Sometimes called the insulation layer, the mid-layer provides your warmth. Usually, your mid-layer is either a fleece or an insulated jacket. Like a base layer, a mid-layer should be breathable and able to retain heat.
Fleeces are available in different thicknesses and are quick-drying and breathable, offering warmth by retaining heat and removing moisture. Down insulated jackets are packable with an excellent warmth-to-weight ratio, whereas synthetic insulated jackets are able to retain heat even when wet.
It’s best to avoid cotton mid-layers, as they will retain moisture which can lead to you getting cold. The most important consideration is to choose the right level of warmth to suit your activity and weather conditions.
In milder conditions, you may not need anything on top of your mid layer, but when the weather closes in, an outer shell will be necessary.