Precipitation Climatology of Lake Michigan Basin

A detailed climatological study shows that the average annual precipitation over Lake Michigan is 6 percent less than that of the land portion of the basin. This is contrary to results of most previous studies which indicated that lake and land precipitation were equal. However, the results of this study are more accurate than the earlier findings because they are based on more detailed information from recent lake precipitation studies, a greater knowledge of the effect of the lake on the atmosphere, and a greater volume of weather data than existed when the previous studies were made. The chief purpose of this study was to obtain the best possible measure of the average precipitation over the lake in order to estimate more accurately the total water resource of the Lake Michigan Basin. The 6-percent lower precipitation over the lake means that on the average the Lake Michigan Basin receives about 700 billion gallons less water from precipitation per year than would be received if the average lake precipitation were equal to that over the land. This report presents the results of four separate investigations. The first three quantify the effect of the lake on thunderstorms, hailstorms, snowfall, and the amount of precipitation from these conditions. In the final investigation these results were combined with land and limited lake precipitation data to derive the average lake precipitation values. The lake both suppresses and enhances precipitation activities, its effects varying seasonally and areally. Lake-effects suppress summer thunderstorm activity by 20 percent over the southern end of the lake, but increase fall thunderstorm activity by 50 percent. During fall, lake-effects cause 400 percent more hail days in lower Michigan than occur in surrounding areas The lake also causes 25 to 100 percent more snowfall and days of heavy snow along the eastern shore than occur along the western shore. The amount of annual thunderstorm precipitation on the east side of lower Lake Michigan averages about 10 percent less than that on the west side. The west-east decrease in thunderstorm precipitation actually begins over the land area immediately west of the lake, and most of the lake-effect decrease occurs over the western half of the lake. Average annual precipitation from snowfall on the east side of the lower lake is 33 percent greater than that on the west side. Most of the west-east increase in snowfall precipitation occurs near the eastern shore with maximization 10 to 25 miles inland in western Michigan. Average lake precipitation in summer is 8.4 inches, which is 14 percent lower than that of the surrounding area. The average winter lake precipitation is 5.4 inches, which is 4 percent higher than that of the surrounding land area. The average spring lake value is 7.4 inches, 7 percent less than the land average, and that in the fall is 8.4 inches, which is equal to the land average. The average annual precipitation over the lake is lowest, less than 27 inches, in the westcentral portion, and highest in the extreme southeastern and northeastern portions. The lake average annual value is 29.6 inches. I N T R O D U C T I O N An exhaustive climatological study of the precipitation regime of the Lake Michigan Basin has been performed to obtain the best possible estimates of the average precipitation over the lake. Increasing national concern with water resources has focused greater attention on the Great Lakes. Scientific evaluation of Lake Michigan as a resource is limited because the water budget of the lake has never been properly measured. The lake surface comprises 30 percent of the basin and only a very few short-term measurements of precipitation have been made on the lake. If accurate calculations of the average annual precipitation could be derived, the lake's water budget could be estimated immeasurably better. The lake comprises 22,400 square miles and is a major influence on the lower atmosphere. At most times of the year the lake water temperature is quite different from the temperature of air masses passing over the lake, so that the lake is constantly acting to either warm or cool the overlying air. Through evaporation the lake also serves as a moisture source for overlying air masses. This