The Mid-Atlantic region that includes the Virginias and Carolinas shows a strong summer increase between 1960 and 1973, followed by a decline. The winter haze was virtually unchanged over the 30-year period. A representative seasonal pattern for Raleigh/Durham, NC, Greensboro, NC, and Charlotte, NC is depicted in Figure 8. In 1961, the haze pattern at these sites was roughly constant throughout the year. By 1970, a strong summer peak emerged that was twice the magnitude of the winter haze. The shape of the strongly summer peaked seasonal pattern has remained throughout the 1990s.
The haziness over the Gulf states increased between 1960 and 1970, and remained virtually unchanged since then. The seasonal pattern for Houston, TX, Port Arthur, TX, and Lake Charles, LA reflect a seasonally uniform pattern throughout the 30-year period (Figure 9).
The central Midwest, including Missouri and Arkansas, exhibits virtually no change during the winter season and a slight increase in the summer (1960-1970). The upper Midwest (Figure 10) shows opposing trends for summer and winter. While summer haze has increased, mostly between 1960 and 1973, the winter haze has declined. This phenomenon is also illustrated in the seasonal charts for Toledo, OH, Flint, MI, and Detroit, MI. In 1961 all three sites exhibit a strong winter peak which transformed into a summer peak after 1973.
Similar seasonal shifts can be observed for the Los Angeles basin (Figure 11). In the 1960s both Los Angeles International Airport and Long Beach Airport show a winter peak. By 1980 the smog-haze seasonality has declined, and the overall Bext has declined (visibility has improved).