Great Lakes lake-breeze circulations, which occur most often during the spring and summer months, can have large economic, societal, and climatic impacts on coastal regions. Although lake breezes are mesoscale weather phenomena, they can significantly modify the summer climatic conditions in the Great Lakes coastal regions by frequently providing cooler temperatures several 10s of kilometers inland from lake shorelines. Lake breezes have also been shown to influence the dispersion of atmospheric pollutants and nuisance airborne biota in lake coastal regions.

Several variations of the classic lake-breeze index, LBI, were evaluated using identified lake-breeze events along the eastern, western, and both shores of Lake Michigan during a 15-year period (1982-1996). Indices computed using offshore or shore-perpendicular wind speeds at inland sites, U or |U| respectively, were found to provide the best prediction of lake-breeze events. The different values of LBI calculated using either U or |U| resolved > 95% of identified lake-breeze events based on critical LBI values of 2 through 6. When wind speed, irrespective of wind direction, was used to calculate LBI the success of the critical indices decreased by as much as 26%. Our finding confirms those by previous studies that showed the lake-breeze index has a significant tendency to overestimate the number of lake-breeze events. Based on our analyses of values calculated using wind speed, a critical threshold of LBI=3 would tend to underestimate the actual number of lake-breeze events by about 20% and identify nearly 60% of non lake-breeze days as lake breezes. Additional results of the classic LBI confirm the theoretical finding of Walsh (1974) and demonstrate that LBI may not be significantly affected by changes in location as suggested by both Biggs and Graves (1962) and Lyons (1972). Further changes in the prediction of lake breezes through modifications to the classic lake-breeze index using data from over-lake and inland measurement sites (e.g., boundary-layer stability) will be presented and discussed.