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.