Winter Trousers 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 trousers are pulled on as at any other time of the year. Winter trousers make a big difference to warmth and comfort.
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Winter trouser basics - Much of the time this is 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 legs.
Traditional materials - moleskin and corduroy
Moleskin and corduroy are heavyweight, soft cotton fabrics. They have a tight dense weave and a soft nap on the outside that adds to comfort and wind resistance. 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.
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.
The whole time I was in Antarctica I wore
moleskin trousers, something I still
wear in the colder months when I can and wouldn't consider wearing
Softshell fabrics are a good choice for outdoor winter trousers, these have a reasonable 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 wind. "Hard shells" are fully wind and waterproof, though can be incompliant to wear for extended periods.
Hiking and insulated trousers
There are a whole range of modern materials such as polyester, nylon and polyamide which can make great outdoors trousers for winter hiking.
Hiking trousers are often a more relaxed fit than many casual trousers 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 trousers. Hiking trousers may also come with additional insulation, lined with a separate thermal layer or with more substantial insulating material.
Cotton outer, fleece-lined
Cotton is a very practical material for trousers, 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 trousers with synthetic fleece for extra warmth, like having built-in thermal underwear allowing you to wear your favourite trousers style year-round, don't use these for hiking, skiing or where they may get wet though. 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 trousers 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 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.
Traditional wool formal or and casual trousers
If you have to wear formal trousers, then you're going to be indoors at least for 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 trousers 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.
Tweed is a traditional style of woven wool used for more casual wear, especially outdoor pursuits, it has good windproof properties and while not waterproof will still insulate if it gets wet. It makes an excellent insulating and weatherproof layer for the legs.