Strange as it may seem, the most recent Image of the Week entry from the Climate and Radiation Branch at Goddard Space Flight Center suggests that air pollution does indeed exacerbate lightning storms. The graphic, created by Goddard meteorologist Thomas Bell, shows that rainfall and lightning rarely peaks over the weekend in the southeastern United States. In fact, lightning hasn't peaked on a weekend during any year since 1998, Bell has shown after combing through meteorological data from 1998 to 2009. After publishing a number of scientific papers on the topic, Bell thinks he knows why: air pollution (which is at its highest levels midweek and lowest levels on the weekend) can strengthen thunderstorms, particularly in the unstable and humid air of the Southeast.
The figure shows the day of the week favored by rainfall and by lightning in each summer from 1998 to 2009 in the southeastern United States (click here for a map of the area included in the analysis). The "clock plots" on the left and right indicate the day of the week when mean activity was at a maximum. The numbers indicate the year the data comes from. The rain estimates are based on TRMM and other satellite observations. The lightning data were collected by ground stations that are part of the National Lightning Detection Network. Credit: NASA/Bell
To find out more, I ran some questions about the connection between pollution and lightning by Bell via email:
What On Earth: Why does air pollution have any bearing on lightning or rainfall, and why is the connection more noticeable over the Southeast than other parts of the country?
Thomas Bell: Our explanation is that the storms need to start their growth in a hot, humid environment to give the pollution “something to work with.” The pollution causes the storm to climb to higher altitudes, because it causes the cloud droplets being formed in the storm to be smaller than they would be in a clean environment. They’re lighter and are carried up higher than usual, where they freeze (releasing latent heat), which pumps the storm up more than would happen in clean air.
The environment needs to be hot in order to have the capacity to push the storm up to altitudes where freezing can occur. The environment also needs to be humid because when the storm grows more vigorously it “sucks” air at its base up more strongly, pulling in more moisture, which then provides additional energy to the storm as the moisture condenses during its climb. The western half of the country is fairly dry—even though it can be hot, there isn’t much “fuel” (moisture) to feed a storm when it tries to grow more vigorously. The Southeast is especially hot and humid in the summer, so that’s where the effect shows up best, according to our theory.
What On Earth: Are there particular types of air pollution that have more or less impact on rain and lightning?
Thomas Bell: We don’t have a good answer to this as yet. The pollution should be the kind that affects cloud droplet growth. If we had to finger something, we’d probably choose the kinds of particulates that are emitted by diesel engines, because it seems that the weekly cycle in pollution is due, to a considerable extent, to the weekly cycle in transportation (probably trucking). More trucks are on the roads from Tue-Thu than they are the rest of the week. But this is more conjecture than a well-documented explanation.
For more context on Bell's findings, see these earlier Image of Week entries, take a look at these news stories, or read these peer-reviewed journal articles. And, when you've done all that, head over to the Earth Science Picture of the Day page (which we've written about previously) and enjoy these spectacular pictures of lightning. Here's an eerie one taken in the German town of Bad Mergentheim:
The figure shows the day of the week favored by rainfall and by lightning in each summer from 1998 to 2009 in the southeastern United States (click here for a map of the area included in the analysis). The "clock plots" on the left and right indicate the day of the week when mean activity was at a maximum. The numbers indicate the year the data comes from. The rain estimates are based on TRMM and other satellite observations. The lightning data were collected by ground stations that are part of the National Lightning Detection Network. Credit: NASA/Bell
To find out more, I ran some questions about the connection between pollution and lightning by Bell via email:
What On Earth: Why does air pollution have any bearing on lightning or rainfall, and why is the connection more noticeable over the Southeast than other parts of the country?
Thomas Bell: Our explanation is that the storms need to start their growth in a hot, humid environment to give the pollution “something to work with.” The pollution causes the storm to climb to higher altitudes, because it causes the cloud droplets being formed in the storm to be smaller than they would be in a clean environment. They’re lighter and are carried up higher than usual, where they freeze (releasing latent heat), which pumps the storm up more than would happen in clean air.
The environment needs to be hot in order to have the capacity to push the storm up to altitudes where freezing can occur. The environment also needs to be humid because when the storm grows more vigorously it “sucks” air at its base up more strongly, pulling in more moisture, which then provides additional energy to the storm as the moisture condenses during its climb. The western half of the country is fairly dry—even though it can be hot, there isn’t much “fuel” (moisture) to feed a storm when it tries to grow more vigorously. The Southeast is especially hot and humid in the summer, so that’s where the effect shows up best, according to our theory.
What On Earth: Are there particular types of air pollution that have more or less impact on rain and lightning?
Thomas Bell: We don’t have a good answer to this as yet. The pollution should be the kind that affects cloud droplet growth. If we had to finger something, we’d probably choose the kinds of particulates that are emitted by diesel engines, because it seems that the weekly cycle in pollution is due, to a considerable extent, to the weekly cycle in transportation (probably trucking). More trucks are on the roads from Tue-Thu than they are the rest of the week. But this is more conjecture than a well-documented explanation.
For more context on Bell's findings, see these earlier Image of Week entries, take a look at these news stories, or read these peer-reviewed journal articles. And, when you've done all that, head over to the Earth Science Picture of the Day page (which we've written about previously) and enjoy these spectacular pictures of lightning. Here's an eerie one taken in the German town of Bad Mergentheim:
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