At the beginning of winter, 26 degrees seems bitter cold, but after days and days of temperatures below freezing, 26 seems bracing. Does this phenomenon have a name (and if not, please consider mine).
— Maja Ramirez, Avondale
The phenomenon does indeed have a name: acclimation. It’s the process of becoming habituated to a new climate or environment. You are absolutely correct to observe that the first genuinely cold weather of the winter season seems brutally cold, but after at least a few days of chill, the harshness of the temperature seems much less.
Acclimation works in other ways as well. A winter day in Chicago with a reading in the 60s will seem, to most of us, genuinely mild, but a midsummer day with an identical temperature will seem unpleasantly chilly, even cold.