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. We can do better than that though and make a big overall difference to warmth and comfort.
Much 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 blood circulation can be cut down somewhat to reduce heat loss, but they do have a large surface area and so will 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. The usual approach of another top layer makes less difference than doing something about those thinly insulated pants.
Some winter pants options:
Cotton outer, fleece lined
Cotton is a very practical material for pants, whether they be for work, casual, chinos or jeans. It is hard wearing, smooth and comfortable, but isn't good when it gets cold.
A great solution to this is to line the pants with synthetic fleece for extra warmth, like having built-in thermal underwear allowing you to wear your favourite pants style year-round.
Cotton is not good when it gets wet, so don't use these for hiking or if you are unable to dry them overnight. The worst of all materials for winter pants is plain denim, cold and unforgiving to the touch at low temperatures it loves to hang on to water making it downright miserable to wear and possibly dangerous adding to the risk of hypothermia.
Formal or dress pants
If you have to wear formal pants, then you're going to be indoors, though many 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 upper thigh area for comfort but also adds extra warmth.
Softshell fabrics are a good choice for outdoor winter pants, these have a reasonable degree of weatherproofing against wind and light rain. "Hard shells" are fully wind and waterproof, though can be incompliant to wear for extended periods, soft shell is designed for cold conditions with extra surface weatherproofing though are not sufficient for extended rain or very strong cold wind.
It works better as a choice for pants material than it does for jackets.
Hiking and insulated pants
There are a whole range of modern materials, polyester, nylon and polyamide which can make great outdoors hiking winter pants.
Hiking pants are often a more relaxed fit than many casual pants allowing for greater freedom of movement and also if necessary making it easier to accommodate thermal underwear for extra warmth. The relaxed fit means that circulation is not affected and traps a layer of warm air within the pants.
Like many on this page, hiking pants may also come with additional insulation, lined with a separate thermal layer or with more substantial insulating material.
moleskin and corduroy
Moleskin and corduroy are both heavyweight soft cotton fabrics. They are tightly and densely woven 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), they are not as long-term hardwearing as synthetic materials and tend to cost a little more.
They 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
pants,,, something I still wear in the
colder months when I can and wouldn't consider wearing
Bib overalls, bibs
High pants with a chest panel and maybe a back panel too, they add extra insulation and keep you covered while bending or stretching, especially good for activities such as working 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 an 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 in January or climbing a Himalaya in winter.
An Antarctic Scenario - layering for versatility
This is me in Antarctica on a 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 as it did 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.