Visibility Trends in the Midwest

Visibility in all of the cities fluctuate from year to year. No trend in visibility is detected from 1973-1978, but visibility in six of the seven cities appears to have improved since 1979. Visibility in Chicago, the other city, appears to have remained level.

Along with examining overall visibility trends the influence of meteorology is also investigated. Three factors appear to have a significant impact on visibility: relative humidity, wind speed and wind direction. When the data set is examined, it is seen that as relative humidity increases visibility decreases. It has been observed that for many atmospheric aerosols, size increases as water is absorbed under high relative humidity conditions, which causes more light to be scattered. As wind speed increases, visibility also increases. This is probably due to mixing and clean air being brought into the area. Visibility is also affected by changes in the wind direction. Certain wind directions are associated with specific weather types, and some monitors may be affected by local sources under specific wind conditions.

Because of the impact weather has on visibility, trends observed may be the result of meteorological trends. Future work will involve attempting to distinguish trends due to the effects of weather from trends due to anthropogenic factors.

References

Sloane, Christine S. (1982) "Visibility Trends-I. Methods of Analysis", Atmospheric Environment, 6, 41-51.
National Research Council (1993), Protecting Visibility in National Parks and Wilderness Areas. Washington DC: National Academy Press.