With an ultra-fast video camera, and the asset of being in the right place at the right time, physicist Marcelo Saba, a researcher at the National Institute for Space Research (Inpe), and doctoral student Diego Rhamon obtained an unprecedented image of a lightning strike, showing details of its connection to various lightning rods located in the immediate vicinity.
The rarity of the image caused it to be reproduced on the cover of Geophysical Research Letters (GRL), one of the most important scientific journals in the area. The work was supported by FAPESP.
“The image was captured on a summer night in São José dos Campos. [ SP ]when a descending ray of negative charge approached the ground with the speed of 370 kilometers per second [ km/s ]. At the moment when the discharge was just a few tens of meters from the ground, several lightning rods and ledges of buildings located in the region produced upward positive discharges, competing to connect with the descending lightning. The final image before the connection was obtained 25 millionths of a second before the impact of lightning on one of the buildings”, says Saba. It was this spectacular image that the editors of GRL reproduced on the cover of the publication.
The researcher reports that his camera captured 40,000 images per second. rotated in super slow motion, the video shows how lightning rods behave. And also that lightning can be a danger if these protective equipment are not correctly installed.
This is because, despite there being more than 30 lightning rods nearby, the lightning did not connect with any of them, but with the chimney of an oven located on the roof of one of the buildings. “An installation failure left this area unprotected. And the impact of a current of 30,000 amperes produced impressive damage,” he says.
On average, 20% of lightning is constituted by exchanges of electrical charge between the clouds and the ground. The remaining 80% is composed of electrical discharges inside the clouds. Of those that touch the ground, almost all are descending rays: they start in the cloud and come to the ground.
Upward rays also exist, but are rare. And they only happen from tall structures like mountain tops, skyscrapers, towers and antennas. Depending on the charge they transfer to the ground, lightning can be further classified as negative or positive.
“Lightning can reach up to 100 km in length. And carry currents of around 30,000 amperes. This is equivalent to the current drawn by 30,000 100-watt bulbs working together. In some cases, the current can reach 300 thousand amperes. The temperature of lightning, 30,000 °C, is five times greater than the surface temperature of the Sun,” says Saba.
How rays form
The researcher explains that it all starts with the electrification of the clouds. Its mechanism is still not entirely known. But it stems, roughly speaking, from the friction between ice particles, water droplets and hail, which releases charges and creates polarities between different regions of the clouds, with differences in electrical potential ranging from 100 million to 1 billion volts.
“It is necessary to take into account that storm clouds are huge structures, which have their base 2 or 3 km from the ground and whose top can reach up to 20 km in altitude. Their diameters range between 10 and 20 km”, he says.
The branched form assumed by the rays is explained by the fact that the electric charges seek the easiest path, that is, that offers less resistance, and not the shortest path, which would be the straight line.
The easiest path, usually zigzag, is determined by different electrical characteristics of the atmosphere, which is not homogeneous. “A beam composed of several discharges can last up to 2 seconds. However, each discharge only lasts for fractions of a millisecond,” adds Saba.
He points out that the lightning rod neither attracts nor repels lightning. Nor does he discharge the clouds as formerly thought. It simply gives lightning an easy and safe path to the ground.
As it is not always possible to have the protection of a lightning rod and summer is the time when most atmospheric electrical discharges occur, it is convenient to consider Saba’s recommendations.
“Storms happen more in the afternoon than in the morning. So beware of outdoor activities on summer afternoons. When you hear thunder, seek cover. Never stand under trees or poles. Not even under poor covers. If you don’t have a sturdy place to protect yourself, stay in your car and wait out the storm. If there is no car or any other place to take shelter, squat down with your feet together. Never standing, never lying down. Inside the house, avoid contact with appliances and the use of wired telephones.”
The researcher claims that a person struck by lightning can survive. And there are many examples of this. Chances increase when the person is promptly attended to. “Cardiac arrest is the only cause of death. In this case, the recommended care is cardiopulmonary resuscitation, ”he teaches.
Saba started the systematic study of lightning with high-speed cameras in 2003. This study, which is still in progress, provided the largest video bank of lightning filmed at high speed in the world. The researcher and his advisees have already been contemplated with 17 grants or scholarships provided by FAPESP.
| Science