The real cold descended overnight from the Arctic, and by the time I headed outside to see what the frigid temperatures had wrought, it was in the mid-teens and windy: a wonderful break from global warming. I certainly didn't need the thermometer to gauge the situation: the Rhododendrons did that job for me. One of their adaptations to winter weather is to roll their evergreen leaves when the mercury heads below freezing, and when it really drops, the leaves can be thin and tight as cigars, even pencils. The curling movement is known as thermonasty, and it helps to minimize surface area and, in so doing, exposure to the potentially desiccating impact of cold and wind. Darwin wrote about this in the nineteenth century, and when it starts happening, I usually feature thermonasty in my weekly natural history column. So the natural thermometers have started speaking. It's clearly frigid. Most of my walking today will be confined to treks back and forth to the wood pile.