It was 20 degrees F when I left for work on my bike this morning. But that number doesn't tell the whole story. Taking into account the self-generated wind a biker creates by moving through the air, the chill factor made it feel like 3 degrees F. Brrrrrrrrrrrrrr!
Here is a great chart you can use to determine your daily Chill Factor:
To be exact, you need to add/subtract the headwind/tailwind during your trip to get the actual chill factor. For example, if it is 20 degrees, you are averaging 15mph, and you have a 10mph headwind (as I did today), then the wind to use is actually 25mph. If you have a 10mph tailwind, then use 5mph in the calculation.