the earth begins to conduct electricity because the "voltage" between the cloud and the ground is very high: several million volts (the "voltage" of the jacks is 110 or 220 volts). The beam causes the short-circuit the cloud to the ground and the path formed by the ray passes an electric current of thousands of amperes. A weak beam current is about 2,000 A, an average radius of 30,000 A and the strongest rays has current of more than 100,000 A (a shower has a 30 A current). Despite the current of lightning are very high, they circulate for a very short time (usually lasts less than a second distance) .The rays can exit the cloud to earth, earth to cloud or cloud and then out of the earth and meet halfway. Worldwide, there are about 360,000 per hour rays (rays 100 per second). Brazil is one of the countries in the world where they fall more rays. In the state of Minas Gerais, where they were made precise measurements of the number of rays falling on the ground, we have close to 8 rays per square kilometer per year. Many rays occur within the clouds. Generally this type of ray offers no danger for those who are on earth, yet it creates danger for aircraft. The rays fall on the higher points because they always try to find the shortest path between the cloud and the earth. Tall trees, towers, television antennas, church towers and buildings are points preferred by lightning.

Lightning is a very intense electric current that occurs in the atmosphere with typical duration of half a second and typical trajectory with a length of 5-10 kilometers. It is a consequence of fast-moving electrons from one place to another. The electrons move so fast that they make the air around you to light, resulting in a flash, and heating, resulting in a sound (thunder). Lightning is typically associated with cumulonimbus or thunderstorm clouds, although it can occur in association with active volcanoes, snow storms or even dust storms. Within storms, various ice particles become charged through collisions. It is believed that small particles tend to acquire positive charge, whereas the largest gain predominantly negative charges. These particles tend then to separate the influence of ascending and descending air currents and gravity, so that the cloud top acquires a net positive charge and the lower part a net negative charge. The charge separation then produces a large electric field both within the cloud and between the cloud and the ground.

When this field eventually breaks the electrical resistance of the air, lightning begins. In general terms there are two types of lightning: lightning cloud and the ground lightning. Lightning cloud originate within the cumulonimbus clouds, typically in the region where water droplets are transformed into ice, and propagate within the cloud (intra-cloud lightning) or out of the cloud, toward another cloud (cloud-cloud lightning) or in any direction in the air (discharges into the air). Lightning in the soil, in turn, may originate in the same or other regions within the cumulonimbus cloud (cloud-to-ground lightning) or on the ground, below or near the storm (ground-cloud lightning). Over 99% of lightning in the soil are cloud-to-ground lightning. Lightning ground-cloud are relatively rare and usually occur from the top of mountains or tall structures, or they may be generated by rockets fired towards the storms. Lightning in soil can also be classified in terms of the leading load signal, negative or positive, initiating the discharge. About 90% of cloud-to-ground lightning occurring on our planet are negative. This percentage, however, can change substantially in certain storms. About 70% of lightning are lightning cloud. Although they are most lightning, they are less well known that lightning in the soil, partly because they are less dangerous, in part because they are hidden by cloud. A rarer form of lightning is not included in the above categories are as ball lightning. A ball lightning is the name given to a luminous sphere that usually occurs near the storms, but not necessarily simultaneously to a normal lightning. They are, in general, red, yellow, blue, orange, or white, has a 10 a 40 cm diameter, appear near the ground or the atmosphere, and maintain a relatively constant brightness during their lifetime. They may move rapidly or slowly, or stand still may be silent or produce crackles, last from seconds to minutes (average of 4 seconds) and slowly disappear suddenly or silent or producing a noise. Although they have been observed for more than a century, they are not well known and remains a mystery.

Lightning may comprise one or more discharges, called return strokes. In the first case, it is called simple lightning and in the second, multiple lightning. Each discharge lasts return few hundred microseconds, and multiple flashes, the time interval between successive return stroke is typically 40 milliseconds. When the interval separating the return stroke is close to 100 milliseconds, the lightning is seen flashing in the sky, because the human eye can identify them individually. The following figures illustrate the various processes contained in a negative cloud to ground lightning (indicating the typical time intervals), accompanied by a detailed description of these processes. Other types of lightning in the soil have similar steps with small differences, mainly in relation to the initial process. Lightning in the cloud, however, present a different development and that is still not very well known. Almost nothing is known about the development of rare lightning, as ball lightning or lightning related to volcanoes, snowstorms or dust.

On the influence of the electric field established between the cloud and the ground, the negative charges (electrons) then move in steps of tens of meters long called leader of the steps. Each step has a typical duration of 1 microsecond, with a pause between them of 50 microseconds. After a few milliseconds, the stepped leader emerges from the cloud base, moving toward the ground. During the movement, some loads following new paths under the influence of loads in the surrounding atmosphere of the channel, forming the branches. The loads on the channel move towards the ground in stages with an average speed of about 100 km / s and producing a low brightness in a region with a diameter between 1 and 10 m along which the load is deposited. Most of the light is produced during steps 1 microsecond, with virtually no light during breaks. As loads leader propagate along the channel towards the ground, electric and magnetic field variations are also produced. Altogether, 10 carries a stepped leader or more load coulombs and reaches a point near the ground in tens of milliseconds, depending on the tortuosity of the way. The average current is leading the scaled about 1 kA and is transported in a core channel with few centimeters in diameter.

When the stepped leader channel approaches the ground, the electrical charge contained in the channel produces intense electric field between the end of the leader and the ground, corresponding to an electric potential of about 100 million volts. This field causes the air break stiffness close to the ground causing one or more ascending positive discharges, discharges known as streamers and leaders, leaving the ground, in general, the highest objects. The distance between the object to be achieved and the end of the leader in instant the conectante leader leaves the ground is called distance of attraction. The distance type tends to increase with the increase of peak discharge current return. The point of junction between the leader and the scaled conectante leader is usually considered to be in the middle distance of attraction. When one of streamers leaders find the negative downward leader, generally between 10-100 meters from the ground, the lightning channel is formed. Then, the charges stored in the channel start to move toward the ground and a wave propagates as a visible flare upward along the channel with a speed of about 100,000 km / s, one third the speed of light, illuminating the channel and all other branches. The wave velocity decreases with height. This discharge is called a return stroke lasts a few hundred microseconds and produces most of the light that we see. The light of the return stroke stems from continuous and discrete emissions of atoms, molecules and ions after being excited and ionized by the wave and moves up due to the fact that the first electrons to move down toward the soil are those closest to the ground. As electrons higher up in the channel move, the upper channel parts become visible. Due to the upward movement of light along the channel occur too fast to be able to be seen, the channel as a whole seems to be lit at the same time. The branches of the channel that do not connect to ground usually are not as bright as that of the channel below the junction of the branch. This is due to the fact that fewer electrons pass through them than through the channel. The light of the return stroke is usually white. However, just as the sun can have various colors, distant lightning may also have other colors such as yellow, purple, orange or even green, depending on the properties of the atmosphere between the lightning and the observer. The load placed on the channel, as well as those around and on top of the channel, moving down along the center of the channel in a region with a few centimeters in diameter, producing a peak in the soil medium current of about 30 -40 kA, varying from a few to hundreds of kA. Current measurements in manned towers has registered maximum values ​​of 400 kA. In general, the current reaches its peak in a few microseconds and half decay value from about 50 microseconds. The average negative charge is transferred to the ground of about 10 coulombs, with a peak around 200 coulombs. In the process, electrical and magnetic fields with temporal variations from nanoseconds to milliseconds are produced. These fields are generically called sferics. The waveform of sferics is similar to the form of the current wave, peaking at almost the same instant the peak current and a second inverted peak associated with the reflected field at the base of the ionosphere. At distances greater than 10 km of lightning, the peak of fields tends to decrease inversely with distance, in the absence of significant propagation effects.

For distances greater than about 50-100 km, the peak of the fields is significantly attenuated due to the spread on the surface not perfectly conductive earth. At the time of peak of the fields, the average total radiated electromagnetic power is about ten times greater than that in the optical spectrum. In general, the peak of the fields produced by lightning in clouds is less intense than those produced by lightning strikes the ground. In the frequency domain, the field has a maximum intensity around 5-10 kHz for lightning in the ground and around 100-200 kHz for lightning in the clouds. The return stroke also violently heats the air around you. The air reaches maximum temperatures of about 20,000 to 30,000 degrees Celsius in about 10 microseconds, corresponding to 1020 electron densities of electrons per cubic meter. When air is heated, it expands, and this expansion generates in a distance of a few hundred meters, supersonic shock wave and longer distances, an intense sound wave moves away from the channel in all directions. These waves are the thunder we hear. Thunder produced by lightning in the soil typically has a maximum intensity around 50-100 Hz, while those produced by lightning in the clouds has a maximum around 20-30 Hz. Next lightning, the sound is intense snap and can cause damage to the human ear. Distant lightning, the sound is relatively weak bang. The duration of Thunder is a measure of the difference between the distances from the nearest point and the farthest point of the channel to the observer. Typical durations are 5-20 seconds. Most thunder has bangs and creaks because the channel is bent, causing sound waves to reach the observer at different times and from different directions. Snaps can also be produced by branches. The greater the number of branches, the greater the number of crackling thunder. If lightning occurs at a distance around 100 meters from the viewer or less, he will hear a heavy snap like the crack of a whip (sometimes preceded by one click, similar to a click of fingers) which is associated with the wave shock that precedes the sound wave. Thunder produced by lightning in the soil in general can be heard at distances of 20 km. Produced by thunder lightning in clouds are similar to those produced by lightning strikes the ground, but are generally weaker. During periods of heavy rain and winds, this distance will be lower while in quiet evenings, thunder can be heard at greater distances. Part of the acoustic energy of thunder this concentrated at frequencies below those that the human ear can hear usually a few tens of Hz.

This part is called thunder infrassônico and is believed to be associated with changes in electrostatic energy inside the cloud upon the occurrence of lightning. Thunder can be used to calculate how far away lightning. When you see the lightning, start counting the seconds until you hear thunder. Divide the number of seconds for three (3) and you have the approximate distance of lightning in kilometers. The average error associated with this method is 20%. In part, the source of this error is due to the fact that most of lightning have long chain branching. Thus, a lightning three kilometers away can produce thunder after three seconds, indicating that a branch is only one kilometer away. If you see the flash and not hear the thunder, the lightning probably this more than 20 kilometers from you.

After the current return stroke traverse the canal, lightning can finish. However, in most cases, after an average pause 30-60 milliseconds, more charges are deposited on top of the channel discharges into the cloud, called K and J. The J processes process is responsible for a slow variation of the electric field soil with duration of about tens of milliseconds, while the process produces K type field variations pulses (called K variations) at intervals of a few milliseconds, with individual pulses with a duration of tens to hundreds of microseconds and electric field peak around ten times lower than those produced by return strokes. These processes are indicative cargo transport within the cloud. Since there is already ionized path of air produced by the stepped leader, another leader can spread into the soil by the channel. This leader is usually not scaled, but continuous and is called continuous leader. It approximates the soil within a few milliseconds, propagating at speeds of about 3000 km / s. It is not visible and usually has no branching. The leader lays continuous few coulombs load along the channel resulting from a current of about 1 kA. When continuous leader approaches the ground, one has again a return discharge, called discharge subsequent return, which normally is not as bright as the first discharge return, nor branched. The peak current subsequent return stroke is usually, but not always, lower than that of the first discharge return. The subsequent return stroke current also takes less time to reach its peak (approximately 1 microsecond) and the decay half value (about 20 microseconds) than the first return stroke. Consequently, the induced fields are also usually smaller in amplitude and has a shorter duration than the fields associated with the first return stroke. Sometimes, when the time after a return stroke is greater than 100 milliseconds, of the channel can be dissipated and a new leader that starts its path as a continuous leader can, after some time, change to staggered leader. In these cases, the leader is called solid-phased leader and reaches the ground at a different point from the previous leader. The subsequent return stroke then follows a different path in the atmosphere with respect to the first flush of return and the lightning has a bifurcated channel. About a quarter of lightning into the ground show this effect. This process leader / subsequent return stroke can be repeated several times, causing the lightning flashes in the sky at each new downloading return. All return strokes that follow at least partially the same channel are the same cloud-to-ground lightning. Then, a flash can be formed by up to tens of return strokes. The average number of return strokes in a negative cloud-to-ground lightning is about 3-5 and the maximum number ever recorded is 42. Often, a chain of 100 The order goes through the canal for several milliseconds or even tens or even hundreds milliseconds following the first discharge or return some subsequent return stroke.

This current is called the DC and typically carries 10 coulombs of charge to the ground. Continuous currents produce slow, intense field variations in electric field measurements near lightning and not visible continuous light channel. Sometimes, during the occurrence of direct current, the brightness increases channel for about 1 millisecond following a momentary increase in current, an M component called process variation The term M is used to denote the electric field variation accompanying occurrence of M. component.

Lightning in soil can also be started by leading positive progeny , i.e. , positively charged leader . In fact, positive leaders descendants correspond to upward movements of negative charges (electrons) . The resulting return stroke effectively transports positive charges from the cloud towards the ground. In this case , the lightning bolt is called positive . In general , there are no subsequent return strokes in positive lightning , that is, they are simple lightning. The average peak current of the return stroke positive flashes as well as the average load placed on the ground , however, are generally larger than the corresponding values ​​for return stroke of lightning negative , so that they usually cause greater damage than the negative lightning. A large proportion of forest fires and damage to power lines caused by lightning are due to positive lightning.

It is believed that lightning has a large effect on the environment. They were probably present during the emergence of life on Earth, and may even have participated in the generation of molecules which have given rise to life. Lightning causes fires participating, thus, the balance of composition of trees and plants. Lightning modify the atmosphere of the features around the areas where they occur. They break the air molecules, which recombine to produce new elements. These new elements change the chemical balance of the atmosphere, affecting the concentration of important elements with ozone and mingle with the rain and precipitate as natural fertilizers. Lightning play a role in maintaining the electric field of good weather in the atmosphere, which is a consequence of existing net negative charge on Earth and the net positive charge in the atmosphere. Lightning produce transient phenomena in the upper atmosphere known as sprites, blue jets and elves. These phenomena are weak lights almost invisible to the human eye that occur in the mesosphere, troposphere and the lower ionosphere, respectively. Sprites and blue jets of observations have been made with high sensitivity cameras and more recently by telescopes on top of mountains, pointed in the direction of storms hundreds of kilometers away. Lightning also play a significant role in maintaining the balance between waves and particles in the ionosphere and magnetosphere, acting as a source of waves.

During the last two decades, cloud-to-ground lightning have been detected and mapped in real time on large areas by several lightning detection system. Some countries, like the United States, Japan and Canada, are entirely covered by such systems. Over the United States, an average 20-30 million cloud-to-ground lightning has been detected every year since 1989, the year that such systems began to fully cover the entire country. Other countries such as Brazil, are partially covered. Rough estimates indicate that about 100 million cloud-to-ground lightning strikes occur in Brazil every year. Lightning has been generated by small rockets attached to long copper wire thrown in the direction of storms. When the rocket is launched, the wire attached to it is unrolled creating a conductive path through which the Lightning after started, spreads. This technique has allowed the measurement of electric and magnetic fields and near the lightning channel. Lightning have also been detected in space, during the last two decades by optical sensors on board satellites and spacecraft. The satellites can not distinguish between lightning in the ground and clouds. They have shown that about 50-100 lightning occur every second on our planet, mostly in the tropics (about 70%). Finally, spacecraft have shown that the Earth is not the only planet where lightning occurs. Lightning has also been detected on Venus, Jupiter and Saturn, and probably occur in Uranus and Neptune. The rays are dangerous? Yes. The rays bring a number of risks to people, animals, equipment and facilities.

Even before lightning fall hazard exists. Before falling a distance, the clouds are "charged electricity" and if below the cloud have, for example, a very long fence, the fence wires also will be "carried by electricity". If a person or animal touching the fence will receive an electric shock, which in some cases can be fatal. The electric shock occurs when an electric current flows through the body of a person or animal. Depending on the amperage and the time in which it circulates through the body, there may be various consequences: tingling, pain, violent contractions, burns and death. If a lightning strike directly on a person or animal, there is hardly salvation. In most cases people are not directly affected. When lightning strikes a fence or a building causes a current flow through the metal parts of the affected installation.

Even in case of a lightning strike on a structure that does not have metal , as, for example, a tree , a person near this tree can have a shock. The values ​​of the voltages and currents involved in radius are so great that it makes the tree behave as a conductor of electricity.

The rays can cause mechanical damage, such as cutting down trees or even start bricks and tiles of a house. One of the great dangers that the rays create are fires. Many forest fires are caused by lightning. In the case of silos and flammable material deposits, a lightning strike can cause catastrophic consequences.

The lightning current can cause burns and other damage to the heart , lungs , central nervous system and other body parts , through heating and a variety of electrochemical reactions . The extent of damage depends on the current intensity , the body parts affected , the physical condition of the victim, and the specific conditions of the incident. About 20 to 30% of the victims die lightning , mostly by cardiac and respiratory arrest , and about 70 % of the survivors for a long time suffering from serious psychological and organic sequelae . The most common consequences are decrease or loss of memory , decreased ability to concentrate and sleep disorders. In Brazil it is estimated that about 100 people die each year hit by lightning.

In order to avoid the above accidents, personal protective rules listed below must be followed. If possible , do not leave the street or not to remain on the street during storms unless absolutely necessary. In these cases , seek shelter in the following places :

a) non-convertible cars, buses or other non- metallic convertible vehicles;

b ) in houses or buildings have lightning protection ;

c ) in underground shelters, such as meters or tunnels;

d ) in large buildings with metal structures ;

e) on boats or closed metallic vessels;