A Mid Layer is worn on top of your Base Layer and underneath your Outer Shell. There are many differant forms of Mid Layer available, but the primary purpose of this type of outdoor clothing is to insulate and retain heat.
Fleece has excellent insulating properties and is often worn as a Mid Layer. The level of insulation a fleece provides is indicated by its weight. The most common fleece weightings generally available are 100, 200 and 300 weight, with 300 weight fleece being the most insulating. When choosing a fleece weighting, it is important to consider that although a higher weighting like 200 or 300 weight offers the advantage of being warmer, this weight of fleece will be heavier and will not pack down as small as fleece with a lighter weighting like 100 weight.
Mid Layers constructed of 100 weight fleece are often worn as part of a flexible layering system. When the temperature is cold enough, the 100 weight fleece is worn as a Mid Layer (or sometimes the Top Layer if the weather is dry) to provide warmth. However, if the temperature of the person wearing the garment increases as a result of higher levels of exercise or a general environmental increase, the fleece can be taken off and carried inside the Rucksack with ease. 100 weight fleece is light and packs down well. As such, it offers great flexibility if you are taking part in high energy activities such as trekking, scrambling, skiing or mountaineering.
Mid Layers are available in many differant styles. One of the most popular styles currently available is the 1/4 zip fleece top, which is light, packs down well and is easily carried in a rucksack. Mid Layer full zip fleece jackets are also available. It is also possible to find full zip fleece jackets that zip in with outer shell jackets. An outer shell that includes a fleece that zips in is known as a 3-in-1 jacket. It is worth noting that only compatible Mid Layer and Outer Shell jackets can be connected to form a 3-in-1 garment.
Most Mid Layer manufacturers offer their own branded fleece fabrics. However, garments constructed using well known branded fabrics such as Polartec and GORE WindStopper are also available. Outdoor clothing constructed of Polartec and GORE WindStopper comes at an additional price, but these brands are widely recognised as high quality.
