Severe thunderstorms were more common downwind than over the city-as was excessive rainfall. During Metromex, the summer precipitation was 25 percent higher downwind of St. Louis than over the city. Fall precipitation was 17 percent higher downwind, but winter and spring precipitation was about the same in the city and downwind. St. Louis industry injects large quantities of aerosols into the air that reduce visibility downwind and increase summertime cloudiness. The largest aerosols settle to the ground within four to five hours of downwind travel.
Precipitation washes aerosols out of the air.
This dry and wet deposition has contributed to the pollution of soil and streams, making them increasingly acidic.