So-called because Chicagoans, who live under gray skies for about 70% of the year, break out summer clothes at any glimpse of sunshine.
Suprisingly, they are usually quite comfortable in t-shirts when it's 20 degrees Farenheit, as the long, harsh winters give them opportunity to become acclimated to bitter cold.
2. When winter is not quite over, but it's sunny and no longer snowy. Still cold enough for a jacket.
I'm so happy it's almost spring! The snow's finally starting to melt, and we've been getting Chicago t-shirt weather instead of blizzards.