A new study suggests that in the summertime, tornadoes and hailstorms in the eastern US occur significantly more often during the middle of the week. Why? There's more pollution during the workweek due to commuting and other factors. From National Geographic:
…Moisture gathers around specks of pollutants, which leads to more cloud droplets. Computer models suggest these droplets get lofted up to higher, colder air, leading to more plentiful and larger hail.
Understanding how pollution can generate more tornadoes is a bit trickier. First, the large icy particles of hail that pollutants help seed possess less surface area than an equal mass of smaller "hydrometeors"—that is, particles of condensed water or ice.
As such, these large hydrometeors evaporate more slowly, and thus are not as likely to suck heat from the air. This makes it easier for warm air to help form a "supercell," the cloud type that usually produces tornadoes and large hail…
The pollution-storm pattern is not seen in the western U.S. because the air is too dry and the cloud masses too high and cold for air pollution to influence weather the same way, said study co-author Daniel Rosenfeld, of the Hebrew University of Jerusalem in Israel.
Overall, the research "provides yet another good reason for reducing air pollution," Rosenfeld said.