Why tornadoes take the weekends off in summer

National Geographic : An analysis of summertime storm activity in the eastern US from 1995 to 2009 revealed that the occurrence of tornadoes and hailstorms peaked in the middle of the week, when human-made summertime air pollution also peaked. Pollution can help breed storms because moisture gathers around specks of pollutants, which leads to more cloud droplets, which get lofted to higher, colder air, where they’re more likely to produce hail. The process by which pollution can increase the number of tornadoes is more complex. The large icy hail particles seeded by pollutants have less surface area than an equal mass of smaller particles of condensed water or ice, and they evaporate more slowly and are less likely to suck heat from the air. It then becomes easier for warm air to help form a supercell, the storm type that usually produces tornadoes and large hail.