It is always challenging to ride your bike in cold weather.
This is mainly due to starting out cold, then sweating which makes you wet. This in turn makes you susceptible to all kinds of nasty illnesses and injuries. Wind chill and wet skin are a horrible combination that can take their toll not only on your riding, but also your health. Thankfully you can overcome cold weather riding through the use of special mountain bike clothes.
Socks
You don't want overly heavy or thick socks whilst riding as it will tighten your shoes on your feet and lead to circulation problems.
Lightweight socks made specifically for riding are the ideal choice. You can also try ski or snowboarding socks to see how they feel whilst you're riding; these are sure to be warm.
Underwear
Many people overlook underwear for riding in cold conditions however it is vitally important that you consider it.
Consider polypropylene underwear which is great for cold weather and also lightweight.
When riding you are creating more wind against your body than if you were standing still or walking. This is where extra heat can be lost within your body, particularly when the wind is extra chilly.
Your outer clothing should be waterproof and windproof without being excessively heavy and bulky.
Your helmet should also be cold weather proof or at least made with the cold in mind. You can buy bike helmets with extra insulation. Humans lose heat through their head so a good quality helmet is absolutely vital if you are to retain heat and minimize heat loss.
Winter mountain bike clothes are a necessity if you live in or regularly ride in cold climates.