Cloudless sky or clear sky is the most common weather condition on the earth, when no water condenses from the atmosphere to the earth surface. However, water vapor in cloudless sky is still a key part of the hydrosphere, i.e., the basis of atmosphere-surface water cycling. Because of the temporal and spatial heterogeneity of water vapor holding capacity of the atmosphere, the move of water vapor, either horizontally or vertically, will lead to the generation of clouds and then precipitation. Therefore, a climatological view of the macro-scale reference value of water vapor content in the atmosphere under non-precipitation condition is substantial for hydrosphere research. In this paper, we will define this reference value as azure water, quantify it as the total column atmospheric water in all the gaseous, liquid and solid phases under non-precipitation condition, calculate its climatology values, and discuss its global pattern. Since azure water is the threshold for the appearance of precipitation, it will be important in studying the atmosphere-surface water cycling in the hydrosphere.

Due to the rapid deterioration of global climate, the study of its impact on atmospheric water resource is becoming an urgent issue in many regions. Extensive research has been done in the field of atmospheric water resources, but a consensus is yet to be reached to make this concept widely adoptable. Based on the concept of azure water, this paper proposes a novel concept representing the precipitable atmospheric water resources, being named as the “white water”, where an assumption is made to quantify the white water with an evaluation approach. Taking Tarim Basin in Xinjiang and Sanjiangyuan Area in Qinghai as study areas, the white water is found to be able to explain the differences in precipitation of these two areas, in spite that they have the same level of total atmospheric water flux. Further analysis also shows that the feature variables derived from the white water can intuitively explain the differences in precipitation conversion potentials from different regions. The proposed research work provides a new and effective approach to establish the link between the white water and the atmospheric water resources.