Winter Pants Mid layer insulation
In cold weather the legs are often the most poorly insulated part of the body, while warm tops and coats get a look in, as often as not, the same pants are pulled on as at any other time of the year. Winter pants make a big difference to 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 with thicker material 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 cut down somewhat to reduce heat loss, but the legs do have a large surface area and so will still potentially lose a lot of heat and feel cold in the process. 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, many people will simply add another top layer but this makes much less difference than doing something about those thinly insulated legs.
Insulated and windproof warm winter pants are what you need.
Softshell fabrics are a good choice for outdoor winter pants, these have 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.
Hiking, insulated and snow pants
There are a whole range of modern materials, polyester, nylon and polyamide which can make great outdoors pants for colder weather hiking and/or snowsports.
Hiking pants are usually a more relaxed fit than many casual pants allowing for greater freedom of movement and making it easier to wear 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.
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.
Traditional materials - moleskin and corduroy
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 inbetween 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
something I still wear in the colder months when I can and wouldn't
consider wearing anything else
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. That bib can be almost not there when loose with nothing over it or a useful extra layer when closed down and under a jacket.
Available with tough cotton outers for work, with snow gaiters at the ankle for skiing and varying amounts of insulation for building things, working on cars or climbing a Himalaya during the winter.
Formal or dress pants
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 here of course as is a lining which may be present on the front of thigh area for comfort but also adds extra warmth.
An Antarctic Scenario - layering for versatility
This is me in Antarctica on a relatively mild and calm late winters day at about -15C (+5F). The sun and lack of wind meant it felt unusually warm for a short while, with the potential for the wind to get up and temperature to go down especially when the sun started to set early in the afternoon. This is a realistic use of cold weather gear, in the worst weather it is dangerous to go far outside, so people generally stay on base.
We went out on a trip to some icebergs frozen into the sea-ice. This entailed a 3 mile very rugged overland hike to get to the edge of the sea and then more hard work over a couple of miles of broken flat sea-ice with a substantial snow covering.
This was a day of varied temperatures, changing wind speed, and different activity levels, hard uphill walking and then easier on the flat. There was always the possibility that some hardcore Antarctic weather could arrive in a pretty short time that we needed to be prepared for. The clothing had to be versatile for changing conditions and up to the job of fending off the worst the weather might bring.