Azure water: The reference atmospheric water under non-precipitation condition [version 1]

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.