I hope to address several things here. Some are matter of fact and others are …well … natural. So I apologies ahead of time if the natural issues are out of your reading scope.
Yes even after all the clothes we put on, we will still need to think about taking breaks and warming up. Remember please that you are simply standing in the cold. You are not burning energy like a hiker or a skier so you will have to compensate. The first thing you will need is an egg timer. I usually set it for 20-30 minutes if the weather is bearable, but I will go down to 15 minutes if it is very cold. When the timer goes off, you will need to drop what you are doing and either get back to the car to thaw out for 10 minutes, or if that car is not close enough you will need to get your body warm by jogging or jumping or exercising in place for 5-10 minutes.
Trust me this is not optional if you want to be out there for long. You will not beat the cold. You have to either generate heat or else get heat from the car. If you don’t you will be rattling your teeth in 45 minutes and then you will be finding a reason to head back in. If you go past one hour without a break, you will have to head back in as there is no way to recapture enough heat to sustain you outside. Be smart. Break it up and you can be out all day I promise you. I have done it many times before.
When you get back in the car for a break, get some warm liquid into you. Soup is the best thing. Tea is OK, but it will make your bladder active, and coffee is a no-no. We will talk about that and alcohol later. I also carry seeds and nuts with me for snacks. You want simple snacks that don’t fill you up or get your digestive system working too hard. More on that later too.
By all means carry a few of those heat sachets for your gloves. They even make some for boots but I never need them. I do use a seat heater, the kind you get for football stadiums in winter. If you can build a small fire or if you have a camping stove to warm up the soup and your hands on all the better. What ever you do, don’t get so immersed in painting that you loose track of basic survival issues.
Foods and Drinks To Avoid
Coffee is a diuretic especially if you have it with cream so avoid it like the plague when you are out there. Also please avoid any greasy foods or heavy foods. The more your digestive system has to work, the more blood gets drawn from your extremities towards your abdomen and that is not a good thing if you plan to stand and paint in the cold.
You should completely avoid alcohol when you are out painting. I know the old myth about the flask of vodka or brandy and how it can make you feel warm all over. It is a myth and a bad one at that. The fleeting warmth that you feel as the alcohol goes down is soon replaces with alcohol in your blood which acts to lower the blood temperature very rapidly. Keep the booze for the evening at the cabin. You will enjoy it there much more and you won’t have to drive under the influence either!
As I mentioned before soup is the best liquid you can have. Chicken soup with noodles, a few salt crackers a hand full of seeds and nuts should keep you going every couple of hours.
I am surprised that no plein air book or article ever talks about this basic human function. It is hell to have to answer a major call of nature in the cold. So avoid it. basically (excuse the details here please but someone needs to talk about these things) I force myself to go before I leave for a plein air outing in winter or summer. A number one is not a major issue, but if you force yourself to a number two before you leave and you follow the eating and drinking recommendations above, you should be just fine.
First Aid Kit and Medical Issues
I carry a basic first aid kit with me at all times in the car. I augment that with Tylenol, antacid and sun cream for winter. I use the sun cream both as a barrier from the sun and snow glare and as a moisturizer against the cold drying temperature and wind.