Your final kit list will vary depending on the trek and the season, but there are some key guidelines that apply to all treks in any mountain area.
Layering: while trekking it’s more practical and more comfortable to wear at least three thin layers rather than one or two bulky, thick layers. The layering system allows you to adjust your body warmth quickly and easily based on changes in the weather and your activity level. During your trekking day you can add or remove these layers as needed: staying warm and dry doesn’t just keep you comfortable, it also keeps you safe in the mountains. Each of your three layers serves a purpose,
The base layer, which lies against your skin, keeps you comfortable and warm by wicking moisture away from your body. Base layers are usually made either of merino wool or synthetic fabrics such as polyester. Merino wool is effective and has the advantage of being virtually odour-free, but is expensive; synthetic fabrics are cheaper, equally effective but not odour-free. For high altitude treks you should have a base layer not just for your torso but also your legs. You should choose the thickness of your base layer depending on where you’re trekking and the likely temperatures.
Next is your insulating layer, which retains your body warmth and protects you from the cold. Merino wool, goose down, Thinsulate and classic fleece are the best fabrics for your insulating layer. Thinsulate and fleece have the advantages of being breathable, lightweight and continue to insulate even when wet. Fleece comes in various thicknesses and you should choose yours depending on the temperatures you’re likely to encounter. For added protection you could choose a ‘wind fleece’ insulating layer, which has a high level of wind resistance.
Finally your shell layer is an outer layer that protects you from wind, snow and rain. The quality of your shell layer is important as it should be a breathable fabric that allows your sweat and body moisture to escape, while also preventing rain from getting in to reach your insulating and base layers. Most use breathable fabrics treated with a water repellent that causes rain to bead up and run off. You will need a shell layer to cover your legs as well as your body.