Why Ride Year Round?
According to Bob Matter, the BW meteorologist, the Chicago Bike Winter season is defined as the 181 day period between November 1st and April 30th. During that period you can expect 117 days, or 65% of the time, to be +21F or warmer. That means the LOW of the day will be +21F or warmer. Typically the difference between the daily low and daily high temperature is 15 degrees. So on a day when the low temperature is in the low 20's, you can expect the high temperature to be in the mid 30's. That is an excellent temperature range for outdoor activities like cycling and cross country skiing. See below for the 2010-2011 stats.
Bike Winter Weather Update