Winter has a beautiful sky (when it's not cloudy) and low humidity - a recipe for good observing.  But it can also be bitterly cold - especially since observing tends to be a rather sedentary activity.  Yet observing can be done comfortably, if you dress for the weather.  Let's review some common tips for warm winter dressing.

Wear layered clothing that's loose-fitting.  Loose clothing traps air inside, which helps insulate you from the cold.   In extreme cold, be sure to protect all uncovered skin.  However, the most common mistakes people make are wearing layers of cotton on top, tight-fitting jeans below, and shoes that leave their ankles exposed causing cold feet. Layers are important, but they must be the right layers, made out of the right fabric.

In cold weather, you should never wear cotton next to your skin. That goes for ALL underwear & socks! The reason? Cotton doesn't hold in heat very well even when dry and it absorbs moisture, so once it's damp, the moisture conducts heat right through it. If there is any wind, you'll feel a chill, making it worse. Once it's wet, it takes forever to dry. So the layer right next to your skin should be a synthetic fabric, such as a treated polyester.  Since polyester is incapable of absorbing moisture, it won't lose its insulating properties when wet and will dry rapidly.  

There actually is a natural fabric that will work next to your skin and women have known about it for a long time: Silk. It's extremely . . . well silky, won't absorb moisture and allows other layers to easily slide over it. It also feels great next to your skin. The warmth it provides given its weight is amazing.

So the first layer next to your naked body should be underwear made of either treated poly or silk. The second layer should be a long sleeve top, bottoms, sock liners for your feet & glove liners for your hands. These last two items should be lightweight to make room for additional layers. If you get cold easily, put on a short sleeve crew neck poly shirt on top of this layer. This will help keep your chest & back warm. The next layer could be a heavier layer of polyester, or a poly turtleneck, depending on the weather and your own comfort level.

Thinking wool for that outer layer?  Think again.  Wool is a much better insulator than cotton, but wool will absorb moisture and when it does, it will lose all of its insulating ability. It also takes forever to dry so don't allow it to get wet.   idea for the outer layer on top is a fuzzy fleece pullover or jacket. They also have pockets.  If it is made of polyester, it won't absorb moisture and will trap heat very well. Look for one that has large enough pockets on both sides to hold your hands with gloves on.   If you get cold easily, wear a poly or down vest underneath this layer. The outer layer can also be a nylon down jacket or a full length coat that extends below your rump. Realize that even down will lose it's insulating ability when wet or compressed too much. It needs room to puff out.   Finally keep in mind that this outer layer will need to be large enough to accommodate the layers beneath. You need space for trapped air in these layers and so you can move.

Next, let's talk about your legs. If you layer up your chest well, but only wear cotton jeans, you'll freeze. Why?  First, your rump & legs have more surface area than your trunk. Two, cotton doesn't insulate very well and the warm blood that leaves your core will loose its heat as it travels down your legs to your feet and back again, where your core has to warm it up again. You'll be alright for awhile, then you'll start to get cold and then you'll start to shiver. Third, once you reach this point, your body will constrict the blood vessels going to your legs & feet to maintain your core temperature, making the situation worse.  Fourth, jeans don't block the wind.

So what to do?  Silk bottoms under jeans is a first step. Adding poly bottoms between the silk ones and your jeans is another. Wearing flannel lined jeans is another option. Get them roomy enough so you can layer underneath them if need be. Or forget jeans altogether and go with nylon insulated bib overalls, like those you ski in. These have the advantage of covering your lower back when you sit or bend over, so that no skin is exposed. Or you can wear sweat bottoms made of fuzzy polyester and wear plastic or nylon raingear or snowpants over them. This combination works well because it allows you to move, it traps warm air, the nylon shell keeps wind from cooling you off, and the suspenders don't constrict your waist like a belt does. With your rump & legs covered like this, you'll be very comfortable for long periods of time and your feet will stay warm.

Now about your feet... Protect them well; if they're cold, the rest of you will be cold too!  How to keep your feet warm?  Layer them. First a very light long sock made of either polyester or silk. Next, a heavier sock that will cover your calves. Next, you need a shoe that is large enough to let you wiggle your toes and trap heat. If your shoes are too small for these layers, you'll compress the insulation making it less effective and your toes will be jammed together, so they'll get cold. Insulated after-ski boots work well as they also cover your ankles. Regular shoes leave your ankles exposed and you'll loose critical heat. Plus cold air always sinks and that's where your feet are. Whatever boots you decide on, don't over tighten them.  Also, in extreme cold, hard ground seems to rise up through your boots if you stand there for long periods. Even a simple throw rug or doormat can help insulate you from the cold ground.

Let's move on up to your hands. Layering works well here also. Start with a lightweight poly or silk glove liner, then an intermediate glove if you need it and finally a ski glove. Don't make the mistake of trying to cram your hand with these additional layers on into a glove that's too small. Your hands will be cold and uncomfortable and then you will. You need airspace around your fingers to keep them warm. Since we have to focus, change eyepieces, turn on flashlights and such, many will want to use gloves. But if your hands get cold easily, insulated mittens with a nylon shell over them are warmer because your fingers are all together and mittens have less surface area by far exposed to the cold than gloves. Another tip for keeping your hands (and feet) warm is to apply a little greaseless lotion to them first. It'll act as an additional insulation layer. Matter of fact, you can do this to your entire body (including your back) if you get cold easily. A little goes a long way here; you don't want to be sticky.

Finally, we come to your neck & head. A polar fleece neck gaitor (it looks like a short, wide tube that's open on both ends) will cover your neck and prevent heat loss, which can be substantial in this area of your body. If you wear a turtleneck, pull it up all the way under your chin as a first layer over your neck then put the neck gaitor over it. For your head, there are many choices. If you get cold easily, you should wear a silk or poly balaclava then a ski cap or the hood from your jacket (if it has one) over it. A balaclava looks like a sock, sized for your head, with a oval hole in the side for your eyes. It covers your head, ears, nose, mouth and neck. Even a lightweight one will keep you very warm

Think you're overdressed?  Remember, if you're going out on a walk, you can get away with less, because your muscles will generate heat as you exercise. For non-aerobic activities, you might have to add a layer here or there. And a few last things to remember:

• Astronomy is not usually considered an aerobic activity, though I have seen a few rare exceptions. If you start to sweat, unzip your outer jacket, take off your hat, peal off an outer layer, whatever it takes. The goal is to not sweat. You may want to remove a layer before you start setting up and then put it back on when your finished.
• Don't sit on a metal lawn chair, bench or even an aluminum framed beach chair in cold weather without first covering the metal with an insulated stadium seat or blanket. Metal is an excellent conductor of heat and you'll lose lots of it from your rump. Remember that when you sit, you compress all the air out of your insulation layers covering your rump, reducing their effectiveness. Don't sit on the tailgate of a truck for the same reason.
• For a quick warm up, sit on the hood of your car if it's still warm.
• Don't hold onto cold eyepieces, even with gloves on, any longer than necessary. Heat will flow out of your hands right into them.
• Bring something warm to drink in a thermos.
• If your hands get cold easily, don't wear a metal watch or rings on your fingers, they'll conduct heat away from your body. Put your metal watch in your jacket pocket. Likewise no earrings, noserings, liprings/studs, tongue studs, eyebrow studs, etc. Don't stick your tongue on any cold metal, it WILL stick! Lastly remove all alien implants before observing in cold weather.
• Eat well a few hours before so your energy reserves are full. Your body burns more calories in colder weather because it's constantly losing heat and it needs to maintain its core temperature.
• If you wear metal framed glasses to see at night, they might give you a headache over a period of time in very cold weather as they draw heat out of your temples where they touch. To prevent this, obtain some 1/8 or 3/16 inch adhesive backed foam weather stripping and attach it to your frames where they touch the side of your head. Or you can use a flexible piece of heatshrink tubing to slide over the end. You don't need to shrink it in place as you'll want to remove it later. 
• Take breaks every hour or half hour, depending on the weather, and go warm up. Keep an extra pair of dry socks warming on the dash of your car, or go in and throw a pair in the dryer for a few minutes. It’s amazing how a nice toasty pair of socks can change your attitude!
• Get out of the wind. Most of the time it’s not the air temperature that gets you, it’s the wind-chill. Put a building or a hedge or an observatory between you and the wind and you will be able to endure the cold for twice as long. The added bonus of not having the telescope shake will save you time in making estimates.