Skiing in the dark has been pretty interesting, this winter. Especially on downhills, there's an "oh shit" factor of not being able to see anything much beyond the five feet my headlamp can illuminate, but that's more than balanced by the sense that any falls will be into some nice cushy snow - much preferable to the pavement of rollerskiing. (Ignore those trees, please: they're usually quite small, and often cloaked in nice springy branches.)
The cold is a different matter, more subject to management than to probability. Thanks to the right clothing, the cold hasn't driven me off the tracks, despite a couple outings in temperatures at or below 0° F (which is much more impressively rendered as -18°C). But every time I head out, my body goes through a very odd but orderly and now predictable routine of adjustment to the cold.
An illustration: over the first six and eight minutes of skiing (six if it's colder, eight if warmer), my fingertips get very numb, to the point tonight (1°F air temp) that it was hard to properly grip my ski poles. The fingers stay that way for about another six minutes, but then, after almost exactly a quarter-hour of skiing, the blood rushes back in and they warm up rapidly, which is where they stay for the rest of the session, whether that's 15 or 50 more minutes.
Thinking about this phenomenon - which doesn't happen on my bike trips to or from work, since I'm only riding for about eight minutes, maximum - I wondered exactly why the body reacted this way: why not just try to maintain body temperature the whole time? Coincidentally, today I saw a report from the International Ski Federation on cold-weather health which at least partly explains it. The body initially reacts to cold by trying to use the available blood to maintain the necessary warmth in the core (all those organs) and the brain. As exertion causes an increase in body temperature, there's simply more warmth to go around, and the body sends it back out to the extremities. The lessons for me are: 1) Warm up thoroughly and/or intensively, to jack up the core temp rapidly, and b) Be aware that, from the perspective of a brain during a winter workout, some extremities are apparently less valuable than others.