Thunderstorm as a mercury shower

Rain from a storm cloud can contain twice as much mercury as normal rain

Storm clouds seen from orbit: When they rain, their water often contains a lot of mercury. © NASA
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Heavy metal in the downpour: A thunderstorm rain can contain significantly more mercury than normal precipitation, as US researchers have found. The reason for this is the enormously towering thundercloud: it reaches into heights of the atmosphere, in which particularly much mercury occurs - and this heavy metal then dissolves into the raindrops.

Thunderstorms are an everyday phenomenon - and at the same time an exciting research object. Because the sudden discharges of electrical energy still hold some riddles. Researchers have just recently made the thunder visible and found out that there are more flashes in a solar storm. Still unexplained, however, is the emergence of sprites - luminous phenomena high above the thunderstorms.

Mercury in the rainstorm

A thunderstorm phenomenon that tends to affect the ground has now been discovered by Christopher Holmes of the University of Florida and his colleagues. For their study, they investigated whether rainstorms and normal rain differed in their chemical composition. Instead, they collected rainwater at 800 different locations in the southern United States.

The analysis came as a surprise: when the rainwater came from a thunderstorm, it contained about twice as much mercury as normal rainwater. The toxic heavy metal mercury gets into the atmosphere mainly in the form of its vapors. It is released, for example, during the combustion of coal, during cement production or through refuse-fire, but also gets into the environment through electronic waste and industrial wastewater.

Cloud height is crucial

But why does thunderstorm rain contain more mercury than normal rainfall? To clarify this question, the researchers additionally evaluated radar and satellite data from the thunderstorms that occurred during the study period. It showed that the higher the storm clouds reached, the more mercury they rained off. display

Storm clouds reach to the upper limit of the troposphere - and there is a lot of mercury. P Tim Peake, ESA / NASA

The reason for this is that "the strong convection of these clouds reaches into the upper troposphere and thus into the areas in which the soluble, oxidized methylmercury reaches the highest concentrations, " explains Ren Holmes and his colleagues. This accumulation of mercury at high altitude has been measured by airborne surveys both over the US and across Europe.

In a normal rainstorm or in wet weather, the precipitation falls from very low-hanging clouds. Therefore, less heavy metal is dissolved in these raindrops.

More mercury in the south

According to the researchers, this effect could also explain why certain areas of the United States, for example, are more exposed to mercury than others. In the last 20 years, in some of the most southern states of the USA, in places twice as much mercury levels were measured in the water than in the north.

Holmes and his colleagues suspect that this is due to the significantly higher thunderstorm frequency in the US southern states: Because there are often warm, humid air from the Gulf of Mexico in the summer It accordingly more thunderstorms and therefore more frequent mercury rain. (Environmental Science and Technology, 2016; doi: 10.1021 / acs.est.6b02586)

(Florida State University, 02.09.2016 - NPO)