Hugh, recently B.B. wrote asking for readers' input on how a listening post/observation post (LP/OP) occupant can stay warm during extremely cold weather. His letter, and your reply, generated some self-reflection as I begin retirement in a few months after a 30+ year Army career, which spanned both active duty and National Guard service. During that time I served on active duty as an artillery forward observer to both Infantry and Armor companies and was branched Infantry for most of my National Guard service.
I suspect that B.B. will find that his biggest problem will not be one of staying warm, but it will rather be one of staying awake. I have been constantly amazed over my three decades, while watching both my soldiers and those of other units as well as those of other countries that I have served with, at how misused and misunderstood the monitoring of sleep is. People are hard-wired to sleep at night and be up during the day. More importantly, to be functional they need a minimal amount of sleep. Put them in a situation outside their normal routine, and their body will shut down. The bottom-line is: Don't make shifts too long because sleep is a double-edged weapon. They will be worthless the next day, either for driving, security, or doing simple farm work.
Let's start with B.B.'s initial question, keeping in mind that most people will utilize LP/OPs in two settings: Permanent and temporary, with temporary being what most would think of as patrol bases. Let's deal with the patrol base, or temporary halt, LP/OP. You simply can't put a person out in the dark for a long length of time and expect them to function well the next day. You have to balance a number of things, including exactly how cold it is, how exposed to the wind the location is, and how many people are available for duty. In reasonably temperate temperatures with a limited number of people, I think two hours is the absolute limit you should have someone on duty. They will need to function the next day and any longer will degrade their mental and physical abilities. In really cold weather one option is to put him in a chair inside a sleeping bag. If he wears a pair of sneakers (or even moccasins), it will help keep the bag clean, yet allow him to maintain mobility and the ability to quickly fight back. We never did this in the military; however, many of my friends do this on the deer stand, especially those with children hunting for the first time. The children stay warm, thus don't take an immediate dislike to hunting, yet it remains a simple thing to unzip the bag enough to shoot. Interestingly, the German army has a sleeping bag, still available at Army surplus stores, that has a quick opening slit in the back designed for you to put your feet out and run. It even has arm sleeves and a hood so you can fight from it. I have had one for about 20 years. While it is not the warmest bag I own, it will keep you comfortable enough for an hour or two in the harshest weather. Be sure the chair isn't metal or it will sap the warmth right out of everything it touches. We would take pieces of thick carpet and put on the floor of our tracked vehicles, otherwise in winter the cold metal would quickly turn your feet into blocks of ice.
We have a saying in the Army, "Soldiers don't do what you expect; they do what you inspect." There is no higher calling than standing guard over your buddies while they are asleep and defenseless. Yet, soldiers are also human and will succumb to things like discomfort due to cold and sleepiness. That is why close supervision is paramount. Rarely will a single LP be able to cover your entire area, thus a radio watch or sergeant of the guard to manage the entire perimeter is important. When you first stop at a location, do a thorough analysis of the location. The goal is to have as few LP/OPs as possible, while still maintaining security. I remember once, as a young Lieutenant, waking up late (after daylight) because a sentry had gotten tired of waiting on his replacement and simply gone back to sleep. When my very irate platoon sergeant looked into it, he found that our SOP needed a little tweaking. It called for only one LP/OP, which was to rotate every hour. We had enough soldiers that you only had to stand guard at night once every couple of days, insuring you got a reasonable amount of sleep. The problem, however, arose when the outgoing sentry would wake his replacement, who would need a few minutes to get dressed and ready. Many times, since they were both junior soldiers and there was no "adult" to insure prompt compliance, they would lay back down "for a few minutes to get their thoughts together." Needless to say, they would go back to sleep, generating a return visit by the outgoing sentry. Over the course of the night we found each tour got progressively longer, until some of the sentries had pulled double their allotted time in a single night watch. My platoon sergeant's answer, after a thorough reaming out by the first sergeant, was to confiscate one sleeping bag from the collective platoon, meaning that the outgoing sentry couldn't get warm until he got in the sleeping bag of the soldier who replaced him. Although we were in Germany at the time, it wasn't winter. So, although it did get chilly at night it wasn't dangerously cold. The first time someone was slow in getting up they were quickly reminded by a boot to get up so the outgoing guy could crawl in. Perhaps it was not the most hygienic of answers, but it worked. Remember, this was during peacetime maneuvers whose purpose was primarily to ensure we got up on time the next morning, as well as to keep thieves (yes, the military has them) from coming in and stealing anything they could find. During TEOTWAWKI you will probably not want to trust your life or the lives of your family to a single point of failure. Later in my career, after three mobilizations, we never used only one person. The Romans realized the importance of this and made it a death penalty for any sentries who fell asleep. The sentence was carried out immediately (the same day) and by his own unit, who were the very people he was charged with guarding. Obviously, you aren't going to do this to a family member or close friend, but it does show the importance of putting extra thought into it. Another answer, had we needed more than one guard, would have been to add a radio watch or sergeant of the guard. Their job is to be sure everyone stays awake, are focused, and to stay in touch with your higher element. During Desert Storm, my unit was short-handed. Every night I took both the first and last radio watch. That allowed my platoon sergeant a bit more sleep and allowed me some quiet time to write out operation orders and do other paperwork. After several months in the desert, before the war even began, we were all pretty exhausted, but you have to maintain a balance of security with work performance. During TEOTWAWKI you are going to have a multitude of daily tasks, including dealing with food procurement, and will require a clear mind. The task is to balance the two. You just have to put some thought into it.
On another train-up for a mobilization, we solved the problem another way. We were not yet in an active war zone, so having only one sentry wasn't a life or death situation. I had one soldier, from the deep southern woods, who was terrified of the desert to the point he couldn't sleep at night. This was made worse by the fact that when we first got to Fort Irwin and hadn't become acclimated to the local conditions (one of which was the local coyote population), he has a late night visitor. He had eaten his dinner and put his paper plate down beside his head before laying down to sleep. He woke about an hour later and opened his eyes only to see a coyote eating his leftovers from about 12 inches away. He woke the whole platoon up screaming and wouldn't leave the top of his tracked vehicle at night for the entire time we were at Fort Irwin. Every night my platoon sergeant gave him a pair of night observation devices with fresh batteries, and he stood watch over us until morning. He took countless catnaps during the day and somehow continued to function for the three months we were there. Once again, a solution that isn't really optimal, but you have to find solutions that fix your unique situation.
Optimally, your LP/OP will hunker down in total silence and hear anyone approaching from a long distance. Sometimes, however, they will be required to move around, once again perhaps if you don't have enough people. This is not really a good idea as it makes you much more visible but is necessary at times. . If you don't have enough people to have a radio watch designee, the LP/OP may have to wake up their replacement. A special note about safety: During the peacetime in the Army, we always lost more soldiers to accidents than any other thing. We were always careful where we parked the vehicles, especially the tanks and tracked vehicles that required cranking several times at night to maintain battery charge, especially if the radios were running. If possible, you don't move a vehicle, even a few feet, at night for fear of crushing someone. This happened several times to neighboring units while I was stationed in Germany. The same will be a concern in TEOTWAWKI. Night vehicular traffic will be dangerous from both an ambush and blackout driving considerations. When you laager up (camp in a defensive encirclement), it might be a good idea to park with an eye on not only safe, easy, movement in the dark if you have to move, but also in such a way that it facilitates the sentry in locating someone in the dark when noise discipline is paramount. One friend of mine had rivets welded to the right side of his track, to which he snapped his pup tent. That way everyone always knew where to find him.
LP/OPs for more permanent locations, such as I believe B.B. is talking about, have additional concerns. One is that it will be very difficult to completely camouflage your LP/OP. In snowy conditions, there is really no way around the obvious path through the snow to and from the location. The less you move back and forth the less obvious it will be, but there is no way to make it completely hidden in snow. One option is to dig it in so he can move around at least a little to stay warm. Although not a cold weather concern, we got around this in Iraq by draping camo nets over our guard towers. The Iraqis knew what and where they were, but couldn't tell if or when they were occupied, much the way deer can't tell if an enclosed deer stand has a hunter in it. While not all a retreat's LP/OP locations can be from an elevated building, it is worth the effort to strategically locate your positions in such a way that the occupant can move around some to stay warm, but not in such a way as to give away that someone is actually in it. You also need to stay away from taking coffee to it. Not only do smells carry at night, but the added heat makes it easier to pick up if the bad guys have a thermal imaging device. I never realized this until one of the tankers I was supporting invited me into his tank after dark to scan the local area with his thermals. It was amazing to go from pitch black darkness to actually being able to tell small details, including someone who was obviously drinking coffee from a canteen cup and another who was urinating off the top of his tank. If we were near a German town, you could even tell which cars had people in them, even if the motor was not on, due to the glow through the windows. It was amazing the difference the devices made.
Not all aids have to be high tech. If the LP/OP is at ground level, take a dog with you. Not only can they be used as a large, warming device (just ask any duck/goose hunter), they are much more likely to detect something out of the ordinary. I know that during my year in Iraq many of our nighttime raids on suspected insurgent hideouts were ruined by common untrained dogs giving the warning something wasn't right.
In closing, I see the answer to B.B.'s question as not a simple technological fix. Rather, it is a matter of balancing multiple ideas: proper location of the LP/OP itself, the amount of time each person spends in it, risk in its being detected versus facilitating the job of providing early warning, and, yes, whatever tricks you need to use to stay warm.