Running in the cold

Cold weather is friendlier to performance than heat — many runners set personal bests in the 40s°F — but it brings its own hazards, and the real number to watch is wind chill, not air temperature. A still 20°F day and a windy 20°F day are completely different runs: moving air strips heat from exposed skin far faster, which is why RunWeather applies a wind-chill governor to the score and flags genuinely dangerous conditions.

Dress for about 15–20°F warmer than the thermometer reads, because you'll heat up once you're moving — the classic mistake is overdressing. Layer so you can regulate, protect the extremities (hands, ears, and face lose heat fastest), and watch for ice and low light, which are often the bigger winter risks than the cold itself. Below roughly 0°F wind chill, cover all exposed skin; in extreme wind chill, move the run indoors.

The flip side: cold, dry air usually means low dew points and low heat strain, so winter mornings often score very well for running. Use each city's month-by-month table to find the winter days that are crisp and runnable versus the ones where wind chill tips into the danger zone.