Impacts of the Great Lakes on regional climate conditions

Estimates of lake-induced spatial changes of six climate variables (precipitation, mean minimum and mean maximum temperatures, cloud cover, vapor pressure, and wind speed) were derived for the entire Great Lakes basin. These patterns were estimated by a comparison of maps of each weather variable using: (1) all regional climate data, and (2) regional data when observations within an 80-km zone around the lakes were removed. Results generally confirm expectations and prior findings, but point to inadequacies in data collection that limit a highly precise analysis. Lake effects are most noticeable in precipitation and temperature and vary considerably by season, time of day, and lake size. Greatest lake influences are found near Lake Superior where up to 100% more precipitation falls downwind of the lake in winter compared to that expected without its presence. During summer, all lakes cause a downwind decrease in rainfall of 10% to 20%. Mean minimum temperatures in the basin are higher in all seasons and over all lakes. Lake-induced reductions in mean maximum temperatures in the region are observed during spring and summer. Effects on cloud cover are greatest during winter and show increases of approximately 25% in areas downwind of Lakes Superior and Michigan. Conversely, the cool summertime waters of Lakes Michigan and Huron reduce cloudiness roughly 10%. Variations in vapor pressure are consistent with observed changes in temperature. Amounts in winter are estimated to be 10% to 15% higher across the center of the basin, but decrease by roughly 5% to 10% at many lake shore sites in summer. Seasonal wind speed data were considered to lack an appropriate number of quality long-term climate stations to determine spatial lake effects. Surface elevations, increasing east of the basin, complicated detection of effects due solely to the lakes.