I live in the Midwest US (specifically southern Wisconsin). Our temperature generally ranges from maybe -10 to +90 F or roughly -20 to +30 C, with occasional weeks that go beyond this range.
I commute to work by bike throughout the year. Admittedly, I avoid the worst of the summer heat by adjusting my work hours so I leave for work before it gets too hot. Also, I'm kind of a freak in the sense that I don't sweat a lot.
If you're curious about this, the weather history feature for Weather Underground will produce nice graphs for most places worldwide.
The midwest has some of the coldest winters of any region of the US because it doesn't have the ocean nearby to moderate temperatures. Most of the midwest would spend most or all of December, January and February well below 40 degrees.
