Layering your clothing is a tried-and-tested way to ensure your comfort in the outdoors, Mallory and Irvine wore 7 layers each on Everest. The beauty of this simple concept is that it allows you to make quick adjustments based on your activity level and changes in the weather.
Each layer has a function. The base layer (against your skin) manages moisture; the insulating layer protects you from the cold; the shell layer (outer layer) shields you from wind and rain. You simply add or subtract layers as needed.
For a deeper understanding, read on.
Your Base Layer: Moisture Management
This is your next-to-skin layer. It helps regulate your body temperature by moving perspiration away from your skin.
Keeping dry helps you maintain a cool body temperature in the summer and avoid hypothermia in the winter. If you've ever worn a cotton T-shirt under your raincoat while you hiked, you probably remember feeling wet and clammy, even though you weren't getting wet from the rain itself. Cotton is a fabric that retains perspiration and can leave you chilled.
For outdoor comfort, your base layer is normally made from synthetic fabrics. Rather than absorbing moisture, these fabrics transport (or "wick" like wax on a candle) perspiration away from your skin, dispersing it on the outer surface where it can evaporate. The result: You stay drier even when you sweat, and your shirt dries faster afterwards. Remember wet skin leads to 25 times faster heat loss, if you don’t believe me ask an otter.
A base layer can be anything from briefs and sports bras to tights and T-shirts in long and short sleeves. It can be designed to fit snugly or loosely, generally closer fitting base layers work better, this “second skin” leaves no uncovered areas where sweat can remain to cause a chill. Base layers also vary with regard to their thermal properties, so you would choose a thicker weight option for winter and a thinner one for summer.
Your Middle Layer: Insulation
The insulating layer helps you retain heat by trapping air close to your body.
Natural fibers such as wool and goose down are excellent insulators. For very cold and dry conditions, goose down is best. It offers an unbeatable warmth-to-weight ratio and is highly compressible. Down's main drawback is that it must be kept dry to maintain its insulating ability.
Alternatives to down are synthetic fibres that have down-like properties, these include Primaloft and Jack Wolfskin’s Fibre Cloud, whilst not quite as warm or compressible as down they offer unrivalled warmth in wet or damp conditions and so are ideal fro the UK, where, lets face it is often like this.
Classic fleece such as Polartec 100, 200 or Thermal Pro polyester and other synthetics such as Thinsulate and Nanuk provide warmth for a variety of conditions. They're lightweight, breathable and insulate even when wet, although nowhere near as well. They also dry faster and have a higher warmth-to-weight ratio than even wool. Classic fleece's main drawbacks are wind permeability and bulk (it's less compressible than other fabrics).
Your Shell Layer: Weather Protection
The shell or outer layer protects you from wind, rain or snow. Shells range from high end mountaineering jackets to simple windproof jackets. Most allow at least some perspiration to escape; virtually all are treated with a durable water repellent (DWR) finish to make water bead up and roll off the fabric. Which if well maintained allow the under layers to breath and deal with moisture and sweat.
An outer shell is an important piece in bad weather, because if wind and water are allowed to penetrate to your inner layers, you begin to feel cold.
Furthermore, without proper ventilation, perspiration can't evaporate but instead condenses on the inside of your shell.
Fit is another consideration. Your shell layer should be roomy enough to fit easily over other layers and not restrict your movement. You may need to try a fleece or insulating jacket with your shell, so either bring yours or borrow one of ours.
Shells can be lumped into the following categories:
Waterproof/breathable shells: The most functional (and expensive) choices, these are best for wet, cool conditions and alpine activities. Shells using laminated membranes such as Gore-Tex and Texapore offer top performance; as do Paramo Analogy garments, those using fabric coatings are a more economical alternative.
Soft shells: These emphasize breathability with some level of water resistance. Most feature stretch fabric or fabric panels for added comfort during aerobic activities. Many offer both shell and insulative properties, so they in effect combine 2 layers into 1. Soft shells include cold- and mild-weather options.