Uncommon and more destructive, positive rays of lightning account for only 5% of such rays. In Brazil, these rays of lightning prevail in the Southeast, the country’s most closely monitored region. Data collected over the last three years by sensors of the Brazilian Network of Atmospheric Discharges/BrasilDat and field measurements conducted during the last two summers portray a distinct and worrisome situation in another part of the country. In the west and north of the State of Rio Grande do Sul and in the west of the State of Santa Catarina, the formation of positive lightning is up to five times more frequent and discharges of this kind account for 25% of the total number. “Similar rates are known in only two other places on our planet: in the American Midwest and in Japan,” says Osmar Pinto Junior, coordinator of the Atmospheric Electricity Group/Elat of the National Aerospatiale Research Institute/Inpe), in the city of São José dos Campos, São Paulo State. The institute is at the forefront of research studies on rays of lightning. “We have to pay attention to this special situation in the South,” he says. Forest fires and damage to the electric power network caused by lightning are generally blamed on these positive discharges.
Some specific characteristics render these positive rays potentially more dangerous. Although not an absolute rule, the intensity of the electric current tends to be higher than that of the negative discharges. In extreme cases, the discharge can be even ten times higher and correspond to 300 thousand amperes. Even though the intensity of the current is not necessarily stronger, the discharge lasts for a longer time on the ground. “The positive bolts hit the ground and last for hundreds of milliseconds,” says physicist Marcelo Saba, from Elat, who is participating in field work that chases electric discharges. In the municipal regions of Uruguaiana and Santa Rosa in the State of Rio Grande do Sul, researchers caught positive rays of lightning that transferred electricity to the ground for more than 500 milliseconds, twice the average amount of time of a negative ray of lightning. Long-lasting discharges are more destructive than shorter ones, even though both kinds have the same electric current. The air around such a powerful ray of lightning can achieve, for fractions of a second, temperatures that are five times higher than those on the solar surface.
Comprised of droplets of water and particles of ice in different sizes, storm clouds of the cumulonimbus kind, which are the origin of most rays of lightning, can be compared to a battery. Because of the action of updrafts, downdrafts and gravity, these particles collide, become electrically charged and separate into two poles: the smaller, positively charged ones (crystals) remain on top, while the bigger, negatively charged particles (hail) remain on the bottom. This system is balanced, because electric charges with different polarities attract each other. Storm clouds give rise to rays of lightning when some kind of imbalance in this system causes the electric field produced by all these charges to go beyond the insulating capacity of air at a given point inside the cloud. This is when a ray of lightning is formed. Approximately 70% of the rays remain inside the cloud or in the atmosphere, while only 30% find their way to the ground. If the ray of lightning going down to the ground is negatively charged, this ray will look for the best pathway in the atmosphere that will lead it to a positively charged point on the ground, and vice versa. As everybody knows, opposites attract each other.
This brief explanation on how electric charges are distributed in the cumulonimbus clouds makes it relatively easily to understand why the occurrence of positive rays that hit the ground anywhere in the world is not as frequent as the occurrence of negative rays. As the lower part of the clouds is closer to Earth’s surface, the negatively charged discharges are easier to observe and are a common occurrence on days when it is raining heavily. Most of the rays originating at the top of these clouds, where the positive charges are found, remain inside the clouds at a distance of 5 to 20 kilometers above the ground. However, some of these discharges that originate on the top of the clouds escape from the thundercloud and hit the ground. As they come down, the positive rays burn through the air as if though they were a single light and electric wire, usually without the branches that are commonly seen around the negative discharges. They can travel through relatively long distances in the atmosphere and fall at a distance of more than ten kilometers from the cloud that generated them.
There is no scientific evidence that the higher occurrence of positive rays in the south of Brazil is a recent atmospheric phenomenon or is linked to the climate changes apparently taking place on Earth. “We do not have the historical series of the occurrence of the discharges in the State of Rio Grande do Sul, but I don’t think that this is anything new,” says Pinto Junior, whose research studies were partially funded by a theme project run by FAPESP. This region has probably been prone to this phenomenon for many years, maybe even for many centuries, and nobody had ever noticed it. Until late 2006, no reliable methods existed to measure this kind of event in the country’s southern region. Since that time, however, the scope of BrasilDat, which had been limited to the southeastern states, has included the southern states. That is when the first detailed information on lightning in this part of Brazil began to be generated.
Researchers suspect that the higher occurrence of positive discharges in the region of Uruguaiana and Santa Rosa might be connected to a local climatic characteristic: the collision of cold and dry fronts coming from Argentina with the hot and humid fronts coming from the Amazon. The result of these collisions, which also occur in Argentina, Paraguay and Uruguay, are heavy thunderstorms which theoretically might set off an abnormal percentage of positive discharges. There are some signs indicating that storms in the south have become heavier in the last ten years, with higher amount of rain and lightning concentrated in a few hours. But it is still too early to relate one thing to the other. And now, in the middle of spring, a team from Elat, equipped with electric field sensors that produce 8 thousand images per second, is working in Santa Maria, a town in the middle of the State of Rio Grande do Sul, to observe, on site, more positive rays of lightning. “Since 2003 we have filmed roughly 1,500 rays in Brazil,” says Saba. “But only 50 were positive.”
National Lightning Ray Monitoring Project (Pronar) (nº 03/08655-4); Modality Theme Project; Coordinator Osmar Pinto Junior – Inpe; Investment R$ 673.089,30