DISCUSSION: In order for lightning to occur, ice of different sizes and supercooled liquid water (liquid that occurs below 0°C) must be present in a cloud. A thunderstorm cloud (i.e., a cumulonimbus cloud) is quite turbulent with all the ice getting tossed about and colliding with one another. The environment inside a thunderstorm is somewhat similar to the environment inside a washing machine. The collisions amongst the ice allow charge to transfer between them. Since charges can typically move easier through liquids than solids, the supercooled water helps speed up the charge transfer process. Larger pieces of ice tend to acquire charges of one sign, while smaller ice tend to accumulate the opposite sign of charge. A strong updraft is required to produce all this mass of ice and liquid in the cloud. It is also necessary to help transport the smaller ice crystals toward the top of the cloud, while large ice settles toward the bottom of the cloud. This charge transfer and separation may eventually build up large enough to overcome the electrical resistance of air, resulting in a lightning flash.
So, how does air pollution play into this? It is very difficult to get water vapor to spontaneously condense or freeze in the atmosphere. There must be something (e.g., an aerosol) to condense or freeze onto. When there are more aerosols or more pollution in the atmosphere, more droplets can form. This large number of droplets are all competing for a limited amount of water vapor, and thus, cannot grow to raindrop size. Instead, they remain in the cloud and may get carried upward above the freezing level supplying the supercooled liquid and/or ice required for lightning. In contrast, clean air with relatively few aerosols only allows a smaller number of droplets to form. In this case, there is more water vapor available for each droplet, potentially allowing them to grow to raindrop size and fall out of the cloud before they can freeze and contribute to lightning formation. Hence, less air pollution may result in less lightning and vice versa.
Observations support this relationship between air pollution and lightning. The response to the COVID-19 pandemic, especially in the beginning, resulted in less travel, industrial activity, etc., which resulted in a cleaner atmosphere. A recent study by Yakun Liu showed that global lightning flashes like the one shown above in Spain occurred much less often in association with this decrease in air pollution. Local increases in lightning have also been observed near the Philippines in association with increased aerosols from a volcano, along major shipping lanes due to pollution from ships, and in association with smoke from wildfires in Australia.
Reducing air pollution can obviously, lead to better health outcomes. Given that lightning can start fires and/or cause fatalities, the reduction of lightning associated with a reduction in air pollution may be another beneficial aspect of actions and policies that seek to limit air pollution.
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©2022 Meteorologist Dr. Ken Leppert II