The title of our presentation is adapted from Report A-1 of the MIT Planetary Circulations Project, by Eugene M. Rasmusson, published 40 years ago in 1966. Gene used monthly mean sounding data from North American radiosonde stations to derive new insights into the sources and large scale transport pathways of atmospheric water. In this presentation, which is more geographically focused than Gene's continental-scale effort, we review a long-standing debate over the sources of moisture for one region (southwest North America), which has been resolved only partially using sonde-based data. Dynamical diagnostic tools such as trajectory models allow for more detailed examination of large scale moisture transport paths. Additional insights into the hydrological cycle are gained from a new geochemical data set that we have derived from analyses of the deuterium concentration of sampled water vapor. With these new tools we can extend the research that launched Gene's long career, and begin to constrain the sources and transport paths of water vapor in much more detail than was possible 40 years ago.