As with skiing itself, choosing the right outfit for a day on the slopes comes with a learning curve. You need to find a system of clothing that works for you - with the "system" part emphasized. Because everyone is different, and weather conditions are never a certainty, it's important to develop a system of layers to help keep you warm and dry while allowing you to adapt to changing conditions. So, we'll start from the base up to your ski jacket and break down what goes into layering for skiing.
Sure, a waterproof ski jacket is important, but your base layer is also one of the most important parts of your kit. Since your base layers come into contact with your skin, they serve two major functions - comfort, and wicking moisture (sweat) away from your skin. Even though it's cold out, you're active while skiing and therefore sweating, and moisture means cold!
Ski baselayers are typically thin wicking layers made from wool or synthetic materials. It's very important to avoid cotton here. Cotton soaks up moisture and does not provide insulation when wet.
One base layer that might get overlooked is socks. A good pair of ski socks can keep you comfortable and warm, while a bad pair of ski socks can ruin your day. Compared to a shell layer like your snow pants, socks are cheap, so this is an easy place to increase your warmth and comfort. It might seem contradictory, but thin socks will actually be warmer than thick socks, as thicker socks often cut off the circulation to your feet. Similar to your other base layers, look for wool or synthetic fabrics.
So, once you have your base set, it's time to get warm and cozy. Your middle layer or layers are your most adaptable and warmest layers. If it's warm out, if you run warm, or if you have an insulated jacket, you might not even need to wear a mid layer. Conversely, if it's very cold, you can add another mid layer. Most skiers are fine with just a baselayer and their ski pants, but layer up on their torso.
The best ski mid layers are, once again, not cotton. Any wool or synthetic jacket that does not compromise your mobility can be a great choice. Think about layers like fleece jackets and puffy jackets.
Your ski jacket is one of the keys to staying warm and dry on the slopes. Aside from the nice little details, ski jackets have two main features, they're waterproof and breathable. This means that snow can't get in, but perspiration can get out. If a jacket is only waterproof, but not breathable, you'll just get wet from the inside from your sweat, instead of wet from the outside. The technology that goes into ski jackets can make them pretty pricey. If you don't want to go all-in and invest, you can look to other jackets you might have that are waterproof and breathable, like a rain jacket. These other jackets might be a little bit colder though, requiring more layering underneath.
There are a couple of dynamics to consider when choosing a ski jacket. First is shell jackets versus insulated jackets. Shell jackets are as the name suggests, just an outer layer - they provide protection but not warmth. Shells allow you to fine-tune your setup. Meanwhile, insulated jackets build warmth into the waterproof jacket. Insulated jackets are good for colder regions, but are less-flexible for layering than shells. Both are great options, pick based on your preference and where you ride.
The next important factor is the level of waterproofing and breathability. These stats are normally measured in the thousands, like 20k/20k waterproof/breathable. While we won't go into the details behind these numbers, we'll break it down. Ski jackets should be at least 10k/10k to keep you protected. For wetter climates like the PNW, 20k is preferred.
Choosing ski pants is similar to choosing a jacket - insulated vs shells, and various waterproof ratings. There is, however, one additional question, pants vs. bibs. Ski pants are great, but many skiers prefer the added protection of bibs. Bibs are comfortable, have some more storage options, and keep help keep snow out of your jacket and underlayers. This is another personal decision where you can't go wrong either way.
So, it might look complicated, but once you get the hang of layering, you'll be warmer, dryer, and happier than ever out on the slopes. Recapping the important points: your ski outfit is a system, all the pieces work together, avoid cotton, and be adaptable - that's the whole point of dressing in layers!