A fleece jacket is a lightweight casual jacket made of a polyester synthetic wool such as polar fleece. It is typically designed to function as a mid-layer for outdoor activities in a range of weather conditions. Fleece is a soft-napped, insulating fabric made from polyethylene terephthalate (PET), a hydrophobic fiber. It is very comfortable due to its light weight and anti-perspiration qualities, and allows moisture to evaporate while blocking humidity from the outside. Fleece provides thermal insulation but is not normally weatherproof and so it will not effectively keep out wind and rain. A fleece jacket will typically have a zipper up the middle, rather than buttons or other fasteners. Fleece jackets work best when worn over a moisture-wicking baselayer material like polypropylene, Capilene, silk, or merino, and under a waterproof and/or windproof hardshell jacket. Fleece is a synthetic fabric thats woven from polyester, a type of plastic derived from petrochemicals. It is a vegan alternative to wool and is less expensive than down. Fleece technology is continuously improving, and the yarn is now as fine as cashmere and can even be made from recycled plastic bottles.