Imagine that you are lying on a green hill watching the clouds go by on a beautiful day. The clouds you are probably thinking of are cumulus clouds, the ones that resemble fluffy cotton balls . They seem innocent enough, but they can grow into the most formidable cumulonimbus, the storm cloud.
These are the monsters that produce thunder and lightning. They are powerful, destructive, and intensely mysterious. They can also be increasingly common, making understanding how they work – and their effects on the human world, including how we build buildings or power lines – more important than ever.
Many clouds form when hot, humid air rises to high altitudes where it cools and condenses into water droplets. Thunderstorms occur when a cloud that forms in this way quickly grows very large, absorbing more and more water vapor. Precipitation and strong gusty winds almost always follow. And, of course, lightning. Lightning bolts may seem strange enough, but they have occurred about 700 times – we have about 100 hits per second – somewhere around the globe in the time it took you to read this sentence.
Lightning and thunderstorms appear to be increasingly common and there are signs that this will continue as a result of global warming. In 2014, Professor David Romps from the University of California, Berkeley, USA, developed an atmospheric model that predicted that lightning will increase by 12% for every degree of global warming. There is some evidence that this could be happening already. Researchers in the Netherlands have looked at the number of fires started by lightning in the forests of Alaska and Canada and have found that these have increased between 2% and 4% annually over the last 40 years.
We do not understand lightning well. If, for example, a ray is filmed and reproduced in super slow motion, it will be perceived that the ray is advancing in steps. It pauses for a while at intervals before moving on, says Dr. Alejandro Luque of the Instituto de Astrofísica de Andalucía in Granada. But we don’t know why this happens. He says there are some papers on it, but essentially no accepted theories.
Sprites
Dr. Luque thinks he might have some ideas about the problem from his work studying an even more incredible and better understood electrical phenomenon: sprites .
Sprites are huge jets of colored light that occur between 50 and 90 kilometers above the ground, much higher than thunderstorms. Their existence was questioned for years as they are difficult to see from the ground. Dr. Luque studied them primarily by looking at photos taken by research aircraft.
Although they are less familiar than lightning, sprites physics is easier to study because, at such a high altitude, there is little air and therefore electrical discharges occur more slowly and colder temperatures. Lightning creates temperatures hotter than the surface of the sun. But Dr. Luque said that the drainage channels of the sprites have “more or less the same temperature as the surrounding air.”
Sprites channels are made of many tiny filaments called streamers. And as the streamers spread, some spots within them glow brighter and more persistently. In sprites , the bright glow is due to the behavior of electrons, says Dr. Luque. In some areas of the serpentine, electrons adhere to air molecules and this increases the strength of the electric field, producing a brighter light.
Steps
This explanation is not controversial, Dr. Luque says, but what we don’t know is whether – as he suspects – an analogous process could explain why the lightning itself proceeds in steps . In the context of lightning, at lower altitudes, there are more air molecules and the binding of electrons to them could work in a slightly different way to produce the pattern of steps. Dr. Luque wants to find out if this is correct through his eLightning project.
He and his student Alejandro Malagón-Romero established this hypothesis in 2019. His team is now working on building a computational model of rays to test whether the process can explain passing behavior.
Understanding why lightning proceeds in steps is not going to help us make it less dangerous, but Dr. Luque says that better understanding the phenomenon could be helpful in other areas. For example, discharges can form around power lines and therefore must be designed to minimize this. These discharges are also used in industry, for example in the disinfection of industrial waste gases and even in photocopiers. A better understanding of how they work could lead to better designs.
Lightning may seem like the most dangerous weapon in a thunderstorm arsenal, but these storms also create unusually strong winds.
Europe’s climate is dominated by air systems known as extratropical cyclones, spiraling air currents that bring with them wind and rain as they sweep across a region. The average European city experiences between 70 and 90 a year and scientists have a good understanding of how they work. These storms can be strong, although they are not always.
Whenever a building is constructed in Europe, designers must ensure that it can withstand strong winds and the models they use for this are based on extratropical cyclones. The problem is that winds that are believed to be rare, such as those in thunderstorms, are not taken into account.
Electric storms
To understand why this matters, you have to understand the difference between cyclones and thunderstorms. For starters, thunderstorms are more intense than cyclones. While a cyclone can last three days, a thunderstorm can end in 20 minutes . So instead of a moderate and sustained wind, there is a gust of very strong gusts. Second, and more important, is how the strength of the winds varies depending on the altitude. Cyclones get stronger and stronger at heights. Thunderstorms, meanwhile, tend to produce winds that start at around 100 meters high and blow downward, with the wind getting stronger as it descends. “A normal wind blows parallel to the ground, but a thunderstorm blows downward. It’s completely different, “said Professor Giovanni Solari of the University of Genoa in Italy.
Putting all of the above together, the result, says Professor Solari, is that we are falling into the over-engineering of our tallest buildings, especially skyscrapers, and the under-engineering of low-rise buildings and structures like shipyard cranes . The top 200 meters of a 300-meter skyscraper are probably not being hit by a thunderstorm, but we designed them as if they were because our models assume the winds are strongest at height. “We are making buildings too safe,” he said. On the other hand, small cranes can be tipped over by electrical storms, which produce their strongest wind at ground level.
Prof. Solari’s goal, through the THUNDERR project, is to correct this, which could make construction more efficient and less expensive, producing a storm wind model that can be used to help design buildings . The first step was to take a synthetic thunderstorm created in a world-class wind tunnel at the University of Ontario in Canada and model it. That’s already done, Prof. Solari says, and his models do a good job of capturing what these synthetic storms produce. But that was the easy part.
You are now moving to modeling real storms, in which there is a lot of variation. To help, Prof. Solari and his team have built a network of 45 meteorological towers spaced around the Mediterranean coast designed to capture data on the winds created by thunderstorms.
“People used to think that thunderstorms were rare,” Prof. Solari said. That was because we couldn’t see them. The network has now logged a database of 250 thunderstorm records. The plan is now to adjust the initial model to account for all these different storms and to be truly representative.
Artículo original
This article was originally published in Horizon, the EU Research and Innovation Magazine
