The examination of historical data on standard visual range observations made at local airports yielded:
*Lower visibility in the 1940's than in the 1930's, with the lowest between 1944-1947. This could possibly be a result of the invention of the private vehicle.
*Between 1932-1937, there were very clear afternoons with a visual range in excess of 35 miles. This occurred on 21% of summer days.
*By 1943-1947, the number of such very clear afternoons dropped to 0.2% of the summer days!!!
*1950-1961: Near to Los Angeles, a minimum visual range of 3 miles or less was recorded on more than 140 days/yr when the humidity was less than 70%.
*In the 1970�s there was visibility improvement in downtown L.A., possibly due to the Clean Air Act, which was implemented in 1963. From 1971-1973, approximately 25% of the days of the year showed minimum visibilities below 3 miles at less than 70% humidity.