Few drivers realize that winter washer effectiveness depends as much on surface tension as on alcohol content. On a cold windscreen, a fluid that wets uniformly lifts road salt and soot without leaving residues that scatter headlight glare. Mixing with hard tap water introduces minerals that neutralize surfactants, increasing streaks and night halos; use demineralized water. Brief preheating with the defroster prevents flash-freeze, and fan nozzles spread thinner films that resist re-icing. Avoid dish soap additions, which foam and smear. For dependable dark-night clarity on snow and ice, see Winter –40 °C ethanol.