Short- or long-sleeve baselayer top underneath a tee. Running jersey plus vest. Shorts and boxers with extra long legs for extra warmth around the thighs. Or a thin and wind-resistant running jacket worn over a tee plus tights.

A general rule of thumb is to dress as if it's 10 degrees warmer outside. This is because your body temperature will rise as you exercise. So, if it's 40 degrees and you're heading out for a run, wear what you normally would in 50-degree weather. For most runners, this would call for a long-sleeve tech tee with shorts.