It had always been assumed that the rarity of lightning in ocean storms was due to cleaner air over the vast seas，but keen observations have shown that even highly polluted air is associated with reduced lightning at sea when sea spray aerosols（气溶胶） are abundant. In their paper published in the journal Nature Communications，an international team of researchers describes their multi-year study of atmospheric conditions over the oceans bordering Africa and what it showed about salt in sea spray aerosols and its impact on lightning.
The researchers obtained atmospheric data on Africa and the oceans around it for the period 2013 to 2017. The data included numbers of lightning flashes，rainfall，temperatures and cloud representations. They looked for differences in atmospheric conditions in storms that occurred over land versus those that occurred over the sea and found one main difference：the amount of salt in the air.
Lightning is produced when upwardly moving air in clouds forms ice crystals（冰晶）—aerosol particles then begin to bump into one another，creating an electrical charge. Lightning happens when the electrical field in one part of the cloud becomes positively charged （ usually at the top of the cloud） and another negatively charged （usually at the bottom of the cloud）. And clouds form，of course，when vapor evaporates（蒸发） from the surface of the Earth and the water drops bond with aerosols.
The researchers found that things are slightly different over the ocean. When water evaporates from the sea，it bears a load of salt. When the salt water bonds with aerosols to form water droplets，they tend to be bigger and heavier than those that form over land due to the salt—and that results in more of the water in the clouds falling as rain before it can rise up and form ice crystals. The result is far fewer lightning discharges.
The researchers suggest their findings could be useful in improving the accuracy of both climate models and meteorological reports. It could also possibly lead some to attempt to seed storm clouds with salt to reduce their severity.
