March 28, 2023

How do lightning rods work?

José Tadeu Arantes | FAPESP Agency

With an ultra-fast video camera and the advantage of being in the right place at the right time, physicist Marcelo Saba, a researcher at the National Institute for Space Research (Inpe) and PhD student Diego Rhamon obtained an unprecedented image of a lightning bolt, showing details of the connection of with 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 region. The work was supported by FAPESP.

“The image was taken on a summer night in São José dos Campos. [SP]when a descending ray of negative charge approached the ground at a speed of 370 kilometers per second [km/s]. By the time the discharge was only a few tens of meters off the ground, several lightning rods and building overhangs located in the area produced positive upward discharges competing to connect with the descending lightning. The final image before connection was taken 25 millionths of a second before the lightning struck one of the buildings,” says Saba. It was this striking image that the editors of GRL reproduced on the cover of the publication.

How lightning rods work

The researcher reports that his camera recorded 40,000 images per second. Shot in super slow motion, the video shows how lightning rods behave. And also that lightning can be a hazard if this protective equipment is not properly installed. This is because, although there were more than 30 lightning rods nearby, the lightning did not connect with any of them, but with the chimney of a furnace located on the roof of a building. “An installation failure left this area unprotected. And the impact of a current of 30,000 amps caused impressive damage,” he says.

On average, 20% of lightning consists of exchanges of electrical charge between the clouds and the ground. The remaining 80% consists of electrical discharges within the clouds. Of those that do touch the ground, almost all are downward rays: they start from the cloud and come to the ground. Upward rays also exist, but are rare. And they only occur from tall structures like mountain tops, skyscrapers, towers and antennas. Depending on the charge they carry to the ground, lightning bolts can be further classified as negative or positive.

“Lightning can reach up to 100 kilometers in length. And they carry currents of about 30,000 amperes. This is equivalent to the current drawn by 30,000 100 watt light bulbs working together. In some cases, the current can reach 300 thousand amperes. The temperature of lightning, 30,000°C, is five times higher than the surface temperature of the Sun,” says Saba.

How rays are formed

The researcher explains that it all starts with cloud electricity. Its mechanism is not yet fully understood. But it comes, roughly, from friction between ice particles, water droplets and hail, which releases charges and creates polarities between different regions of 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 at a distance of 2 or 3 kilometers from the ground and whose top can reach up to 20 kilometers in altitude. Their diameters vary between 10 and 20 km,” he says.

The branched form that the rays take is explained by the fact that electric charges seek the easiest path, that is, the one that offers less resistance, and not the shortest path, which would be the straight line. The easiest path, usually zig-zag, is determined by different electrical characteristics of the atmosphere, which is not homogeneous. “A burst consisting 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 it discharge the clouds as formerly believed. It simply gives the lightning an easy and safe path to ground.

Summer is the time when most electrical discharges occur.

Since it is not always possible to protect 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 be careful with 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 durable place to shelter, stay in your car and wait out the storm. If there is no car or any other place to take refuge, squat with your feet together. Never standing, never lying down. Inside the house, avoid contact with appliances and the use of corded telephones.”

The researcher claims that a person struck by lightning can survive. And there are many examples of this. The chances are increased when the person is followed up early. “Cardiac arrest is the only cause of death. In this case, the recommended care is cardiopulmonary resuscitation,” he teaches.

Saba began the systematic study of lightning with high-speed cameras in 2003. This study, which is still ongoing, provided the largest video bank of lightning filmed at high speed in the world. The researcher and his advisors have already been studied with 17 grants or scholarships provided by FAPESP.

The article Close View of Lightning Attachment Process Reveals Fine Structure of Streamer Zone can be accessed at:

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