Abstract
Airport visibility data for the period 1958–78 or 1979, depending on the availability of data, for 18 cities in the western and southwestern United States are examined. A threshold visibility below which visibility is deemed “poor” is determined for each station. The number of poor days divided by the total number of days with unlimited ceiling is calculated for each year. Linear trend regressions are done on each of the cities for the entire 21- or 22-year period and also for the periods 1958–72 and 1973–78 or 1979. No general trends are apparent over the total period. Visibilities generally decreased, however, from 1958 to 1972 and generally increased from 1973 to 1978. This suggests positive effects from implementation of the Clean Air Act of 1973. Correlation coefficients calculated between all possible pairs of cities yielded only 11 correlations significant at the 1% level. Only a few of these seem physically plausible. Correlations done between the cities and SOx emissions from Arizona copper smelters suggest a positive correlation between increasing SOx emissions and poor visibility for three Arizona cities. Other causes of the observed apparent visibility trends are also discussed. These include changing procedures of data gathering, increasing population, and regional and synoptic-scale meteorological conditions.
