Changes in visibility and the occurrence of smoke or haze during the last three decades are identified for eight locations in and around Illinois. The analyses utilize individual daily data and are performed on both seasonal and annual bases. Visibility variation is investigated using cumulative percentiles and mean ridits.

Summer is the season that experienced the greatest 1950–80 visibility change. Except at Chicago, this was dominated by a pronounced overall decline that coincided with a marked increase in the frequency of smoke/haze. Superimposed on these trends are 1) a strong early-1960s visibility maximum and smoke/haze minimum for Indianapolis and the northern half of Illinois and 2) particularly pronounced visibility degradation and increased smoke/haze occurrence during the late 1960s at most stations. The 1950–80 summer visibility decline at Chicago was much smaller than elsewhere and coincided with a marked downward smoke/hue frequency trend.

The extra-Chicago visibility results for spring are less pronounced versions of their summer counterparts; those for autumn contain the same overall decline, but not the foregoing smaller-scale variations. The spring and autumn occurrence of smoke/haze outside of Chicago exhibits little spatially coherent trend for the study period. Chicago's spring visibility improved slightly during 1950–80 and was accompanied by a stronger decrease in the number of smoke/haze days than occurred for summer. Autumn is the season in which Chicago visibility has degraded most in the last three decades, even though the concurrent reduction in the frequency of smoke/haze has exceeded that of summer and spring.

The winter results differ substantially from those for the other seasons. The 1950–80 winter visibility trends for individual stations range between a moderate decrease and a noticeable improvement, and are associated with strong reductions in smoke/haze frequency. These favorable changes are greatest at Chicago. Superimposed on them are 1) strong visibility maxima and smoke/haze minima during the mid-1950s and mid-1960s at most stations, 2) marked visibility degradation and increased smoke/haze occurrence outside of Chicago (especially in northwestern Illinois) in the late 1960s, and 3) some improvement in that situation in the early 1970s, followed by renewed deterioration.

The extra-Chicago annual results are determined by the similar patterns for summer (especially), spring and autumn. Their Chicago counterparts are the product of larger season-to-season variation, and accordingly reflect winter results to a greater extent.

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