What you are seeing is probably cold induced vasodilation https://pmc.ncbi.nlm.nih.gov/articles/PMC4843861/ https://pmc.ncbi.nlm.nih.gov/articles/PMC4843861/

What you are seeing is probably cold induced vasodilation https://pmc.ncbi.nlm.nih.gov/articles/PMC4843861/ https://pmc.ncbi.nlm.nih.gov/articles/PMC4843861/ Incidentally there are some studies that show you get better at it with more frequent exposure. I have kayaked for many years and have found this to be the case - if my hands get cold now, dipping them into the water to further cool then hence opening the veins is very effective if counterintuitive way of warming my hands up.