Cold and Parkinson’s aren’t directly linked, but the cold does (also) increase muscle stiffness because the body tries to reduce heat loss and cut blood flow to the skin and, to some extent, the muscles. Muscle work generates heat, but the jaw muscles do not generate particularly much, and this is a muscle that is both strong and more exposed to the cold than, for example, other muscles covered by clothing.
I would think it has to do with a certain fatigue phenomenon linked to lower dopamine levels, which plays a small part in it, but mostly it’s cold-induced stiffness. A face mask and breaks to warm up the body – the face is probably more effective than more L-dopa.
//Håkan Widner