The onset of the doldrums of summer in the Great Lakes is due largely to weather patterns that set up a repeating scenario that draws very warm and moist Gulf of Mexico air directly into the Great Lakes basin. The forecast map below (courtesy NCAR) for July 5, 2010, illustrates precisely the establishment of the “Mississippi Heat Pump” – an atmospheric engine that churns this moist warm air northward.
Low pressure systems (“L” on the map) are a standard feature of the continental weather system for North America (the infamous “Colorado low” for example), and due to the rotation of the earth, they form in the lee of the Rocky Mountain cordillera and advance their way eastward.
In the northern hemisphere, low pressure areas (areas of rising air) rotate counterclockwise (“cyclones”), and high pressure areas rotate clockwise (“anticyclones”). Since air nearer the ground tends to contain more moisture, as its moves upward in a low, gets cooler, forms clouds, rain and stormy weather. In highs, the opposite is true -upper air, being cooler, descends and warms. As it does so, its capacity to hold moisture increases. Clouds dissipate, and if the air dries enough, we get the clear blue skies of a great summer day. Where the pressure difference is not great between the high and an adjacent low, we get hazy, more humid weather.
Turning back to the map, we see several areas of low pressure stretching from Texas to northern Quebec. The yellow arrows show the rotation and air flow about the lows. Note that there is a tendency in each case to draw Gulf air northward in the rotation up the Mississippi valley. What is important though, for the heat pump, is the formation of a strong high off the Atlantic seaboard (“H”- sometimes called a “Bermuda high” if they centre far enough off the coast). These highs, when they form, tend to be very stable and last several days . Note that rotation and air mass flow around the high is clockwise, as shown by the yellow arrow. You therefore have a situation where rotation about the lows over the central US coincide with the rotation around the seaboard high, forming a strong flow northward from the Gulf (magenta lines), up the Mississippi and into the Great Lakes basin. In this particular case, the draw is further augmented by a strong low over northern Quebec.
Since the lows are also drawing cooler air from the north (“cold fronts”), the interface between the lows and the high can set off very turbulent weather including strong convective and tornadic storms. From June on, these are characteristic for the upper US Midwest, upper Mississippi and Ohio valleys. Eastward flow of weather brings them across the Great Lakes, especially in the lee of lakes Erie and Ontario. Its easy to see that this repetitive seasonal weather pattern goes a long way to maintaining the rainfall load that sustains the Great Lakes, the Mississppi, and other large mid-continent river systems.