Every winter I suffer from sketcher’s abstinence. It gets too cold for drawing outdoors, the ink in my fountain pens stop flowing, and watercolours freeze on the paper surface. Some years I just give up, and try to find nice environments and interesting subject matter indoors, but after a while it is not quite enough. I tend to look for spaces and situations when I draw, not just things.
The last few years I have been trying to find reasonable methods to go out sketching in spite of low temperatures, but without freezing my fingers off. Pencils always work, of course. Especially the carpenter´s pencil, which is thicker, and therefore easier to hold with mittens. Alcohol is also a neat solution to be able to work with watercolours in cold temperatures. Not drinking it (although that might help too, I guess, to a certain extent), but mixing it in the watercolour water.
This works reasonably well in temperatures just below the freezing point. What you need is uncoloured alcohol without flavouring, that you mix in your water. Cheap vodka has worked for me. I usually start mixing 50/50 (which doesn´t say much, since the vodka is about 40-45% alcohol to begin with), and then add more alcohol depending on how cold it is. Trial and error.
I avoid using my more expensive watercolour brushes for this, but have an old squirrel mop that works fine. Depending on temperature I am sometimes painting more with slush than with dissolved liquid watercolour, but it usually works out in the end. Sometimes I get these really cool random crystal patterns in the washes – one of those cool surprises that only watercolour can give you.