How to Choose Rain Pants
Rain pants are an important part of any hiker's inventory. Being able to choose the right pair for you is a matter of understanding the basic features and how they best suit your needs. There are basically three routes you can take when it comes to choosing rain pants:
Full Side-Zip Pants
Marmot PreCip Full Zip Pants
These pant feature a zipper that runs down the side of each leg, allowing you to completely unzip the legs. The primary benefit to this style of pant is that it allows you to don the pants without having to pull the legs over your feet. This is an especially nice feature when your feet are wet and muddy as you don't have to worry about getting any mud or water on the inside of the pants as you put them on. This also means you can put these on without even having to lift your feet off the ground. For this reason, this is really the only acceptable option when it comes to choosing hardshell pants for activities like mountaineering and ski touring, where you generally need to be able to put the pants on without removing boots, crampons, or skis.
The zippers have a couple other benefits. They allow easy access to pants pockets, so you don't have to pull the pants down just to get your camera out. They also allow you to vent by partially unzipping the pants. Of course you probably wouldn't want to leave them unzipped while it's raining, but having that extra ventilation is great in between squalls.
The primary downside to this type of rain pant is the weight. The zippers tend to add a few extra ounces, which may not seem like a lot, but consider the fact that most of the time this item will be on your back. Of course with the added weight also comes added durability. These will last season after season of hard ski touring. Most ultralight backpackers would simply never consider this option though simply because of the weight. If you are willing to pack the extra weight, these are a very convenient and versatile option.
Easy to put on/ take off
Ability to vent
Outdoor Research Helium Pants
I call these featureless rain pants because the designers have chosen to forego features like pockets and zippers in favor of keeping the weight low. And they do a good job of that, these rain pants are often less than half the weight of their full-zip counterparts. They are also highly packable, most will crush down to the size of a baseball and simply disappear into your pack. You won't notice they're there until you need them.
The primary drawback of this type of pant is that they can be difficult to put on, as they require you to slide each pant leg over your feet. This can be difficult, especially if it's raining or your shoes are muddy (or if you're simply not very coordinated). Outdoor Research and Black Diamond both added a small zipper near the ankle to help get the legs over your shoes. ZPacks simply oversized the ankle opening instead of adding weight with a zipper (hence the incredibly low weight). It's not that this type of rain pant is difficult to put on, just that it's not as quick or easy as the full side-zip variety. The lack of side zips also means drastically reduced ventilation. If you have to wear these things for a long time, you're going to get clammy. Get a break in the weather and you have to decide if you want to keep them on and get clammy, or go through hassle of removing them and then putting them back on when the rains start again. But if weight is more important than convenience, then these are a solid choice.
There are also 3/4 zip pants, which have a zipper that runs about 3/4 of the way up the legs. Some would consider this a type of full-zip pants, but seeing as you still have to slide each foot through the legs to get them on, it seems to make more sense to put them in this category. They are easier to put on than typical featureless pants, but not quite as easy as full-zip pants.
More difficult to put on/ take off
No ventilation options
ZPacks Rain Kilt
Coming in as the lightest option, rain kilts provide an exceptional blend of packability and versatility. When it comes to weight, there is simply no comparison. At less than 2oz., you'll never notice it, even if you store it in your pants pocket. Which is easy to do, because these things pack down to the size of a wallet. Along with being ultralight, the breathability is unparalleled. Kilts vent just as well as full side-zip pants, but unlike with full side-zip pants, kilts inherently have a large vent opening at the bottom that is always open. The simple act of walking helps to circulate air, meaning that it's basically impossible to overheat or get clammy while wearing a rain kilt. Which is ironic, because a lot of these kilts are made out of materials that are not breathable at all. Rain kilts have either a large slit or zipper at the back to ensure movement isn't restricted as you walk. I typically leave my kilt about halfway unzipped while hiking.
Kilts are generally made out of two different materials: silnylon or cuben fiber. Cuben fiber is more expensive, but lighter and more durable. Silnylon is cheaper and heavier (though still very light). Fabric choice is a personal preference. And if you're into supporting small outdoor gear companies, buying a rain kilt is a great way to do that, as most kilts are made by cottage companies. Or you can make one yourself! It's cheap and easy to do, here's one tutorial.
Another advantage of the kilt is that when you fully open it, it's basically just a large rectangle of waterproof fabric. You can lay it out on the wet ground while you organize your gear and keep everything dry. It can also be used in camp as an extra footprint in the tent vestibule, giving you more dry space to put things on, again, ensuring that your gear stays dry. There are some thru hikers that will wear their rain kilt (and nothing else!) on laundry day while all the rest of their clothes are getting washed. Rain kilts are supremely versatile and potentially one of the most useful pieces of gear you could bring on an outdoor adventure.
The main drawback comes into play during extreme weather. Because the bottom of the kilt always remains open, it doesn't provide the same level of protection that typical pants do. Your lower legs may get wet during periods of rain coupled with high winds. But this typically isn't an issue unless it's raining sideways - like I said, extreme weather. This is the main reason these simply don't work for skiing and mountaineering.
The only other drawback is really more of a personal preference. After all, it is a....kilt. Some people don't like the idea of wearing such a piece of clothing. And that's fair - these aren't for everyone. If you don't mind the strange looks you'll get, I believe this is the best option for hiking and backpacking. It combines the best of both worlds: the convenience and ventilation of full zip pants with the weight and packability of featureless pants.
Won't win any fashion contests