Traffic causes tornadoes?

Neither correlation nor modulation are causation. reports,

Scientists have released a new study revealing human activity may be the culprit behind an increase in tornadoes and hailstorms during summer weekdays. It turns out severe weather is higher during the week due to reduced air quality. Simply put, it’s linked to our commute.

A research team analyzed summertime storm activity in the eastern U.S. from 1995 to 2009 using data from the National Oceanic and Atmospheric Administration’s (NOAA) Storm Prediction Center. The data revealed tornadoes and hailstorms occurred at a rate of 20 percent above average during the week, and 20 percent below on the weekend.

The findings proved to not be random, and in fact, matched with similar cycles seen in other storms.

The researchers also looked at EPA air quality data. They noticed human-made summertime air pollution over the east coast spikes midweek, just like severe weather. This led the researchers to make the link to the pollution we create while driving to and from work.

Why would pollution have anything to do with tornadoes and severe weather? Explaining the hail is easy. Moisture gathers around specks of pollutants; this in turn leads to more cloud droplets as the pollution increases. Computer models indicate as these heavy droplets get picked up higher into the air they can cause more hail.

So now for the tornadoes. How does that work? It seems the larger, heavier hail producing droplets created by pollutants have less surface area than an equal mass of smaller particles of normally condensed water droplets or ice. The pollutant based droplets evaporates more slowly and are not as likely to pull heat from the air. This makes it easier for the warm air to help form supercells that spawn tornadoes…

But here’s the study abstract:

This study shows for the first time statistical evidence that when anthropogenic aerosols over the eastern United States during summertime are at their weekly mid-week peak, tornado and hailstorm activity there is also near its weekly maximum. The weekly cycle in summertime storm activity for 1995–2009 was found to be statistically significant and unlikely to be due to natural variability. It correlates well with previously observed weekly cycles of other measures of storm activity. The pattern of variability supports the hypothesis that air pollution aerosols invigorate deep convective clouds in a moist, unstable atmosphere, to the extent of inducing production of large hailstones and tornados. This is caused by the effect of aerosols on cloud drop nucleation, making cloud drops smaller and hydrometeors larger. According to simulations, the larger ice hydrometeors contribute to more hail. The reduced evaporation from the larger hydrometeors produces weaker cold pools. Simulations have shown that too cold and fast-expanding pools inhibit the formation of tornados. The statistical observations suggest that this might be the mechanism by which the weekly modulation in pollution aerosols is causing the weekly cycle in severe convective storms during summer over the eastern United States. Although we focus here on the role of aerosols, they are not a primary atmospheric driver of tornados and hailstorms but rather modulate them in certain conditions.

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