Neither, rain, nor sleet, nor dark of night. . .

Well, actually that's not true, all of those things impact my ability to ride into work. The reality of a 2-2.5 hour bike commute is that on most days I will have to ride in the dark for part, if not most of my ride. That is unless I want to show up to work around 9:30 and leave around 2:30, and in that case there wouldn't be much reason to go in at all.

Riding in the dark is more about equipment and attitude than anything else. I found that a good headlight is not enough. Headlights serve two purposes, illuminate the area in front of you, and provide warning to other bikes and cars of your presence. To be useful in the first way, the light has to be pointed downward at a spot about 10 feet in front of the bike, which makes it pretty useless in the second way. Also, how fast I ride determines where in front of the bike I would like to see possibe hazards. Ten feet may work for level ground, but when zipping down a big hill I naturally look farther in advance, and a stationary light doesn't quite work. The solution -- a head lamp, one of those camping hands-free spelunkeresque lights. They work great, are very bright, and solve all of the problems that my simple fixed light can't. It shines light wherever I direct my eyes; down the hill, at the startling noise I heard on the side of the trail, at the scary glowing eyes in the bushes, at the driver of the car who is trying to turn on red while I'm crossing the intersection. I use both, a headlight to be seen, and a head lamp to be able to see. I also use a blinking red tail light so that I don't get run down from behind.

Lastly, I think the most important thing to remember about riding in the dark is to know your limits. There are parts of the trail that I don't want to be on in the dark, and other parts that are okay. For example there are 4 miles of the Custis trail in Arlington, VA that are hilly, full of blind turns, and possibly covered with leaves and tree roots. I time it to ride this section only after it is bright enough to see where I am going. At an hour before sunrise you can discern shapes well enough to avoid them. This is called, "Nautical Twilight" or the point in which the horizon can first becomes visible. I have found it is okay to ride the W&OD trail in these light conditions, because it is straight, and relatively hazard free. I can see just make out other bikers and walkers with enough time to adjust to them, but I can't really see if there are tree roots or branches down that would impede my progress. At a half hour before sunrise you can see figures clearly, and colors. This is the start of "Civil Twilight" otherwise known as dawn. I know I know I'm a geek; the only reason I mention this is because you can look these times up for specific places to plan out the trip. For example, I know that to make it to the Custis trail with enough light to not fall, I need to leave the house no earlier than an 90 minutes before sunrise. That works out to about 5:45 in January. Brrrrr.
It also means that since I don't want to be riding the streets of Fairfax in the dark I need to leave work at around 3:30 in the afternoon. Summer is a lot easier. Day Lights Saving makes for dark mornings, but light evenings.

I can't wait for the year to start, and the snow to melt. Merry Christmas!