Winter Pants - Cold Weather Insulation
The legs are often the most poorly insulated part of the body, while warm tops and coats get a look in when winter arrives, as often as not, the same pants are pulled on as at any other time of the year. Winter pants will make a big difference to your warmth and comfort.
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Winter pants basics
Most of the time your pants are going to be an outer layer, so they need to be wind resistant. A degree of stretch aids comfort as does a soft lining material or brushed inner surface. Tight is bad as it constricts the circulation and doesn't allow for an insulating air gap.
Your legs aren't part of the body core, so when it's cold their blood circulation can be reduced somewhat to prevent heat loss. Your perception may be that it isn't so bad or you may be able to ignore the fact that your legs are cold, but they will be losing a lot of heat that will make you feel cold overall. The response is often to add another top layer but this makes much less difference than dealing with the source of the heat loss.
Hiking, insulated and snow pants
There are a whole range of modern materials such as polyester, nylon and polyamide which can make great outdoors pants for colder weather hiking and/or snow sports.
Hiking pants are usually a more relaxed fit than many casual pants allowing for greater freedom of movement and giving room for thermal underwear for extra warmth. The relaxed fit means that circulation is not affected and traps a layer of warm air within the pants. Hiking pants may also come with additional insulation, they may be lined with a separate thermal layer or have more substantial insulating material.
Softshell fabrics are a good choice for outdoor winter pants, they give a good degree of weatherproofing against wind and light rain. They are good for sport and other active use as they are very breathable. Softshell is designed for cold conditions with extra surface weatherproofing though are not sufficient for extended rain or very strong cold winds. "Hard shells" are fully wind and waterproof, though less comfortable to wear for extended periods.
Cotton outer, fleece lined
Cotton is a very practical material for pants, whether they be for work or casual styles such as chinos or jeans. Cotton is a hard wearing, smooth and comfortable material, but isn't good when it gets cold.
A great solution to this is to line the pants with fleece for extra warmth, like having built-in thermal underwear allowing you to wear your favourite pants style year-round. Don't use these for hiking or skiing or in conditions where they might get wet. Cotton in the wet and cold is unforgiving, it loves to hang on to water making it downright miserable to wear and possibly dangerous increasing the risk of hypothermia.
Bib overalls, bibs
High pants with a chest panel and maybe a back panel too, they add extra insulation and keep you covered while squatting, bending or stretching, especially good for work activities or skiing where there is a chance of exposing the midriff or getting snow ingress at the waist.
Available with tough cotton outers for work, with snow gaiters at the ankle for skiing and varying amounts of insulation.
Traditional materials - moleskin, corduroy and wool
Moleskin and corduroy are heavyweight, soft cotton fabrics. They have a tight dense weave and have a soft nap on the outside that adds to comfort and resistance to wind. For warmth, comfort and practicality they have no rival (imho). The downside is that they are not as long-term hardwearing as synthetic materials and tend to cost a little more.
These fall in between dress and casual wear in terms of smartness, corduroy maintains its look for longer while moleskin becomes faded and a little baggy around the knees in a similar way to denim after a while.
The whole time I was in Antarctica I wore moleskin pants, something I still wear in the colder months, I wouldn't consider wearing anything else.
Formal or dress pants in wool
If you have to wear formal pants, then you're probably going to be indoors most of the time, though work places can be prone to draughts and lower temperatures in the winter than at other times of the year. There's also the process of getting to work and back home again where even if you make the journey by car, it can be really cold until you've been moving a while and it gets warmed up.
Fortunately fine wool is widely used for smart pants and this is what you should look for. Heavy rather than light weight is better for warmth of course as is a lining which may be present on the front of thigh area.
Men's - Genuine German Army Moleskin Pants for Outdoor, Hunting & Workwear