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Meteorology

We are the champions

The number of lightning flashes that are concentrated in the metropolitan area has tripled

For a country that prides itself in not having active volcanoes, for not being in the pathway of hurricanes nor of being the target of large earthquakes, Brazil has, on the other hand a problem that falls from a great height with devastating frequency: lightning, above all from December until March when it rains intensely in the South and the Southeast. It is frightening the high incidence of electrical discharges in the country, estimated at 100 million, which is the highest in the world. The estimate, precarious and indirect, uses indices based on the number of days of storms within a region, the so called “”keraunic level” or thunder day level index. From it, a calculation provides a probable incidence of lightning flashes in the zone.

In the State of São Paulo, the last summer provided good starting material. A survey by the Atmospheric Electricity Group (Elat) of Inpe revealed that in the three regions researched by the Sidres project, the average number of discharges was three times greater (105,000) in the summer of 2000/2001 than in the previous summer (34,000). “The interesting thing is that this increase was not produced in a uniform manner throughout the state, but was concentrated in the metropolitan areas”, reveals Osmar Pinto Junior, the coordinator of Elat, which counts upon the support of FAPESP and of the National Council for Scientific and Technological Development (CNPq).

Motivated by the news of practically weekly deaths from lightning in cities at the beginning of the year, the survey used data from the detection system that covers the Southeast. Pinto Junior believes that the sample reflects the reality of the state, even without covering all of the territory. In the end, the results are the registrations of three large metropolitan areas, the Paraíba valley, São Paulo city and Campinas, which together correspond to close to 50% of the State territory.

In the three regions the amount of lightning increased, above all in the Paraíba valley, with five times more than in the summer 2000/2001 than in the previous one (50,000 lightning flashes as against 10,000) and in Greater São Paulo, where the incidence almost tripled (40,000 as against 15,000). In the metropolitan area of Campinas, the increase was much less: 15,000 discharges last summer as against 9,000 in the previous one. Why did the number of discharges increase in the state? There is no definite answer, but the specialist suspects that the villain is the increase in average temperature of last summer, close to 4 degrees Celsius warmer than the previous summer.

The hypothesis makes sense, since it is based on the fact that tropical regions have more storms and consequently more lightning than in temperate regions. It is simple. Where the temperature is higher, there is a greater evaporation of water, the raw material for the formation of clouds, among them the cumulus nimbus, that are the makers of lightning storms. For this reason, Pinto Junior believes that the extra heat of last summer was the main cause for the increase in lightning. This warmer summer could be a consequence of the phenomenon La Niña, which cools the superficial waters of the South Pacific Ocean and has repercussions on the climate throughout almost the whole world. “It could also be linked to the increase in the temperature of the Atlantic Ocean. However, in this case, we don’t have any data to justify such an association.”

Another impressive finding was that the increase in lightning was not uniform throughout the researched regions. It concentrated itself in the more urbanized zones, hotter because of the effect of “hot spots” and with a higher urbanization of the ground, while there was only a more discreet presence in the rural portions, which are less industrialized and cooler. In this way, the locations with the greatest amounts of lightning in Greater São Paulo were exactly those that were the hottest and basically were the following zones: downtown, north and east of the capital and the areas densely populated in the divisions with ABCD suburbs and Osasco. Also, in the regions of Campinas and the Paraíba valley, the most effected areas were the large cities: Campinas, São José dos Campos and Jacareí.

If a very hot summer provokes more lightning, there is consequently a tendency for a large part of this extra lightning to direct itself exactly to the “hot spots” of these already warmed environments? In reality, towards the large cities? “It is possible, but one cannot be certain of this. This theory, based on what happened during last summer in São Paulo, has a problem. It seems just to make sense to explain what happened during this specific period.” Pinto Junior recalls that during the two previous summers the correlation between lightning and metropolitan regions was not so clear. “It seems that there are other factors involved and it is possible that this correlation could indeed to true but only occurs in years with extremely hot summers.” In the milder summers the relationship would not be so direct.

More studies
The question requires lengthy studies on a large number of reports. “The quality of the data on lightning only became clearly trustworthy beginning with 1998. We cannot use older data.” If the excessive heat is still the main suspect for the extra lightning, Pinto doesn’t rule out other factors playing a role in large cities such as atmospheric pollution, though nothing so far has been proven scientifically.

During the year, up until the middle of May, there were 56 deaths due to lightning within the country, and of those 22 were in the State of São Paulo. “Unless there is a significant change, the estimate is that 120 Brazilians will be killed by electrical discharges this year. This is our historical average”, says Dr. Pinto Junior, author of the book Storms and Lightning in Brazil. In 1996, there were even more fatal victims, close to 150 in total.

For him, the fear of lightning has grown because many victims were in large cities where the incidence increased. “Only in the city of São Paulo, there were five deaths in January. Logically this calls the attention of the media even if there had been the same number of fatal victims in remote areas throughout the entire country. Death in an urban center is what really shocks people.”

The origin of “The Wrath of the Gods”
Humanity has always been looking for an explanation for lightning. Ancient peoples such as the Greeks and the Vikings, as well as the first followers of Buddhism, believed that the lightning bolt was a weapon of the Gods to punish human beings. In the second half of the 18th century, the North American Benjamin Franklin demonstrated that lightning rays are events of natural electricityIt is known that volcanoes in action, snow or sand storms and nuclear explosions can provoke lightning storms. However, the absolute majority of cases come from electrical storm clouds, the cumulus-nimbus type of cloud formation, which works like a hydroelectric power plant gliding along at a height of close to 5 kilometers and with a vertical extension of between 7 and 13 kilometers.

Formed from droplets of water, crystals of ice, and dust particles of various sizes, these clouds remain electrically charged due to internal collisions resulting from the strong ascending and descending currents of air. It is believed that the particles of smaller size acquire a positive charge and the larger particles a negative charge. Gravity and the air currents make these charges separate, creating in the cloud a positive pole at the top and a negative pole at the bottom. “The cloud begins to take on the function of a battery, which also has this electrical make up”, says Odim Mendes Junior.Conditions are thus produced so that a change in equilibrium generates a lightning ray in less than 1/50 of a second.

More than two thirds of the rays that reach the ground begin at the base of the cloud i. e. at the negative pole. Here, intensive electrical fields rupture the electrical insulation of the air causing accelerated electrons and ionized photons in the air. On propagating itself, the flux of electrons looks for the best way to re-establish the electrical equilibrium, often involving branching and twisting, until it hits the point of the surface where it forms the opposite pole (positive) to that of the original pole.

Charges of opposite polarity attract each other, and it is for this reason that at 50 to100 meters from the ground, the negative descending flux causes a positive flux to emerge from the electrically charged surface. The joining of the two completes a channel, creating the discharge of high intensity, in general of 30,000 amperes, and produces the lit up pathway. Less frequently, the rays originate from the top of the storm cloud and thus come from a region of positive charge. When they fall they drain electrons all the way down the channel all the way to the end of the passage, making a negative polarity flux ascend upwards.

Light and sound
The lightning ray is visible because the current produced by the cloud overheats and ionizes the air all the way down the channel generating a thread of light. Besides this, the abrupt expansion of this very hot air generates a shock wave that, on moving itself at different velocities, produces the sound that is typical of thunder. Inseparable companions, thunder and lighting are simultaneous, but it seems that the thunder happens after the lightning because the speed of light (300,000 km per sec) is greater than that of sound (330 m per sec). It has been calculated that observers situated at a distance of more than 20 kilometers from the ray manage to see it, but don’t hear the thunder until later.

There are two groups of lightning rays, those restricted to the atmosphere, more numerous, and those that hit the earth’s surface. In the first group are the intra -cloud lightning rays that happen inside a cumulus-nimbus arrangement and that clear the discharge within itself; the inter- clouds, that leaves one and enters another; as well as the one which throws the discharge into the air, into one side or upwards. In the second group, there is the cloud to ground rays, responsible for 99% of the occurrences that involve the ground, and the ground to cloud rays, of the reverse way and in much fewer numbers.

In terms of polarity, the lighting rays can have a negative charge, more than two thirds of the total, a positive charge or even be bipolar (very uncommon).It is necessary to document the very rare rays, such as those apparently disconnected and with an almost spherical formation, called lightning balls.

The Project
System of Lightning Detection in the State of São Paulo (Sidres) (nº 98/13650-1); Modality Regular line of research assistance; Coordinator Dr. Osmar Pinto Junior – National Institute of Spatial Research (Inpe); Investment R$ 35,530.00 and US$ 51,529.00

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