Visibility Trends in the Midwest

Haze and smog, i.e. visibility degradation, are obvious symptoms of air pollution. With the Clean Air Act and its amendments, the federal government hopes to improve the Nation's air quality. One area of focus is the reduction of particulate matter. If this goal is accomplished, visibility in many areas of the United States will improve. Visibility is important, not only because it can reflect the amount of particulate matter in the air, but also because of its impact on public perception and the quality of life.

Using meteorological data collected by the National Climatic Data Center (NCDC), visibility records for seven cities in the Midwest are examined for visibility trends since the initiation of the Clean Air Act. The cities include St. Louis and Kansas City, Missouri; Sioux City and Des Moines, Iowa; and Moline, Peoria, and Chicago, Illinois. After days with precipitation or relative humidity above 90 percent are removed, yearly 75th percentile visibility are calculated for the one o'clock pm readings. These values for the years 1973-1990 are shown in the following graphs:

Chicago, IL; Des Moines, IA; Kansas City, MO; Moline, IL Peoria, IL; Sioux City, IA; St. Louis, MO