Often it's easy to stay warm when it's very cold and cool when it's very hot, because under those conditions the clothing you wear only has to do a single job. However, when you're really active in the outdoors and it's spring or fall, things can get a little more complex. You might be cold when you start out at the bottom of the hill, then start getting hot and sweaty halfway up. By the time you reach the top the requirements on your clothing will be very different. Now, you could stop and change every ten minutes, but who needs that? Far better to choose an adaptable, breathable item of clothing that will keep you warm without feeling damp and clammy. Patagonia Capilene 3 is the tried and tested midweight choice, and we've also got great options from Icebreaker, Smartwool, and other top-notch brands.