Salty sea spray protects against lightning

Although most of Earth’s rain falls over the oceans, lightning strikes at sea are rarer than expected — and for decades, scientists weren’t sure why. A new study published Tuesday in Nature Communications assumes salt spray can interfere with clouds lightning strike charge.

Dense clouds that form overhead during storms can become electrified when upward-moving air helps them get high enough so that the upper parts of the cloud freeze into a mixture of granular, rounded snowballs called granules. and microscopic ice crystals. As these ice particles bump into each other, they carry electrical charges: larger grains tend to become negatively charged, while smaller ice crystals end up with a positive charge.

Positively charged ice crystals are so light that updrafts carry them to the top of the cloud, while heavier granules tend to sink. Over time, this separation generates an electric field between the positively charged upper part of the cloud and the negatively charged lower part. When the charge difference becomes large enough, lightning strikes.

However, when large, water-absorbing particles of sea salt are present – ​​abundant from ocean spray – the tiny droplets that normally condense on microscopic dust and soot to form clouds grow much faster, becoming heavy enough to fall as rain very quickly. before the cloud can get high enough to charge. Although this mechanism for suppressing marine lightning has been proposed before, evidence for it has yet to be found in global weather observations. To do this, a team of researchers from China, Israel and the US used global measurements of clouds, lightning and the expected distribution of particles such as pollutants, dust and salt in the atmosphere to observe how cloud systems with different combinations of these particles evolve over time. of the weather – documenting when and if there was precipitation and lightning. They found that areas with salt spray saw up to 90 percent less lightning.

“We were able to separate the effects of small and large particles [sea spray] particles,” says atmospheric scientist and study co-author Daniel Rosenfeld of the Hebrew University of Jerusalem. These effects are often overlooked when climate scientists try to predict when and where rain will fall, he adds. “If you don’t take it into account in weather prediction models — and even more so in climate prediction models — you’re not getting the right picture, you’re not getting the right precipitation,” Rosenfeld says.

But fine particles called aerosols aren’t the only factor in the complex interior of clouds. Other differences in the atmosphere over land and oceans due to local weather conditions, such as wind and temperature, can also play a role in how much lightning occurs. “It is very challenging to separate the aerosol effect from the [these other weather conditions] based on observational analysis alone,” said Jiwen Fan, an earth scientist studying interactions between aerosols, clouds, precipitation and climate at the Pacific Northwest National Laboratory. Phan, who was not involved in the new study, suggested that detailed computer modeling of the processes in dense storm clouds would help further clarify the importance of sea salt spray, compared to other meteorological factors, in determining when and where they can for lightning to occur.