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meteorology

The enigmas of the storms

Experiment with weather balloons investigates luminous phenomenon similar to lightning

An unprecedented experiment has begun that is literally going up to the clouds, with the purpose of unveiling one of the strange phenomena associated with lightning, the sprites – weak luminous effects that last an extremely short time, from 5 to 300 milliseconds, and appear during storms. At the end of November, a 40-meter high balloon left Cachoeira Paulista, half way between São Paulo and Rio de Janeiro, as part of a joint project carried out by the National Institute for Space Research (Inpe), and the American universities of Washington and Utah.

Before December 21st, another three unmanned balloons will leave their moorings and rise up to 30 kilometers in altitude, carrying instruments that will analyze the X-rays and visible light emitted by the sprites , a term that has not yet been translated into Portuguese. Monitoring the balloons from the ground or from planes, the researchers will try, for the first time, to get close enough – less than 100 kilometers, if the winds help – to characterize the intensity of the electrical and magnetic fields that these phenomena generate in a band of the atmosphere above the storms – which in the tropical regions are concentrated up to 15 kilometers from the ground – and below the zone in which sprites usually occur, in a layer of the atmosphere located between 40 and 90 kilometers above the ground, the mesosphere.

“The phenomenon, which manifests itself in a high region of the atmosphere, is always associated with lightning in the lower portion”, explains Osmar Pinto Júnior, from Inpe, and the coordinator of the Brazilian part of the project. “We intend to see what happens in this intermediate range.”

In a similar study, carried out in 1999 with balloons that flew over the states of Iowa and Texas (USA), an attempt was made to carry out this analysis, but without success, according to American physicist Robert Holzworth, from the University of Washington and one of the coordinators of the current study, financed by FAPESP and the National Science Foundation (NSF), of the United States. “The balloons stayed over 300 kilometers from the sprites. At that distance, it is impossible to carry out the measurements that we intended to take”, comments Holzworth, who was in Rio de Janeiro at the beginning of November, during the 3rd Brazilian Workshop on Atmospheric Electricity (WAE), organized by the Inpe’s Atmospheric Electricity Group.

If everything goes well, the balloons will give clues for reinforcing the theories that are trying to explain how the sprites are formed and that will make it possible to discover if these flashes of light, which are born during big storms, have different characteristics in the tropics, where 70% of the storms occur. At the moment, the studies of these luminous effects are concentrated in the United States, for a simple reason: nine out of every ten sprites appear over American territory, more precisely over the Midwest of the country, although astronauts in space shuttles have detected these electrical discharges in other regions of the planet – including near to Santa Maria, in Rio Grande do Sul, in 1994.

Aircraft and missiles
Besides bringing theoretical advances, knowing in detail the characteristics of these electrical discharges may in the future have an influence on commercial aviation, should hypersonic flights become a reality, according to Pinto Júnior. Today, civil airplanes travel at about 10 kilometers in altitude and at 900 kilometers an hour. With hypersonic aircraft, the intention is to arrive at speeds ten times higher, but, to do so, the aircraft will have to fly 20 kilometers from the ground.

This becomes important, since recent observations carried out with more sensitive cameras indicate that sprites occur as well – with a lesser intensity, it is true – at much lower layers in the atmosphere, at altitudes lower than 30 kilometers, a little above the storms. According to the researcher from the Inpe, studying the phenomenon also arouses a military interest, particularly in the United States, as it is believed that the radiation emitted by the sprites is similar to that of nuclear missiles.

Foreseen in 1925 by Scottish physicist Charles Rees Wilson (1869-1959), who received the Nobel prize for physics in 1927, these phenomena are regarded as recent: the team led by retired researcher Jack Winkler filmed a sprite for the first time only in 1998, during a night without any moonlight in the state of Minnesota (USA). Published in the early 90’s, the first scientific articles did not yet connect these phenomena to lightning. Accordingly, the variety of shapes and structures – columns or jellyfish, which reach as much as 30 kilometers in height and 40 kilometers in length – determined the choice of a name without any physical association. Today, it is now known that sprites generally appear when there is a storm of major proportions, associated with lightning that leaves the clouds for the ground.

One of the current theoretic models suggests tha tsprites are derived from the electrical field generated between the two poles that form inside a storm cloud – the positive one, close to the top of the storm cloud, and the negative one, at its base – and the consequent ionization of the insulating layer, formed just above the cloud. When there is a positive flash of lightning, this pole is temporarily emptied and as a consequence, the remaining charges spawn an electrical field in the mesosphere – hence the sprite .

But it may not be like that. If the electrical field alone is not capable of creating the ionization and creating the phenomenon, the other model gains strength, according to which the electrical field merely accelerates electrons arising from cosmic rays, which then ionize the medium and create the sprite. Should the experiments with the balloons detect the emission of X-rays, this latter hypothesis will gain strength, since these electrons emit this kind of radiation when braked by the atmosphere.

Heat and pollution
The meeting in Rio de Janeiro did not restrict itself to sprites . Some 200 physicists and engineers also discussed another phenomenon for which there are no clear answers: the incidence of lightning ten times more over the continents than over the oceans, in accordance with surveys carried out in the last few years with the assistance of the Tropical Rainfall Measuring Mission (TRMM) satellite, belonging to NASA, the American space agency. There is something else that intrigues: twice as many flashes of lightning are formed over cities than over uninhabited areas, according to recent surveys carried out by separate centers, such as the Inpe and the University of Texas. Two hypotheses are trying to explain these differences: pollution, or the temperature and humidity profile of the atmosphere.

Although there is no consensus, there are indications that the greater humidity of the oceans is responsible for less lightning, while the greater capacity of the ground for retaining heat – particularly in the urban centers with more than 100,000 inhabitants, where there is a concentration of asphalt and concrete – favors the formation of rising currents of warm air, or thermals, responsible for taking water vapor to the mesosphere, where it condenses into electrically charged clouds over the continents, where lightning originates.

The Inpe team, after crossing over the information from the satellites with the data from lightning sensors installed on the ground, produced the first painstaking map of electrical discharges that hit the ground in Brazil. This new map, according to Pinto Júnior, will be able to replace the one currently adopted by the Brazilian standards for protection against electrical discharges (NBR 5419), drawn up with far less precision: an observer would write down only the thunderbolts he heard in the course of a storm, which does not correspond to the number of flashes of lightning that actually fall in the region.

Presented for the first time at the event in Rio, the new map ranks Brazil as one of the world champions in the total occurrence of lightning in a year. It is calculated that between 50 million and 70 million electrical discharges hit the country every year – double the total detected in the United States, a country with similar dimensions. Even in the calculation for the relative figure for electrical discharges per square kilometer per year (flashes of lightning/km2/year), Brazil is not far behind the champions, the central African nations, far smaller, where the rate of flashes of lightning that strike the ground exceeds 20 flashes/km2/year. Over here, this figure goes to up to 16 flashes of lightning/km2/year, which is not a good thing, as it causes annual losses of up to US$200 billion, besides from 150 to 200 deaths.

The work has brought another novelty. The previous maps indicated the Amazon basin as the region of the country where the greatest incidence of electrical discharges by area during the year takes place. But not any longer. According to the new map, the areas most affected are the south of Mato Grosso do Sul and the western portion of Rio Grande do Sul – with rates of 16 flashes of lightning/km2/year. Precisely for preventing the damage caused by electrical discharges, the Inpe team over the last three years has developed a lightning detector.

The size of a 40 centimeter box, the equipment, produced and patented by a company called Indeleth, which fundspart of the research, sets off an audible and visible signal when it detects storms as far as 60 km away. Presented at the Rio meeting, it starts to be sold this month for R$1,500, half the price of the imported ones. Installed in parks and factories, it may help to save lives and prevent accidents.

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