I ride my bike all year round, and since I live in Canada that means I have to bundle up quite a bit in the winter! When temperatures dip below 5 Celsius or so, I put my "summer" helmet back on the shelf and reach for my custom modified all-weather helmet. It's so effective that despite my mostly bald head I don't need to wear a hat underneath, even at temperatures down to -20C at speeds in excess of 40 km/h (tested and confirmed!)
The All-Weather Helmet is also completely waterproof, so it's great to have when it's raining, sleeting, or snowing.
Virtually any helmet can be modified into an all-weather helmet, the only requirement is that the plastic decorative shell can be removed.
One bike helmet in your size. Make sure the plastic part comes off.
About one square yard (or one square meter) of nylon fabric
Contact cement or some other glue with a long open time, that won't melt foam
A plastic face shield (optional)
A sharp knife
Step 1: Remove the Plastic Shell
The first step is to remove the plastic decorative shell that is stuck to the foam of the helmet. Chances are it will be taped around the edges, and maybe glued in a few spots here and there underneath. If your helmet is more securely attached (like my Giro "summer" helmet) then this may not work so well.
If your helmet has a visor, pop it off and set it aside.
Start by cutting the tape around the perimeter, or just peeling it off. Lift the plastic cover at one end, and feel for the first spots of glue. Gently break the glue connection, being careful not to dent the plastic or pull pieces of foam off the actual helmet. Use a metal shim, such as a ruler, to make this easier. Do this for all the glue spots, until the plastic cover comes loose.