Chicago is along the southwest shore of Lake Michigan and occupies a plain, which for the most part, is only some tens of feet above the lake. Lake Michigan averages 579 feet above sea level, and the city is in a region of frequently changeable weather.
The climate is predominantly continental, ranging from relatively warm in the summer to relatively cold in the winter. However, the continentally is partially modified by Lake Michigan, and to a lesser extent by other Great Lakes. In late autumn and winter, airmasses that are initially very cold often reach the City only after being tempered by passage over one or more of the lakes.
Similarly, in late spring and summer, airmasses reaching the City from the north, northeast or east are cooler because of the movement over the Great Lakes. Very low winter temperatures most often occur in air that flows southward to the west of Lake Superior before reaching the Chicago area.
In the summer, the higher temperatures are with south or southwest flow and are therefore not influenced by the lakes, the only modifying effect being a local lake breeze. Strong south or southwest flow may overcome the lake breeze and cause high temperatures to extend over the entire city.
During the warm season, when the lake is cold relative to land, there is frequently a lake breeze that reduces daytime temperature near the shore, sometimes by 10o or more below temperatures farther inland. When the breeze off the lake is light, this effect usually reaches inland only a mile or two, but with stronger on-shore winds, the whole City is cooled. On the other hand, temperatures at night are warmer near the lake so that 24-hour averages on the whole are only slightly different in various parts of the City and suburbs.
In summer, a combination of high temperature and humidity may develop, usually building up progressively over a period of several days when winds continue out of the south or southwest, becoming oppressive for one or perhaps several days, then ending abruptly with a shift of winds into northwest or northerly. The change may be preceded or accompanied by thundershowers. High relative humidity often results from wind flow off the lake, but the air is then cooler and not oppressive.
In about half the summers, a temperature of 99 degrees or higher has been recorded at Midway Airport (less often downtown), while half the winters have had a minimum as low at -10 degrees. There is a 50% likelihood that the temperature will fall to 32 degrees or lower by October 26, and that the last temperature of 32o or lower in the spring will have occurred by April 20. However, temperatures this low have been recorded as early as September 25 in autumn, and as May 14 in the spring. Normal daily temperatures are below 32 degrees for 87 days during the winter. The normal heating season is from mid-September to early June. Ninety-four percent of the normal heating load is between October 1 and April 30, and 55% during the winter months of December through February. The normal air-conditioning season lasts from about mid-June to early September.
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