There are some hardcore riders out there who will continue to ride year round, regardless of temperature, even in cold climates. We don't recommend it, but those who do it anyway need to be extra careful of the dangers that cold weather riding can bring. YouTuber Dodge Rider breaks it down for us.
As the winter solstice approaches, many of us in northern climates have already put our bikes away for the season. Occasionally, though, an unusually warm day will come along, tempting us to haul the bike out one more time. When temperatures in New England hit 50 last weekend I took a couple of hours for a December cruise myself. The problem with doing that, or riding in winter in general, is that as temperatures drop your body may not keep up with its heat regulation. There is a progression of signs your body will give you that it's getting too cold for your own good. If you learn to recognize them you can save yourself from injury, or even death.
Your extremities are the first to get cold since they're the farthest parts of your body from your heart. It starts with your fingertips, followed by the back of your hands. This is why heated grips exist on some bikes. The next part of your body to get cold is your legs, especially if you don't wear wind-resistant pants. The degree to which any of these body parts gets cold can vary depending on whether you're riding a fully faired touring bike or a sportbike or cruiser with no wind protection at all. It's not so much the outside temperature that will get you, but the windchill from your movement through the air.
After that comes your head. Obviously, make sure your helmet vents are closed. Some less expensive helmets may leak air inside even when fully closed. You can use painter's tape to temporarily cover up these vents and seams without damaging your helmet's finish. A balaclava is extremely useful as well. Not only will it keep your head warm, but if you cover up your mouth, the balaclava will trap moisture as you breathe through it, preventing your visor from fogging up. Even worse, this fog can turn into frost in cold temperatures, which can be a big problem since most riders don't carry an ice scraper with them. Ask me how I know.
Finally, your body will start to shiver. This should be the biggest warning sign that you need to stop riding, now. Your hands and feet no longer have the fine motor control necessary to operate the motorcycle safely, which can lead to a loss of control and a crash.
Dodge Rider also has some recommendations for staying warm on the road. You can avoid highways and stick to lower speed roads to reduce the amount of windchill you will feel. If you're maintaining a steady speed and don't need to cover your clutch, you can stick your left hand behind a windshield or even your leg to get it out of the wind and help it warm up.
I'll add a few suggestions from my personal experience. Heated gear is good. I don't own any yet, but that's a very big "yet." If you do get too cold, grab a hot drink—coffee, tea, hot chocolate, whatever—to warm your body up from the inside. There's no faster way to warm up your core than to put something hot inside of it, and the heat will transfer to your extremities through your bloodstream. A hot shower or bath will also help you warm up from the outside. A mug of hot chocolate along with a relaxing hot bubble bath sounds like just the ticket for me. Don't judge.