Treadmill vs. the ground

It turns out that running on a treadmill and running on the ground are really rather different.

[T]hese biomechanical considerations explain why it is almost always better for serious athletes to train in real conditions rather than on the treadmill when given the choice. Working out on a treadmill in a controlled environment is better than slipping along on icy roads, plodding through snow drifts, fighting gale-force winds or battling with high heat and humidity, but it is unlikely that treadmill training can improve running economy as effectively as overground work. In fact, the movement patterns of the two types of activity are so different that treadmill training might even retard improvements in overground running economy - a particularly troubling thought.

It's also just more fun to run outside. Right now, though, the "icy roads and snow drifts" argument is more than enough to keep me inside.