Azure water: The reference atmospheric water under non-precipitation condition [version 2]

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.