It’s best to be able to add layers if you’re cold and shed layers if you warm up. You can always take off layers, but you can’t put more on if you didn’t bring them.
Bulkier is not always better. Having a base, middle and outer layer is the best way for your body to respond to the cold, wind, and precipitation.
Wool works best for a base layer because of its ability to wick away moisture. Avoid cotton as a base layer because it holds moisture, which can make you colder.
Because fleece insulates your body heat, it’s one of the best fabrics for a middle layer. (It’s also the cheapest.) Other good middle layer fabrics are synthetic, down, and wool. However, fleece as an outer layer doesn’t bode well against the elements because winds can cut right through it.
To protect your body from wind, snow, and rain, wear a waterproof and breathable shell. Wearing an outer layer that soaks in moisture or that is nonbreathable is be okay for dry and cool days, but not for cold, windy, snowy, or rainy days.