A midlayer traps warm air while allowing moisture from your baselayer to continue moving outward. It is the layer you adjust most often: add it when the temperature drops, then unzip or remove it during steep climbs. Fleece works especially well for active use, while insulated jackets provide more warmth for cold, windy, or lower-output moments.

Choose fleece when breathability, quick drying, and comfort during continuous movement matter most. Grid fleece is especially efficient because its structure balances warmth with airflow. Choose a down or synthetic insulated jacket when you need more warmth for its weight, such as during rest stops or colder conditions. Synthetic insulation is generally more forgiving in damp weather; down excels in lightweight, packable warmth when kept dry.

Match warmth to temperature, wind, pace, and how easily you run hot or cold. For fast hiking in mild conditions, a lightweight breathable fleece is often enough. In colder weather, choose a heavier fleece or insulated piece, but avoid over-insulating while moving—sweat can leave you colder later. Useful features to compare include fabric weight, insulation type, breathability, packability, hood design, and pocket placement.