After the base layer (technical underwear) and a mid layer (insulating clothing), your choice of outer layer is important. The type of activity will dictate how lightweight, protective and warm your jacket should be. Above all, the performance of your outer layer is determined by two main indicators: waterproofing and breathability.
A skier who skis mainly on the piste will have different out layer needs to a mountaineer or ski tourer. Similarly a person snowshoeing will have a different type of outer layer to a trail runner. Each sports activity requires a specific outer shell which varies according to a sport's intensity and how it's performed. However, the outer layer must at least be waterproof and breathable to ensure the overall effectiveness of the three-layer system. Other criteria may also apply to specific sports.
The more a jacket prevents water from penetrating, the more water resistant it is. Waterproof levels vary according to the technicality of the jacket, which is measured in water pressure or Schmerber (1 Schmerber = a tube filled with 1mm of water). Water resistance is possible by using special coatings which keep material dry under intermittent, fine rain, with a risk of saturation in heavier or persistent rain, or by using membranes during manufacturing which resist a much more sustained water pressure.
For sports lovers who enjoy outdoor activities occasionally and in mild weather, a moderately water resistent jacket will do. On the other hand, those who go out any type of weather should invest in a more waterproof jacket (minimum 20,000mm pressure, or 20,000 Schmerber).
Required for wicking perspiration away from the body, breathability varies depending on the jacket type. Its level is evaluated by a unit of measurement called the MVTR, which is expressed in gr/m2/24h (the quantity of water that the fabric lets through in 24 hours). Measured from 5,000gr to 30,000gr, this figure rises with a fabric's breathability.
It's highly recommended that active athletes who perform intense activities, such as mountaineering or ski touring, opt for a highly-breathable jacket (minimum 20,000gr) to avoid sweating during exercise.
The weight of the outer layer should also be taken into account. The more intense the activity, the more a lightweight jacket will facilitate movement. This is indeed the case in mountaineering and ski touring, but also when walking medium to long distances. The lightness of a garment often corresponds with its ability to compress. Thus, the lighter and thinner a jacket is, the easier it will be to slip into the a bag — a factor truly appreciated by mountaineers and hikers.
Some outer layers have an insulating layer which reinforces the effect of the mid layer, in addition to the essential features of water resistance and breathability. However, the more intense the activity, the less insulation is required. Apart from athletes who ski or snowshoe in a "calm" manner, it's often better to skip an insulated outer layer to avoid getting too hot or hindering the breathability of the shell.
Athletes who perform intense activities or endurance sports such as mountaineering, ski touring, or freerando (ski touring with more freeriding), should choose a jacket with a high level of water resistance and breathability (minimum 20,000 for each). This advice also applies to experienced skiers and snowboarders who ride in any weather conditions and who use more energy, like when freeriding.
The requirement in terms of outer layer remains the Gore-tex membrane, since it can remain waterproof and breathable in the harshest of environments. It is available in different membranes, from the most versatile to the most effective: Gore-tex classic, Gore-Tex Active, Gore-Tex Pro. Other membranes with similar properties have also been developed by some brands: Polartec Neoshell (Polartec), Hyvent (The North Face), Dry-edge (Millet), Defender (Eider), H2no (Patagonia), and others.
If you're an occasional participant in these sports and all that jargon is lost you, don't panic. When choosing your outer layer, rely on the waterproof and breathable properties advertised on the label. A jacket rated at 10,000mm (water resistance) and 10,000gr (breathability) will perfectly meet your needs. It's now up to you to evaluate your preferences in terms of thermal insulation.