The wind shifted abruptly just before 6pm last night by my house, bringing with it a remarkable drop in temperatures. I live about 1½ km from Lake Michigan, which (with Lake Huron, hydrologically the same body of water) is the second-largest fresh-water lake in the world. In the summer, it keeps Chicago cool. In the winter, it keeps Chicago warm. In the spring, it keeps Chicago paying attention to the weather forecast.
Exhibit 1, temperatures near O'Hare at 6pm yesterday. Note the 13.3°C gap between Chicago and Wheeling, which are just 13 km apart:
Exhibit 2, when the cold front finally reached O'Hare around 8pm:
One tends to notice when the temperature drops 15.6°C (that's 28°F for you philistines out there) in less than an hour.
In other news, after tying a record high temperature yesterday (30.6°C, tied with 1986), today my home office is actually quite comfortable.