So some well-meaning person tells you that to stay warm in cooler weather, you've got to "dress in layers." Well, what is that supposed to mean? Every time someone talks about layers I think about onions and how starting at the outside, you can see the distinct layers that can be peeled away to gradually expose the core. Layering your clothes follows the same principle, only without the strong smell and all the crying. Unless you've been on a particularly difficult ride.
When you dress in layers, you generally have three different types of clothing on your body, each with a distinct purpose. Let's walk through them one by one and with each, talk a bit about what you're trying to accomplish.
The base layer is the one closest to your body. Its purpose is to help you stay warm but not get too hot, and most importantly, should serve as a way to carry perspiration away from your core. Think high-tech long underwear. Believe it or not, wool is an excellent material for this, and some synthetic materials are suitable as well.
The purpose of the middle layer is to insulate and at the same time to still continue to move moisture away from your body. This layer is "fluffier" than the base layer if that makes any sense, keeping warmer air in close to your body yet allowing it to still circulate around you so that you don't get too warm in periods of high activity.
Of the three layers, this one will vary the most depending on the temperature and the individual person. In addition to the actual air temp, your level of activity plus the amount of sunlight and wind combined with your level of fitness and metabolism will all factor into how much insulation you will require in this middle layer.
Polyester fleece is a good choice for an insulating layer. Wool turtlenecks or vests also work well. Some good examples of middle layer clothing are the Pearl Izumi Woolie Mammoth jersey or the all wool Castelli Armando Long Sleeve Jersey.
The outer shell primarily serves as a windbreaker and also works to thwart any precipitation you may encounter. This layer's first job is to keep you dry; the warmth you'll experience comes from what's underneath. The outer shell should be breathable to help get rid of the moisture you're generating through perspiration; strategically placed vents (such as under the arms) are a big help to this end. Nylon is the most common material for lightweight outer shell garments; heavier ones are going to be made from GoreTex or another one of the patented fabrics. A lightweight example of this type of outerwear is the Louis Garneau Winddry Jacket; the Pearl Izumi Barrier jacket is one that runs a bit heavier.
Enjoy Riding in Any Weather
Every day is a great day for riding when you pair this principle of layers with appropriate coverings for your hands, head and feet. With the right gear you can safely and comfortably ride on days when the temperature dips well below freezing and take your bike out in weather that before you might never have considered for riding.