The likelihood of a lightning flash is not the same everywhere. This phenomenon is more common on land than over oceans, and predominantly in areas closer to the equator than in mid and high latitudes. According to recent research by Joel Thornton, an atmospheric scientist at the University of Washington, humans can also influence the onset of electrical discharge in the sky.
The connection between people and lightning can be seen in areas of the two busiest shipping lanes in the world in the Indian Ocean and the South China Sea. The top map shows in orange the particle emissions from the ship's exhaust, calculated from the region's marine traffic database. The second map shows the average lightning density per year from 2005 to 2016, according to the World Lightning Localization Network (WWLLN). Narrow light purple stripes indicate where the increase in lightning has occurred. By comparing these two images, it can be seen that the areas with the most lightning coincide with the routes of the ships.
The researchers found that, on average, the frequency of lightning over shipping lanes is twice that of areas immediately adjacent to these lanes. “It was a surprise to find this feature in the data, and so clearly expressed. Our discovery is one of the most striking examples of human impact on the atmosphere, the consequence of which is an increased lightning speed of storms,”said Thornton. He noted that the idea of the effect of aerosol particles on the intensity of storms and lightning has been discussed for over a decade, but scientists could not agree on the importance of this effect.
WWLLN ground sensors register lightning all over the planet. Recently, the work of Katrina Wirts, atmospheric specialist at the Space Flight Center. Marshall NASA, made the network even more valuable. By reworking the global lightning climatology, she was able to increase the resolution by a factor of five. Now the location of lightning strikes can be determined with an accuracy of 10 square kilometers.
These high-resolution maps led Thornton and his colleagues to hypothesize that the ship's exhaust could be responsible for lightning. The team compared data on surface lightning in the Indian Ocean shipping corridors with data from the Lightning Imaging Sensor tool on the Tropical Rainfall Measuring Mission (TRMM) satellite. They also used TRMM radar data to check if certain areas of the storm clouds actually contained more liquid droplets among water vapor and ice particles. Such an increase could have occurred if particles from ship exhaust altered the cloud structure in the area.
“Our results show that aerosol particles from exhaust gases actually turn a tropical storm into a lightning thunderstorm, increasing the vertical progression of storms,” Thornton concluded.