The concept of water resources is largely limited only to water in rivers, lakes, underground aquifers, and other sources that can be used by humans directly--blue water. It does not normally include water that directly evaporates from the land surface and transpires from vegetation--green water. Generally speaking, the sum of blue water and green water is equivalent to total precipitation. Considering the global land surface, as much as about 65% of the total precipitation is converted into green water, and only the remaining 35% is converted into blue water. The concept of green water clearly extends the scope of research and management of water resources to the total amount of land precipitation, but the source of terrestrial water resources has not been intensively studied. In terms of a complete water cycle, water resources should refer to the renewal/replenishment of the fluxes of fresh water in different processes in the hydrosphere. The storage of water in the atmosphere is relatively small, but the huge flux of water transport in the atmosphere is the fundamental source of terrestrial water resources. In the contexts of existing water shortages, future water demands (including for ecological protection), and global climate change impacts on water resources, accurate quantitative studies on the formation of terrestrial water resources from the atmosphere are becoming increasingly important. Such studies require a clear definition of the concept of atmospheric water resource and particular focus on its precipitation conversion. Such research can indeed enhance our theoretical knowledge about the functions of water resources systems and further broaden our view of traditional water resources management. Motivated by this, the present paper attempts to improve our understanding of atmospheric water resources. This is done through: (1) a review of the concept of atmospheric water resource; (2) proposing a quantitative estimation method for atmospheric water resource; and (3) quantifying the precipitation conversion characteristics of atmospheric water resources. For a given period of time, we define the atmospheric water resource as the accumulated atmospheric water matter with potential to precipitate, and call it as white water.