The surface-water components of the hydrologic cycle usually are considered to be rainfall and subsequent runoff. Yet, in many parts of North America, for a major part of the year, the more appropriate concept is precipitation and storage as snow and ice, followed by melting, followed by runoff. The timing involved in these two concepts is very different: Runoff follows rainfall almost immediately, whereas the time between snowfall and melt can range from days to months for seasonal snowcovers, and from months to millennia for glaciers. The predictive analysis of runoff also is very different: Prediction of runoff from rainfall requires knowledge of the precipitation pattern in time and space, whereas the prediction of runoff from snow and ice melt requires knowledge of the amount and distribution of snow in storage (measurable), and the meteorologic conditions that cause melt. The fact that snow and ice accumulate and melt to produce runoff in a very different way than does rainfall commonly is ignored in a simple hydrologic analysis.