Ken Snodgrass of Baltimore asked last week why frost sometimes forms on his front and rear car windows, but not the sides. Nelson Hyman, a retired thermal engineer, improved on our answer: "Dew/frost form on coldest surfaces first. All the windows radiate heat at the same rate, while they also absorb radiant heat from the environments they face. On balance, the more horizontal front and rear windows, facing the deep cold of space, chill faster than side windows, which face ... warmer terrestrial objects."