The weather in the Great Lakes basin is affected by three factors: air masses from other regions, the location of the basin within a large continental landmass, and the moderating influence of the lakes themselves. The prevailing movement of air is from the west. The characteristically changeable weather of the region is the result of alternating flows of warm, humid air from the Gulf of Mexico and cold, dry air from the Arctic.

In summer, the northern region around Lake Superior generally receives cool, dry air masses from the Canadian northwest. In the south, tropical air masses originating in the Gulf of Mexico are most influential. As the Gulf air crosses the lakes, the bottom layers remain cool while the top layers are warmed. Occasionally, the upper layer traps the cooler air below, which in turn traps moisture and airborne pollutants, and prevents them from rising and dispersing. This is called a temperature inversion and can result in dank, humid days in areas in the midst of the basin, such as Michigan and Southern Ontario, and can also cause smog in low-lying industrial areas.

Increased summer sunshine warms the surface layer of water in the lakes, making it lighter than the colder water below. In the fall and winter months, release of the heat stored in the lakes moderates the climate near the shores of the lakes. Parts of Southern Ontario, Michigan and western New York enjoy milder winters than similar mid-continental areas at lower latitudes.

In the autumn, the rapid movement and occasional clash of warm and cold air masses through the region produce strong winds. Air temperatures begin to drop gradually and less sunlight, combined with increased cloudiness, signal more storms and precipitation. Late autumn storms are often the most perilous for navigation and shipping on the lakes.

In winter, the Great Lakes region is affected by two major air masses. Arctic air from the northwest is very cold and dry when it enters the basin, but is warmed and picks up moisture traveling over the comparatively warmer lakes. When it reaches the land, the moisture condenses as snow, creating heavy snowfalls on the lee side of the lakes in areas frequently referred to as snowbelts. For part of the winter, the region is affected by Pacific air masses that have lost much of their moisture crossing the western mountains. Less frequently, air masses enter the basin from the southwest, bringing in moisture from the Gulf of Mexico. This air is slightly warmer and more humid. During the winter, the temperature of the lakes continues to drop. Ice frequently covers Lake Erie but seldom fully covers the other lakes.

Spring in the Great Lakes region, like autumn, is characterized by variable weather. Alternating air masses move through rapidly, resulting in frequent cloud cover and thunderstorms. By early spring, the warmer air and increased sunshine begin to melt the snow and lake ice, starting again the thermal layering of the lakes. The lakes are slower to warm than the land and tend to keep adjacent land areas cool, thus prolonging cool conditions sometimes well into April. Most years, this delays the leafing and blossoming of plants, protecting tender plants, such as fruit trees, from late frosts. This extended state of dormancy allows plants from somewhat warmer climates to survive in the western shadow of the lakes. It is also the reason for the presence of vineyards in those areas.

To locate more info visit the EPA's web site on The Great Lakes