Science

Led by Joel Thornton, a UW professor of atmospheric sciences, the study mapped lightning around the world locating to find that lightning strokes occur nearly twice as often above heavily-trafficked shipping lanes in the Indian Ocean and the South China Sea compared to nearby areas waters with similar climates.

The difference in lightning activity is not the result of weather patterns alone, the study’s authors argue, but the constant supply of fuel emissions by traveling cargo ships.

The thick layer of smog and haze that covers metropolitan areas is the result of aerosols from compounds of nitrogen and sulfur emitted in the exhaust fumes of your car’s tailpipe. They also act as cloud condensation nuclei – the seeds on which clouds form. Water vapor condenses around aerosols in the atmosphere, creating droplets that make up clouds. The aerosol particles emitted by ship exhaust are similarly changing how storm clouds form over the ocean.

The study is the first to show how particles from ship exhaust fumes are shrinking cloud droplets, shooting them higher into the atmosphere and creating more ice particles and leads to more lightning. According to the study’s authors, these manmade processes are rapidly changing cloud formation to the extent that the planet’s rainfall patterns are out of flux.

Cargo ships crossing oceans emit exhaust continuously and scientists can use ship exhaust to better understand how aerosols affect cloud formation. Co-author Katrina Virts, a former UW postdoctoral researcher and current NASA atmospheric scientist first noticed a virtually straight line of lightning strokes across the Indian Ocean analyzing data from the UW-based World Wide Lightning Location Network whose sensors locate lightning strikes around the world. Comparing the lightning location data to maps of ships’ exhaust plumes from a global database of ship emissions, Virts and her teammates looked at the locations of 1.5 billion lightning strokes from 2005 to 2016.

They found that nearly twice as many lightning strokes on average occurred over major shipping routes across the northern Indian Ocean, the Strait of Malacca, and the South China Sea, compared to adjacent waters with similar climates. Water molecules need aerosols to condense into clouds.

Where the atmosphere has few aerosol particles – over the ocean, for instance – water molecules have fewer particles to condense around, so cloud droplets are large. When more aerosols fill the air, water molecules have more particles to collect around.

Smaller, lighter cloud droplets form as a result and travel higher into the atmosphere. If they reach the freezing line, these aerosols form more ice, creating more lightning. Should these ice particles collide, they electrify storm clouds with unfrozen droplets in the cloud to neutralize the built-up of electric charge.

According to Thornton, the farther they are away from port, the dirtier the fuel ships will burn –spewing more aerosols and creating more lightning strikes across oceans.

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