First, start with the current outside temperature (in Fahrenheit). Then, follow this logic:
If you are not on a road bike AND/OR dont like to pedal hard, subtract 5 deg.
If you didnt subtract for the last one AND its raining, subtract 5 deg. Otherwise, just make sure that your top layer is rain proof.
If the wind is over 15 mph AND NOT in the direction that you are going, subtract 3 deg.
If you take trails or side streets instead of the busier streets for a majority of your ride, subtract 2 deg.
Then find the number in the below listing and dress similar. Its that easy!
70+ : Tshirt or cycling jersey with cycling shorts. Standard socks, thin padded cycling gloves.
70-60: Tshirt with windstopper jacket OR long sleeve shirt, cycling shorts, socks, gloves.
60-50: Long sleeve tshirt with windstopper jacket, cycling tights, standard socks. Switch to thin, non insulated skiing gloves and a thin hat if below 55.
50-40: Thermal, wicking base layer (Under Armor or wool works well), long sleeve tshirt, windstopper jacket. Cycling tights under khaki pants (with leg strap). Thin skiing gloves, neck gator, and thin hat under my helmet.
40-30: Same as 40-50, but with thicker (or wool) socks, and cycling gloves under my skiing gloves.
Under 30: Switch to cycling 'claw' gloves, windproof.
Under 25: Switch to ski goggles instead of safety glasses. Make sure you have some that have clear or yellow lenses, cause you will probably be riding in the dark.
Under 20: Add another layer of warm pants, long john style. Double up the socks on the top so that they are twice as thick around my ankles, otherwise the tiny slit in between my pant and shoe gets hit by the wind, and makes my whole foot cold.
Under 15: Switch to windproof shoes (or booties or garbage bags over them or something to keep the wind out) and add another base layer to the top.
Under 10: Ride slower and in higher gears(or a fixie) so that you generate more heat. Remark at how cold it is. (Note: 9 deg F or below is the point at where is is OK to comment at how cold it is)
Thats as cool as I have gotten. I will keep updating this page as the weather gets colder.
As a rule of thumb, if you are going more than 5 miles and plan on pedaling hard, you should be a little cold when you first start. Otherwise, if you live closer and don't plan on riding hard, then you should be fairly comfortable when you first start.