When riding your motorcycle in winter and bad weather, your first priority is to avoid hypothermia. Your body is in a state of hypothermia when your body temperature drops to a point that your vital organs are threatened. The body will pull heat from your extremities like feet and hands in order to suport those organs.
The initial symptom of hypothermia is shivering. As it worsens your body will not be able to function as it should, you won’t be able to think clearly and your motor skills will be affected. On a bike, these are obviously not good and it can have a high impact on your safety.
The best thing you can do if you start shivering is stop and get off the motorcycle. Get into any nearby shelter, whether that is a store, a restaurant, or a gas station. Just hang out until you warm up sufficiently. A hot beverage can help a lot as well. If there is no shelter nearby, you can still stop and get off the bike. Try doing some simple aerobic exercise to generate some internal heat in your body.
Avoid alcoholic drinks. It might seem like alcohol will warm you up, but it is only temporary and it will soon actually cool you down more than before, plus your judgement and motor skills will be dulled, making a ride even more dangerous.
The best thing to do is plan ahead and make sure you are sufficiently clothed to handle the weather. A full faced helmet can do a lot to keep your head warm. Most of our body heat escapes through our heads so this can go a long way to warming the rest of your body parts as well. You might try a neoprene face mask to take away the wind chill as well.
Obviously you will want to wear a heavy coat, but a scarf and/or a bandana can help a lot as well. Layering your clothes will multiply the insulation. Fleece is an excellent material for internal layers.
Lastly, if you plan on biking a lot in the winter, you might consider clothing that is electrically heated. This type of clothing plugs into your bike’s electrical system.