IMPACTS OF THE GREAT LAKES ON CLIMATE AND WEATHER

No control on Michigan’s climate probably receives more public attention than do the Great Lakes. Their effect is primarily local and is best developed on the lee-shore (eastern and southeastern shoreline) areas of each of the lakes. There this localized effect of the Great Lakes on the climate is of great importance to the state’s agricultural economy.
    During the season when the lakes are generally colder than the air over them--April to August, but particularly in the spring--they extract heat from the overlying atmosphere. This chilling effect may strengthen the polar air masses entering the Midwest from northern areas and allow them to reach Michigan stronger and colder, and more persistent, than they might otherwise be. The Great Lakes have a reverse, though minor, effect on passing cyclonic storms when the lakes are a source of heat, especially October through December. Certain winter cyclones that cross the Great Lakes probably gain strength or size because of the heat and moisture they acquire from the relatively warm water, but the path these storms take is largely determined by the upper-air circulation. So, we cannot blame the lakes for the winter storms in Michigan, but they may strengthen the storms that do occur, prolonging the associated cloudiness and inclement weather.
    The lakes influence on local weather is more varied and dramatic. During the winter months, when cold air crosses the relatively warm water, the air takes up heat and moisture from the lakes. As a result, the lower layers of the atmosphere are warmed, and upward-moving air currents develop. How much warming there is depends upon the length of time the air spends over the water and the temperature difference between the water surface and the overlying atmosphere. Contrasts in excess of 15� F are common, and differences as great as 50� F or more are possible. This warming leads to the formation of puffy-looking cumulus clouds. In general, the stronger the ascending air currents, the more likely it is to produce precipitation, usually as snow. The snow clouds then drift inland with the prevailing low-level winds, resulting in "lake-effect" snow showers within a 15-40 mile-wide zone starting on the lee shore of the lake. With a contrast of less than 15� F between the temperatures of the lake’s surface and the lower atmosphere, precipitation is unlikely, and with a contrast of less than about 6� F clouds may not even develop.
    Some lake-effect snowfalls can be spectacularly heavy; but such snowfalls require not only a great contrast between the temperatures of the water and of the air but also a comparatively long over-water trek to maximize cloud formation. Because most extremely cold winter air masses in Michigan are accompanied by westerly or northwesterly surface winds, the air crosses the narrow axis of Lake Michigan or, in the case of the Keweenaw Peninsula, passes over only the western portion of Lake Superior. Consequently, the time the air spends over the water is much less than if the flow had been parallel to the lake, and the associated snowfalls seldom exceed 6 to 10 inches. Still, because lake-effect snows are frequent in winter, their cumulative effect greatly augments the average annual snowfall in the lee-shore counties. Lake-effect snow seldom falls in significant amounts further inland than about 40 miles, but it is accentuated where inland-moving snow clouds cross higher terrain, as in Otsego County or on much of the Keweenaw Peninsula/Huron Mountains. Often, the lee-shore counties can be getting significant lake-effect snow while the sun is shining in eastern Michigan. Under exceptional conditions (severe cold and strong low-level westerly circulation), snow clouds from Lake Michigan may cover much of the Lower Peninsula far beyond the usual snow belt, though accumulation in the central and eastern counties is usually small.
    A secondary consequence of lake-induced wintertime cloudiness is a moderation of severe cold immediately downwind from the shoreline. Not only do the lakes add heat to the lower atmosphere and hence prevent exceptionally low nighttime temperatures in lee-shore locations, but the resulting cloudiness helps retain heat. Thus, regions about 20 miles downwind from Lake Michigan enjoy somewhat higher minimum temperatures than do locations farther inland.
    Just as the Great Lakes are a source of heat in winter, they remove heat from the atmosphere in spring and summer, and this influence again is strongest over the peninsulas and downwind areas. Thus, maximum July daily temperatures, maybe 10� - 18� F lower along the Michigan shore of Lake Michigan than along the Wisconsin shore. But this effect seldom extends more than a few miles inland.
    Often, the cooling influence of any of the Great Lakes is developed in a rather sharp zone and coincides with what is called a "lake breeze." The lake breeze develops when there is a strong temperature contrast (more than 15� F) between the cool lake water and the warmer air over the nearby land surface-as is typical on almost any summer afternoon. Under these conditions, a shallow layer of lake-cooled air spreads inland in all directions. Its circulation is weak and local, rarely penetrating more than a few miles inland. The breeze progresses farthest inland where it is not opposed by the regional surface wind, but it usually affects only shoreline areas and may not develop at all when the surface wind blows strongly offshore. The effect of the lake breeze is such that daily summertime maximum temperatures in areas near a shore average several degrees lower than in areas farther inland. When the lake breeze develops, it suppresses cloud formation, and the sky is often clear. For this reason, coastal locations in Michigan are sunnier on the average during the summer than are inland locations.
    Places along the shore do have fog more often, however-chiefly during spring and early summer when the Great Lakes are still cold. The surface layers of warm, moist air moving offshore are cooled by the water, and the moisture in the air may condense into fog. A slight shift in the direction of the wind then transports this fog inland. Although usually confined to the immediate shoreline, the fog may extend several miles inland under ideal conditions. The area that is foggy most frequently during the summer is the shoreline of Lake Superior because, of the five Great Lakes, Superior is the deepest, largest, and hence the coldest during this season. Over the open lake, fog is even more prevalent than along the shoreline and has considerable significance for lake shipping.

Parts of the text on this page have been modified from L.M. Sommers' book entitled, "Michigan: A Geography".