Mid layers (or fleeces) are comfortable, lightweight, and more robust than your base layer clothing, and are usually worn over a base layer when the conditions call for a bit of extra warmth. A mid layer will typically feature a zipper for additional temperature regulation, with the option of a half zip for less bulk or a full zip for more ventilation. Highly versatile, mid layers can be worn alone over your base layer in milder conditions, or layered underneath your outer layers for additional warmth in colder temperatures.