Cloudless sky represents the most common weather condition on Earth, yet water vapor in such conditions remains a critical component of the hydrosphere and the foundation of atmosphere–surface water cycling. Because of the temporal variability and spatial heterogeneity of the atmosphere’s water-holding capacity, the movement of water vapor, horizontally or vertically, leads to the generation of clouds and precipitation. Therefore, establishing a climatological, macro-scale reference value of atmospheric water content under non-precipitating conditions is essential. We define this reference as azure water (total column reference water, TCRW)—the vertically integrated atmospheric multiphase water under non-precipitating conditions. Using 6-hourly 0.5° ERA-Interim reanalysis data spanning 1979–2017, we derive a lower bound (TCRW_L) from clear-sky samples (total cloud cover < 10%) and an upper bound (TCRW_U) from weak-precipitation events; we prioritize TCRW_L for its superior climatological stability. Globally, TCRW_L exhibits a strong zonal gradient, decreasing systematically with latitude and elevation, and its high-value band migrates seasonally following large-scale circulation shifts. A latitude–elevation fitting model accurately reconstructs the observed climatological distribution, achieving R² = 0.9371 over land and 0.9234 for the full globe. Latitude and elevation thus dominate the first-order spatial organization of azure water, while atmospheric circulation explains regional deviations. As the threshold moisture state for precipitation onset, azure water provides a climatological baseline that separates non-precipitable background moisture from precipitable water, advancing analytical tools for atmosphere–surface water cycling studies in the hydrosphere and supporting broader hydrological and climate investigations.

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.