A Base™ 4.0 was designed to battle the coldest & most brutal conditions. Facing insane temperatures? The Under Armour Extreme Base Layer Top is what you’re gonna want to put on first. UA Base keeps you comfortable in even the most extreme chill while sitting in a treestand to keeping you cool when the sun decides to finally show. Built in to the Extreme series are strategically placed 4-way stretch panels that allow you to add stretch in places where there wasn't before as well as allows heat to breathe out significantly more giving you all day comfort. No matter if your hiking far it sitting long, rely on the Extreme Base series form Under Armour.