A SYNOPTIC CLIMATOLOGICAL ANALYSIS OF WINTER VISIBILITY TRENDS IN THE MIDEASTERN UNITED-STATES

Long-term trends in wintertime visibility are examined in eight mideastern U.S. cities in light of the underlying climatology. Using an array of meteorological elements known to be related to visibility, a synoptic climatological classification is developed, producing a baseline climate for each city. This approach allows for an analysis of long-term air quality trends which are unrelated to climatic variations. Yearly mean visibility levels are closely related to climatic variations. Years with frequent occurrences of continental polar and arctic air masses and cold front passages exhibit high visibility, while low air quality is related to overrunning situations (a warm or stationary front to the south) and the advection of Atlantic maritime polar air. Winter air quality declined from the early 1950s to 1970, improved throughout the mid 1970s, and either remained steady or improved slightly thereafter.