Life on a bike can be miserable when the temperature drops and speed becomes wind chill. The trick to cycling comfortably in inclement weather is layering and breathable fabrics that wick the moisture away from your skin, so you radiate heat effectively. As they say in Scotland where it rains 300 days a year, there is no such thing as bad weather, just bad clothing.
The main function of the base layer is to insulate the core of your body and wick moisture off the surface of your skin. Often these are surprisingly light garments and many professional cyclists wear them under their cycling shirts even in warm weather. You should not even be aware of the extra layer if it wicks properly. Crash protection is a bonus, because the two layers of fabric absorb some of the abrasion as they slide over each other. To wick effectively, an undershirt should fit snugly. Some are tight enough to have compression properties, but while compression has been proven to be help recovery, it is not clear that you need or want it during your ride. Undershirts specifically designed for cold weather often have higher necklines and fleecy front panels to insulate the parts of the body that are most exposed. Some will also have long sleeves which will keep you warm, but this is not as flexible an arrangement as an undershirt combined with arm warmers, so you might end up feeling too warm if your ride does not end before the day heats up.
The purpose of this layer is to compliment the work of the base layer, and there are separate garments for the torso and limbs. On your torso, the mid layer needs to draw moisture wicked by the undershirt even further away from the skin, and help to regulate your body temperature. Here in South Africa the mid layer will generally be your cycling shirt, even in winter. It is rarely cold enough to justify a heavier fabric, but if you have a choice of sleeve lengths, this may come into play. In mild weather a long sleeved shirt with a light pair of arm warmers and an undershirt is often warm enough. If you have a long sleeved fleecy undershirt you may well find that a sleeveless shirt warn like a gilet as a mid layer is all you need.
While fleecy tights with the same padding as your shorts are wonderful for early morning and night rides, our daytime temperatures rarely justify their cost if that is all the riding you do. Because arms and legs are more sensitive to the cold, money spent on cold weather arm warmers, and knee warmers will never be regretted. Worn with your usual shorts and a cycling shirt, these can be rolled up or down to keep you warm on the descents and comfortable on the climbs, but they are compact enough to be removed and stowed in a jersey pocket as it gets warmer. Don’t forget to put sunscreen on your arms after you remove your arm warmers if you plan to be out for a while. Sunburn happens, in winter too.
Three main features are important when you choose a garment to use as an outer layer: wind protection, moisture wicking and waterproofing. That sounds easy enough, but constructing a garment with equally good moisture wicking and waterproofing properties is very tricky and often this has an impact on the price. On the Highveld, moisture wicking and wind protection are paramount because wet days are rare, but in the Cape riders often have to choose between being a bit too warm and keeping dry. In the dry cold of the Highveld a wind-proof sleeveless gilet is often sufficient to keep you warm, provided your base and mid-layers do their work. A chest warmer, which protects your neck and chest from the wind but allows your back to radiate heat, can also be very useful. With windproof fabric on the outside and fleece on the inside, these do the same work as the newspapers that the professionals stuff in their jerseys on the descents, but they are re-useable.
If anything is going to drive you off the bike, it might be cold hands, feet and ears, as these are most sensitive. Wind and moisture proofing are the most important considerations for your hands and feet, but excellent moisture wicking socks and gloves are available. These are often worn as base layers under lighter garments with more wind stopping than wicking properties. Fairly simple neoprene toe-caps or booties for your feet, and gloves for your hands might also do the job, but these simpler garments will become quite uncomfortable if you are still riding by the middle of the day. The head and ears radiate a lot of heat, so you need to keep them warm if you want to be comfortable on the bike. A light fabric with good wicking properties works best, if you want to avoid overheating and steamed-up glasses.