Layer with purpose
The best training layer works with the forecast, not against it. Think in light, useful pieces that you can adapt as your body warms and conditions change.
A breathable base layer manages comfort close to the skin. Add a wind-resistant layer when the route is exposed, and choose a piece that can be tied around the waist or packed away when you no longer need it.
Fit matters as much as fabric. Look for room through the shoulders and enough length to stay in place, without extra bulk that catches in motion.