Men's long-sleeve base layers

it covers your forearms, where heat escapes, and lets you do without a sweatshirt in transitional weather. In winter, reach for a heavier fabric weight and leave room above it for an insulating layer; crammed layering does not keep you warm, because you compress the air that insulates. A short zip at the neck is subtly useful, as you can ventilate on an ascent before sweating through. You will appreciate a longer back under a backpack, where a t-shirt would otherwise pull out of your trousers.