State Water Survey Division ATMOSPHERIC SCIENCES SECTION

The effect of aircraft contrails on surface climate was assessed by first determining the density of jet traffic across a ten state area of the Upper Midwest, and comparing that distribution with observed changes of various meteorological parameters over the past 50 years, both within and outside of the principal contrail areas. Commercial jet traffic density was found to be greatest in 1979 along a band running from central Ohio westward to central and northern Illinois, with one branch leading westward across northern Missouri, and another branch leading toward the southwest through central Missouri. Another high density region, within the area studied, was eastern Kentucky and Tennessee. The surface meteorological characteristics were compared for stations within the area of high jet traffic density and those areas less affected by jet traffic, from the middle 1950's until the present. Percent possible sun and number of clear days decreased within the areas of high traffic density from about 1935 to 1950, decreasing more slowly thereafter to the present. The same parameters showed no change or slight increases elsewhere in the study area. The frequency of moderated temperatures, i.e., daily temperature maximum minus daily temperature minimum, increased from the late 1940's with the most pronounced increased found over northern Illinois and Indiana. In-flight observations from in excess of 16,000 pilot reports suggest that observations with contrails are more frequently also associated with turbulence. In addition and perhaps of more importance to surface climate, aircraft producing contrails typically observe more frequent cloud cover both