Airflow trajectories were used to create a long-term (40-year) air mass climatology for the lower peninsula of Michigan. The climatology provides a necessary baseline for evaluating the impact of changes in airflow on historical and potential future variations in temperature and precipitation. Five-day back trajectories were calculated four times per day at the 925 hPa level using wind data from the NCEP/NCAR reanalysis fields. A Geographic Information System (GIS) was used to manage and display the large (58,440) volume of trajectories. The analysis revealed spatially coherent airflow pathways, and six resulting air mass source regions were defined. Also evident were large monthly variations in the frequency of trajectories from the major source regions. A residence time analysis performed within a GIS proved to be an efficient and effective means for summarizing the trajectories. Finally, a comparison of the results presented here to those of previous climatological analyses suggests that the choice of methodology has considerable influence on identification of the primary source regions for a location.