During the fall and winter months, we expect Chicago to be cold. Our landlords and building managers (reluctantly) run the heat and we all bundle up and keep our scarves and gloves handy.
But around this time of year, everyone begins to ignore the weather forecast, and starts dressing according to their own expectations of what the weather should be doing. This is stupid. For instance, a few days ago the forecast was for temperatures in the high 40's/low 50's, yet I saw many people dressed in light jackets, sweaters or no outer wear at all. And they looked cold.
Why do Chicagoans do this? I actually hear people insisting that "It's May," as if the weather should warm up on principle. May in Chicago is not a warm month. I've lived here for 15 years and I have only experienced one, ONE Memorial Day on which people did not need to slip on a sweater or jacket by mid-day. Memorial Days in Chicago are chilly. The temperature on that day is always just a little bit too cold to comfortably spend the entire day outdoors. By "comfortably" I mean that at no point do you wish you'd brought an extra pullover. Or ski jacket. That's just how it is, year after year.
This is our coldest month because it's the month during which there is the biggest discrepancy between the weather and the way people cover up. We're cold in February, but we also keep our heaviest clothes on. In May, many people slip into flipflops, switch long pants for light skirts and shorts and prematurely store their jackets. And then they freeze and spend the whole month complaining about how cold it is.
A friend of mine told me there's a Swedish saying that there's no such thing as bad weather, only bad planning. In the spring, most Chicagoans plan horribly. At the risk of sounding like a northern California jerk, maybe it's because I'm originally from the San Francisco Bay Area that I don't put away my long coat until June.