It's all down to the skirt in question and what goes on underneath it. I have a couple of heavy ankle-length cotton skirts that with a petticoat, slip, and tights under are quite good to about -10 F so long as I can stay dry. When it gets really cold, staying dry is key because if you don't things go downhill very, very rapidly. This includes the hem if it'll hit your legs anywhere.denimini wrote:I don't have an answer for cold weather. I have given up on skirts in a comparatively mild winter, temps similar to what Tom is experiencing - I don't know how you all cope so well.
A heavy petticoat is good for keeping the hem "out and away" from the legs; in addition, it adds mass and volume on its own and traps more air underneath. Some have ruffles at the hem, which, believe it or not are actually functional for trapping air and not just decorative. A smooth slip is pretty much mandatory if one is to wear heavy tights as the smooth fabric keeps the heavier fabrics of the skirt and petticoat from "catching" on tights and creating friction problems. All in all, it's an engineered system, and one that works amazingly well; it's just that it's been forgotten about now that trousers are the norm for both men and women alike.
In short (or long, depending on viewpoint), long skirts can be quite a bit warmer in winter-time than trousers. Give it a go sometime. You may be surprised.