Quantitative analysis of summer air masses in the eastern United States and an application to human mortality

A recently developed continental-scale air mass-based classification is used to identify the spatial occurrences of 6 air masses (dry polar, dry temperate, dry tropical, moist polar, moist temperate, and moist tropical) in the eastern United States for summer from 1961 to 1990. This procedure, the spatial synoptic classification (SSC), is based on 'seed' day identification of synoptic events and discriminant analysis to group days together which are within the same air mass type. Thus, the evaluation of the frequency and modification of air masses across a large region is possible. Using the SSC, maps of air mass frequencies and afternoon temperatures and dew points are developed. Rates of modification are determined as each air mass traverses the region. As an example of the environmental applicability of the SSC, the impact of climate upon human mortality is analyzed at 3 climatically different locations (New Orleans, LA; Memphis, TN; and Chicago, IL). Results show how the day-to-day mortality fluctuations are sensitive to air mass type. A particular air mass which is associated with distinctly high mortality is identified for Memphis and Chicago; no such air mass is apparent for New Orleans, where the weather/mortality signal is very weak.