With the recent development of high-quality, digital daily air-temperature archives over North America, flexible approaches that evaluate trends in time-varying percentiles are capable of estimating climate-change parameters from all portions of air-temperature frequency distributions. Using these methods, intense warming is found in the lowest minimum temperatures over western and central North America. During the months of January through March, the lower tail of the daily minimum air-temperature distribution over western North America has warmed at rates exceeding 3°C/50yr. Daily maximum air temperatures also are increasing over western Canada and Alaska at this time, but all parts of the frequency distribution are warming equally. Other times of year in western North America, as well as much of eastern North America, show little change in either minimum or maximum air temperature during the last half-century.