Cold weather makes cotton-poly tee collars pill because dry air stiffens the fibers, so they break and tangle when rubbed by scarves, jackets, or your own neck. To prevent it, wash inside out in cold water, air dry, and avoid rough layers. If pilling has already started, a fabric shaver can clean it up; if it keeps happening, consider a tri-blend tee with rayon or modal for a smoother collar.